Chemistry Comparison

Lets examine the advantages and limitations of today’s popular battery systems. Batteries are scrutinized not only in terms of energy density but service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since NiCad remains a standard against which other batteries are compared, let’s evaluate alternative chemistries against this classic battery type.

Nickel Cadmium (NiCad) – mature and well understood but relatively low in energy density. The Ni-Cad is used where long life, high discharge rate and economical price are important. Main applications are two-way radios, biomedical equipment, professional video cameras and power tools. The Ni-Cad contains toxic metals and is not environmentally friendly.

Nickel-Metal Hydride (NiMH) – has a higher energy density compared to the Ni-Cad at the expense of reduced cycle life. NiMH contains no toxic metals. Applications include mobile phones and laptop computers.

Lead Acid – most economical for larger power applications where weight is of little concern. The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems.

Lithium Ion (Li-ion) – fastest growing battery system. Li-ion is used where high-energy density and light weight is of prime importance. The Li-ion is more expensive than other systems and must follow strict guidelines to assure safety. Applications include notebook computers and cellular phones.

Reusable Alkaline – replaces disposable household batteries; suitable for low-power applications. Its limited cycle life is compensated by low self-discharge, making this battery ideal for portable entertainment devices and flashlights.

	Ni-Cad	NiMH	Lead Acid	Li-ion	Li-ion polymer	Reusable Alkaline
Gravimetric Energy Density (Wh/kg)	45-80	60-120	30-50	110-160	100-130	80 (initial)
Internal Resistance (includes peripheral circuits) in mW	100 to 2001 6V pack	200 to 3001 6V pack	<1001 12V pack	150 to 2501 7.2V pack	200 to 3001 7.2V pack	200 to 20001 6V pack
Cycle Life (to 80% of initial capacity)	15002	300 to 5002,3	200 to 3002	500 to 10003	300 to 500	503 (to 50%)
Fast Charge Time	1h typical	2-4h	8-16h	2-4h	2-4h	2-3h
Overcharge Tolerance	Moderate	low	high	very low	low	moderate
Self-discharge / Month (room temperature)	20%4	30%4	5%	10%5	~10%5	0.3%
Cell Voltage (nominal)	1.25V6	1.25V6	2V	3.6V	3.6V	1.5V
Load Current -    peak -    best result	20C 1C	5C 0.5C or lower	5C7 0.2C	>2C 1C or lower	>2C 1C or lower	0.5C 0.2C or lower
Operating Temperature (discharge only)	-40 to 60°C	-20 to 60°C	-20 to 60°C	-20 to 60°C	0 to 60°C	0 to 65°C
Maintenance Requirement	30 to 60 days	60 to 90 days	3 to 6 months9	not req.	not req.	not req.
Typical Battery Cost (US$, reference only)	$50 (7.2V)	$60 (7.2V)	$25 (6V)	$100 (7.2V)	$100 (7.2V)	$5 (9V)
Cost per Cycle (US$)11	$0.04	$0.12	$0.10	$0.14	$0.29	$0.10-0.50
Commercial use since	1950	1990	1970	1991	1999	1992

Characteristics of commonly used rechargeable batteries.
The figures are based on average ratings of batteries available commercially at the time of publication; experimental batteries with above average ratings are not included.

1.     Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of Li-ion and Li-polymer adds about 100mW.

2.     Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.

3.     Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.

4.     The discharge is highest immediately after charge, then tapers off. The Ni-Cad capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.

5.     Internal protection circuits typically consume 3% of the stored energy per month.

6.     1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference between the cells; it is simply a method of rating.

7.     Capable of high current pulses.

8.     Applies to discharge only; charge temperature range is more confined.

9.     Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.

10.   Cost of battery for commercially available portable devices.

11.   Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

Observation: It is interesting to note that Ni-Cad has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle, but has the most demanding maintenance requirements.

Nickel Cadmium (Ni-Cad) Battery

The Ni-Cad prefers fast charge to slow charge and pulse charge to DC charge. All other chemistries prefer a shallow discharge and moderate load currents. The Ni-Cad is a strong and silent worker; hard labor poses no problem. In fact, the Ni-Cad is the only battery type that performs best under rigorous working conditions. It does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. A periodic full discharge is so important that, if omitted, large crystals will form on the cell plates (also referred to as 'memory') and the Ni-Cad will gradually lose its performance.

Among rechargeable batteries, Ni-Cad remains a popular choice for applications such as two-way radios, emergency medical equipment, professional video cameras and power tools. Over 50 percent of all rechargeable batteries for portable equipment are Ni-Cad. However, the introduction of batteries with higher energy densities and less toxic metals is causing a diversion from Ni-Cad to newer technologies.

Advantages and Limitations of Ni-Cad Batteries
Advantages	Fast and simple charge – even after prolonged storage. High number of charge/discharge cycles – if properly maintained, the Ni-Cad provides over 1000 charge/discharge cycles. Good load performance – the Ni-Cad allows recharging at low temperatures. Long shelf life – in any state-of-charge. Simple storage and transportation – most airfreight companies accept the Ni-Cad without special conditions. Good low temperature performance. Forgiving if abused – the Ni-Cad is one of the most rugged rechargeable batteries. Economically priced – the Ni-Cad is the lowest cost battery in terms of cost per cycle. Available in a wide range of sizes and performance options – most Ni-Cad cells are cylindrical.
Limitations	Relatively low energy density – compared with newer systems. Memory effect – the Ni-Cad must periodically be exercised to prevent memory. Environmentally unfriendly – the Ni-Cad contains toxic metals. Some countries are limiting the use of the Ni-Cad battery. Has relatively high self-discharge – needs recharging after storage.

Nickel-Metal Hydride (NiMH) Battery

The success of the NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to Ni-Cad. There is potential for yet higher capacities, but not without some negative side effects.

Both NiMH and Ni-Cad are affected by high self-discharge. The Ni-Cad loses about 10 percent of its capacity within the first 24 hours, after which the self-discharge settles to about 10 percent per month. The self-discharge of the NiMH is about one-and-a-half to two times greater compared to Ni-Cad. Selection of hydride materials that improve hydrogen bonding and reduce corrosion of the alloy constituents reduces the rate of self-discharge, but at the cost of lower energy density.

The NiMH has been replacing the Ni-Cad in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than Ni-Cad batteries. This is due to environmental concerns about careless disposal of the spent battery.

Initially more expensive than the Ni-Cad, the price of the NiMH has dropped and is now almost at par value. This was made possible with high volume production. With a lower demand for Ni-Cad, there will be a tendency for the price to increase.

Advantages and Limitations of NiMH Batteries
Advantages	30 – 40 percent higher capacity over a standard Ni-Cad. The NiMH has potential for yet higher energy densities. Less prone to memory than the Ni-Cad. Periodic exercise cycles are required less often. Simple storage and transportation – transportation conditions are not subject to regulatory control. Environmentally friendly – contains only mild toxins; profitable for recycling.
Limitations	Limited service life – if repeatedly deep cycled, especially at high load currents, the performance starts to deteriorate after 200 to 300 cycles. Shallow rather than deep discharge cycles are preferred. Limited discharge current – although a NiMH battery is capable of delivering high discharge currents, repeated discharges with high load currents reduces the battery’s cycle life. Best results are achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of the rated capacity). More complex charge algorithm needed – the NiMH generates more heat during charge and requires a longer charge time than the Ni-Cad. The trickle charge is critical and must be controlled carefully. High self-discharge – the NiMH has about 50 percent higher self-discharge compared to the Ni-Cad. New chemical additives improve the self-discharge but at the expense of lower energy density. Performance degrades if stored at elevated temperatures – the NiMH should be stored in a cool place and at a state-of-charge of about 40 percent. High maintenance – battery requires regular full discharge to prevent crystalline formation. About 20 percent more expensive than Ni-Cad – NiMH batteries designed for high current draw are more expensive than the regular version.

Lithium Ion Battery

Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight. Rechargeable batteries using lithium metal anodes (negative electrodes) are capable of providing both high voltage and excellent capacity, resulting in an extraordinary high energy density.

After much research on rechargeable lithium batteries during the 1980s, it was found that cycling causes changes on the lithium electrode. These transformations, which are part of normal wear and tear, reduce the thermal stability, causing potential thermal runaway conditions. When this occurs, the cell temperature quickly approaches the melting point of lithium, resulting in a violent reaction called ‘venting with flame’. A large quantity of rechargeable lithium batteries sent to Japan had to be recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a person’s face.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, the Li-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first Li-ion battery. Other manufacturers followed suit. Today, the Li-ion is the fastest growing and most promising battery chemistry.

The energy density of the Li-ion is typically twice that of the standard Ni-Cad. Improvements in electrode active materials have the potential of increasing the energy density close to three times that of the Ni-Cad. In addition to high capacity, the load characteristics are reasonably good and behave similarly to the Ni-Cad in terms of discharge characteristics (similar shape of discharge profile, but different voltage). The flat discharge curve offers effective utilization of the stored power in a desirable voltage spectrum.

The Li-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery’s life. In addition, the self-discharge is less than half compared to Ni-Cad and NiMH, making the Li-ion well suited for modern fuel gauge applications.

The high cell voltage of Li-ion allows the manufacture of battery packs consisting of only one cell. Many of today’s mobile phones run on a single cell, an advantage that simplifies battery design. Supply voltages of electronic applications have been heading lower, which in turn requires fewer cells per battery pack. To maintain the same power, however, higher currents are needed. This emphasizes the importance of very low cell resistance to allow unrestricted flow of current.

The choice of metals, chemicals and additives help balance the critical trade-off between high energy density, long storage time, extended cycle life and safety. High energy densities can be achieved with relative ease. For example, adding more nickel in lieu of cobalt increases the ampere/hours rating and lowers the manufacturing cost but makes the cell less safe. While a start-up company may focus on high energy density to gain quick market acceptance, safety, cycle life and storage capabilities may be compromised. Reputable manufacturers, such as Sony, Panasonic, Sanyo, Moli Energy and Polystor place high importance on safety. Regulatory authorities assure that only safe batteries are sold to the public.

Li-ion cells cause less harm when disposed of than lead or cadmium-based batteries. Among the Li-ion family, the spinel is the friendliest in terms of disposal.

Despite its overall advantages, Li-ion also has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current is limited and the cell temperature is monitored to prevent temperature extremes. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.

Aging is a concern with most Li-ion batteries. For unknown reasons, battery manufacturers are silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. Over two or perhaps three years, the battery frequently fails. It should be mentioned that other chemistries also have age-related degenerative effects. This is especially true for the NiMH if exposed to high ambient temperatures.

Storing the battery in a cool place slows down the aging process of the Li-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should only be partially charged when in storage.

Extended storage is not recommended for Li-ion batteries. Instead, packs should be rotated. The buyer should be aware of the manufacturing date when purchasing a replacement Li-ion battery. Unfortunately, this information is often encoded in an encrypted serial number and is only available to the manufacturer.

Manufacturers are constantly improving the chemistry of the Li-ion battery. Every six months, a new and enhanced chemical combination is tried. With such rapid progress, it becomes difficult to assess how well the revised battery ages and how it performs after long-term storage.

Cost analysis – The most economical lithium-based battery in terms of cost-to-energy ratio is a pack using the cylindrical 18650 cell. This battery is somewhat bulky but suitable for portable applications such as mobile computing. If a slimmer pack is required (thinner than 18 mm), the prismatic Li-ion cell is the best choice. There is little or no gain in energy density per weight and size over the 18650, however the cost is more than double.

If an ultra-slim geometry is needed (less than 4 mm), the best choice is Li-ion polymer. This is the most expensive option in terms of energy cost. The Li-ion polymer does not offer appreciable energy gains over conventional Li-ion systems, nor does it match the durability of the 18560 cell.

Advantages and Limitations of Li-ion Batteries
Advantages	High energy density – potential for yet higher capacities. Relatively low self-discharge – self-discharge is less than half that of Ni-Cad and NiMH. Low Maintenance – no periodic discharge is needed; no memory.
Limitations	Requires protection circuit – protection circuit limits voltage and current. Battery is safe if not provoked. Subject to aging, even if not in use – storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect. Moderate discharge current. Subject to transportation regulations – shipment of larger quantities of Li-ion batteries may be subject to regulatory control. This restriction does not apply to personal carry-on batteries. Expensive to manufacture – about 40 percent higher in cost than Ni-Cad. Better manufacturing techniques and replacement of rare metals with lower cost alternatives will likely reduce the price. Not fully mature – changes in metal and chemical combinations affect battery test results, especially with some quick test methods.

Caution: Li-ion batteries have a high energy density. Exercise precaution when handling and testing. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse polarity, expose to high temperature or disassemble. Only use the Li-ion battery with the designated protection circuit. High case temperature resulting from abuse of the cell could cause physical injury. The electrolyte is highly flammable. Rupture may cause venting with flame.

Protection Circuits

Most battery packs include some type of protection to safeguard battery and equipment, should a malfunction occur. The most basic protection is a fuse that opens if excessively high current is drawn. Some fuses open permanently and render the battery useless once the filament is broken; other fuses are based on a Polyswitch™, which resembles a resettable fuse. On excess current, the Polyswitch™ creates a high resistance, inhibiting the current flow. When the condition normalizes, the resistance of the switch reverts to the low ON position, allowing normal operation to resume. Solid-state switches are also used to disrupt the current. Both solid-state switches and the Polyswitch™ have a residual resistance to the ON position during normal operation, causing a slight increase in internal battery resistance.

A more complex protection circuit is found in intrinsically safe batteries. These batteries are mandated for two-way radios, gas detectors and other electronic instruments that operate in a hazardous area such as oil refineries and grain elevators. Intrinsically safe batteries prevent explosion, should the electronic devices malfunction while operating in areas that contain explosive gases or high dust concentration. The protection circuit prevents excessive current, which could lead to high heat and electric spark.

There are several levels of intrinsic safety, each serving a specific hazard level. The requirement for intrinsic safety varies from country to country. The purchase cost of an intrinsically safe battery is two or three times that of a regular battery.

Commercial Li-ion packs contain one of the most exact protection circuits in the battery industry. These circuits assure safety under all circumstances when in the hands of the public. Typically, a Field Effect Transistor (FET) opens if the charge voltage of any cell reaches 4.30V and a fuse activates if the cell temperature approaches 90°C (194°F). In addition, a disconnect switch in each cell permanently interrupts the charge current if a safe pressure threshold of 1034 kPa (150 psi) is exceeded. To prevent the battery from over-discharging, the control circuit cuts off the current path at low voltage, which is typically 2.50V/cell.

The Li-ion is typically discharged to 3V/cell. The lowest ‘low-voltage’ power cut-off is 2.5V/cell. During prolonged storage, however, a discharge below that cut-off level is possible. Manufacturers recommend a ‘trickle’ charge to raise such a battery gradually back up into the acceptable voltage window.

Not all chargers are designed to apply a charge once a Li-ion battery has dipped below 2.5V/cell. A ‘wake-up’ boost will be needed to first engage the electronic circuit, after which a gentle charge is applied to re-energize the battery. Caution must be applied not to boost lithium-based batteries back to life, which have dwelled at a very low voltage for a prolonged time.

Each parallel string of cells of a Li-ion pack needs independent voltage monitoring. The more cells that are connected in series, the more complex the protection circuit becomes. Four cells in series is the practical limit for co Commercial applications.

The internal protection circuit of a mobile phone while in the ON position has a resistance of 50 to 100 mW. The circuit normally consists of two switches connected in series. One is responsible for high cut-off, the other for low cut-off. The combined resistance of these two devices virtually doubles the internal resistance of a battery pack, especially if only one cell is used. Battery packs powering mobile phones, for example, must be capable of delivering high current bursts. The internal protection does, in a certain way, interfere with the current delivery.

Some small Li-ion packs with spinel chemistry containing one or two cells may not include an electronic protection circuit. Instead, they use a single component fuse device. These cells are deemed safe because of small size and low capacity. In addition, spinel is more tolerant than other systems if abused. The absence of a protection circuit saves money, but a new problem arises. Here is what can happen:

Mobile phone users have access to chargers that may not be approved by the battery manufacturer. Available at low cost for car and travel, these chargers may rely on the battery’s protection circuit to terminate at full charge. Without the protection circuit, the battery cell voltage rises too high and overcharges the battery. Apparently still safe, irreversible battery damage often occurs. Heat buildup and bulging is common under these circumstances. Such situations must be avoided at all times. The manufacturers are often at a loss when it comes to replacing these batteries under warranty.

Li-ion batteries with cobalt electrodes, for example, require full safety protection. A major concern arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. Such damage often causes the solid-state switches to fuse in a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but does not provide the required safety. If charged beyond safe voltage limits with a poorly designed accessory charger, the battery may heat up, then bulge and in some cases vent with flame. Shorting such a battery can also be hazardous.

Manufacturers of Li-ion batteries refrain from mentioning explosion. ‘Venting with flame’ is the accepted terminology. Although slower in reaction than an explosion, venting with flame can be very violent and inflicts injury to those in close proximity. It can also damage the equipment to which the battery is connected.

Most manufacturers do not sell the Li-ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits. Most battery assembling houses must certify the pack assembly and protection circuit intended to be used with the manufacturer before these items are approved for sale.

Chargers and Proper Charge Methods

To a large extent, the performance and longevity of rechargeable batteries depends on the quality of the chargers. Battery chargers are commonly given low priority, especially on consumer products. Choosing a quality charger makes sense. This is especially true when considering the high cost of battery replacements and the frustration that poorly performing batteries create. In most cases, the extra money invested is returned because the batteries last longer and perform more efficiently.

All About Chargers

There are two distinct varieties of chargers: the personal chargers and the industrial chargers. The personal charger is sold in attractive packaging and is offered with such products as mobile phones, laptops and video cameras. These chargers are economically priced and perform well when used for the application intended. The personal charger offers moderate charge times.

In comparison, the industrial charger is designed for employee use and accommodates fleet batteries. These chargers are built for repetitive use. Available for single or multi-bay configurations, the industrial chargers are offered from the original equipment manufacturer (OEM). In many instances, the chargers can also be obtained from third party manufacturers. While the OEM chargers meet basic requirements, third party manufacturers often include special features, such as negative pulse charging, discharge function for battery conditioning, and state-of-charge (SoC) and state-of-health (SoH) indications. Many third party manufacturers are prepared to build low quantities of custom chargers. Other benefits third party suppliers can offer include creative pricing and superior performance.

Not all third party charger manufacturers meet the quality standards that the industry demands, The buyer should be aware of possible quality and performance compromises when purchasing these chargers at discount prices. Some units may not be rugged enough to withstand repetitive use; others may develop maintenance problems such as burned or broken battery contacts.

Uncontrolled over-charge is another problem of some chargers, especially those used to charge nickel-based batteries. High temperature during charge and standby kills batteries. Over-charging occurs when the charger keeps the battery at a temperature that is warm to touch (body temperature) while in ready condition.

Some temperature rise cannot be avoided when charging nickel-based batteries. A temperature peak is reached when the battery approaches full charge. The temperature must moderate when the ready light appears and the battery has switched to trickle charge. The battery should eventually cool to room temperature.

If the temperature does not drop and remains above room temperature, the charger is performing incorrectly. In such a case, the battery should be removed as soon as possible after the ready light appears. Any prolonged trickle charging will damage the battery. This caution applies especially to the NiMH because it cannot absorb overcharge well. In fact, a NiMH with high trickle charge could be cold to the touch and still be in a damaging overcharge condition. Such a battery would have a short service life.

A lithium-based battery should never get warm in a charger. If this happens, the battery is faulty or the charger is not functioning properly. Discontinue using this battery and/or charger.

It is best to store batteries on a shelf and apply a topping-charge before use rather than leaving the pack in the charger for days. Even at a seemingly correct trickle charge, nickel-based batteries produce a crystalline formation (also referred to as ‘memory’) when left in the charger. Because of relatively high self-discharge, a topping charge is needed before use. Most Li-ion chargers permit a battery to remain engaged without inflicting damage.

There are three types of chargers for nickel-based batteries. They are:

Slow Charger – Also known as ‘overnight charger’ or ‘normal charger’, the slow-charger applies a fixed charge rate of about 0.1C (one tenth of the rated capacity) for as long as the battery is connected. Typical charge time is 14 to 16 hours. In most cases, no full-charge detection occurs to switch the battery to a lower charge rate at the end of the charge cycle. The slow-charger is inexpensive and can be used for Ni-Cad batteries only. With the need to service both Ni-Cad and NiMH, these chargers are being replaced with more advanced units.

If the charge current is set correctly, a battery in a slow-charger remains lukewarm to the touch when fully charged. In this case, the battery does not need to be removed immediately when ready but should not stay in the charger for more than a day. The sooner the battery can be removed after being fully charged, the better it is.

A problem arises if a smaller battery (lower mAh) is charged with a charger designed to service larger packs. Although the charger will perform well in the initial charge phase, the battery starts to heat up past the 70 percent charge level. Because there is no provision to lower the charge current or to terminate the charge, heat-damaging over-charge will occur in the second phase of the charge cycle. If an alternative charger is not available, the user is advised to observe the temperature of the battery being charged and disconnect the battery when it is warm to the touch.

The opposite may also occur when a larger battery is charged on a charger designed for a smaller battery. In such a case, a full charge will never be reached. The battery remains cold during charge and will not perform as expected. A nickel-based battery that is continuously undercharged will eventually loose its ability to accept a full charge due to memory.

Quick Charger – The so-called quick-charger, or rapid charger, is one of the most popular. It is positioned between the slow-charger and the fast-charger, both in terms of charging time and price. Charging takes 3 to 6 hours and the charge rate is around 0.3C. Charge control is required to terminate the charge when the battery is ready. The well designed quick-charger provides better service to nickel-based batteries than the slow-charger. Batteries last longer if charged with higher currents, provided they remain cool and are not overcharged. The quick-chargers are made to accommodate either nickel-based or lithium-based batteries. These two chemistries can normally not be interchanged in the same charger.

Fast Charger – The fast-charger offers several advantages over the other chargers; the obvious one is shorter charge times. Because of the larger power supply and the more expensive control circuits needed, the fast-charger costs more than slower chargers, but the investment is returned in providing good performing batteries that live longer.

The charge time is based on the charge rate, the battery’s SoC, its rating and the chemistry. At a 1C charge rate, an empty Ni-Cad typically charges in a little more than an hour. When a battery is fully charged, some chargers switch to a topping charge mode governed by a timer that completes the charge cycle at a reduced charge current. Once fully charged, the charger switches to trickle charge. This maintenance charge compensates for the self-discharge of the battery.

Modern fast-chargers commonly accommodate both Ni-Cad and NiMH batteries. Because of the fast-charger’s higher charge current and the need to monitor the battery during charge, it is important to charge only batteries specified by the manufacturer. Some battery manufacturers encode the batteries electrically to identify their chemistry and rating. The charger then sets the correct charge current and algorithm for the battery intended. Lead Acid and Li-ion chemistries are charged with different algorithms and are not compatible with the charge methods used for nickel-based batteries.

It is best to fast charge nickel-based batteries. A slow charge is known to build up a crystalline formation on nickel-based batteries, a phenomenon that lowers battery performance and shortens service life. The battery temperature during charge should be moderate and the temperature peak kept as short as possible.

It is not recommended to leave a nickel-based battery in the charger for more than a few days, even with a correctly set trickle charge current. If a battery must remain in a charger for operational readiness, an exercise cycle should be applied once every month.

Simple Guidelines

A charger designed to service NiMH batteries can also accommodate Ni-Cad’s, but not the other way around. A charger only made for the Ni-Cad batteries could overcharge the NiMH battery.

While many charge methods exist for nickel-based batteries, chargers for lithium-based batteries are more defined in terms of charge method and charge time. This is, in part, due to the tight charge regime and voltage requirements demanded by these batteries. There is only one way to charge Li-ion/Polymer batteries and the so-called ‘miracle chargers’, which claim to restore and prolong battery life, do not exist for these chemistries. Neither does a super-fast charging solution apply.

The pulse charge method for Li-ion has no major advantages and the voltage peaks wreak havoc with the voltage limiting circuits. While charge times can be reduced, some manufacturers suggest that pulse charging may shorten the cycle life of Li-ion batteries.

Fast charge methods do not significantly decrease the charge time. A charge rate over 1C should be avoided because such high current can induce lithium plating. With most packs, a charge above 1C is not possible. The protection circuit limits the amount of current the battery can accept. The lithium-based battery has a slow metabolism and must take its time to absorb the energy.

Lead acid chargers serve industrial markets such as hospitals and health care units. Charge times are very long and cannot be shortened. Most lead acid chargers charge the battery in 14 hours. Because of its low energy density, this battery type is not used for small portable devices.

In the following sections various charging needs and charging methods are studied. The charging techniques of different chargers are examined to determine why some perform better than others. Since fast charging rather than slow charging is the norm today, we look at well-designed, closed loop systems, which communicate with the battery and terminate the fast charge when certain responses from the battery are received.

Charging the Nickel Cadmium Battery

Battery manufacturers recommend that new batteries be slow-charged for 24 hours before use. A slow charge helps to bring the cells within a battery pack to an equal charge level because each cell self-discharges to different capacity levels. During long storage, the electrolyte tends to gravitate to the bottom of the cell. The initial trickle charge helps redistribute the electrolyte to remedy dry spots on the separator that may have developed.

Some battery manufacturers do not fully form their batteries before shipment. These batteries reach their full potential only after the customer has primed them through several charge/discharge cycles, either with a battery analyzer or through normal use. In many cases, 50 to 100 discharge/charge cycles are needed to fully form a nickel-based battery. Quality cells, such as those made by Sanyo and Panasonic, are known to perform to full specification after as few as 5 to 7 discharge/charge cycles. Early readings may be inconsistent, but the capacity levels become very steady once fully primed. A slight capacity peak is observed between 100 and 300 cycles.

Most rechargeable cells are equipped with a safety vent to release excess pressure if incorrectly charged. With a resealable vent, no damage occurs on venting but some electrolyte is lost and the seal may leak afterwards. When this happens, a white powder will accumulate over time at the vent opening.

Commercial fast-chargers are often not designed in the best interests of the battery. This is especially true of Ni-Cad chargers that measure the battery’s charge state solely through temperature sensing. Although simple and inexpensive in design, charge termination by temperature sensing is not accurate. The thermistors used commonly exhibit broad tolerances; their positioning with respect to the cells are not consistent. Ambient temperatures and exposure to the sun while charging also affect the accuracy of full-charge detection. To prevent the risk of premature cut-off and assure full charge under most conditions, charger manufacturers use 50°C (122°F) as the recommended temperature cut-off. Although a prolonged temperature above 45°C (113°F) is harmful to the battery, a brief temperature peak above that level is often unavoidable.

More advanced Ni-Cad chargers sense the rate of temperature increase, defined as dT/dt, or the change in temperature over charge time, rather than responding to an absolute temperature (dT/dt is defined as delta Temperature / delta time). This type of charger is kinder to the batteries than a fixed temperature cut-off, but the cells still need to generate heat to trigger detection. To terminate the charge, a temperature increase of 1°C (1.8°F) per minute with an absolute temperature cut-off of 60°C (140°F) works well. Because of the relatively large mass of a cell and the sluggish propagation of heat, the delta temperature, as this method is called, will also enter a brief overcharge condition before the full-charge is detected. The dT/dt method only works with fast chargers.

Harmful overcharge occurs if a fully charged battery is repeatedly inserted for topping charge. Vehicular or base station chargers that require the removal of two-way radios with each use are especially hard on the batteries because each reconnection initiates a fast-charge cycle. This also applies to laptops that are momentarily disconnected and reconnected to perform a service. Likewise, a technician may briefly plug the laptop into the power source to check a repeater station or service other installations. Problems with laptop batteries have also been reported in car manufacturing plants where the workers move the laptops from car to car, checking their functions, while momentarily plugging into the external power source. Repetitive connection to power affects mostly ‘dumb’ nickel-based batteries. A ‘dumb’ battery contains no electronic circuitry to communicate with the charger. Li-ion chargers detect the SoC by voltage only and multiple reconnections will not confuse the charging regime.

The charge efficiency factor of a standard Ni-Cd is better on fast charge than slow charge. At a 1C charge rate, the typical charge efficiency is 1.1 or 91 percent. On an overnight slow charge (0.1C), the efficiency drops to 1.4 or 71 percent.

At a rate of 1C, the charge time of a Ni-Cad is slightly longer than 60 minutes (66 minutes at an assumed charge efficiency of 1.1). The charge time on a battery that is partially discharged or cannot hold full capacity due to memory or other degradation is shorter accordingly. At a 0.1C charge rate, the charge time of an empty Ni-Cad is about 14 hours, which relates to the charge efficiency of 1.4.

Crystalline formation on Ni-Cad cell). Research conducted in Germany has shown that the reverse load method adds 15 percent to the life of the Ni-Cad battery.

After full charge, the Ni-Cad battery is maintained with a trickle charge to compensate for the self-discharge. The trickle charge for a Ni-Cad battery ranges between 0.05C and 0.1C. In an effort to reduce the memory phenomenon, there is a trend towards lower trickle charge currents.

Charging the Nickel-Metal Hydride Battery

Chargers for NiMH batteries are very similar to those of the Ni-Cad system but the electronics is generally more complex. To begin with, the NiMH produces a very small voltage drop at full charge. This NDV is almost non-existent at charge rates below 0.5C and elevated temperatures. Aging and cell mismatch works further against the already minute voltage delta. The cell mismatch gets worse with age and increased cycle count, which makes the use of the NDV increasingly more difficult.

The NDV of a NiMH charger must respond to a voltage drop of 16mV or less. Increasing the sensitivity of the charger to respond to the small voltage drop often terminates the fast charge by error halfway through the charge cycle. Voltage fluctuations and noise induced by the battery and charger can fool the NDV detection circuit if set too precisely.

The popularity of the NiMH battery has introduced many innovative charging techniques. Most of today’s NiMH fast chargers use a combination of NDV, voltage plateau, rate-of-temperature-increase (dT/dt), temperature threshold and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge.

NiMH batteries which use the NDV method or the thermal cut-off control tend to deliver higher capacities than those charged by less aggressive methods. The gain is approximately 6 percent on a good battery. This capacity increase is due to the brief overcharge to which the battery is exposed. The negative aspect is a shorter cycle life. Rather than expecting 350 to 400 service cycles, this pack may be exhausted with 300 cycles.

Similar to Ni-Cad charge methods, most NiMH fast-chargers work on the rate-of-temperature-increase (dT/dt). A temperature raise of 1°C (1.8°F) per minute is commonly used to terminate the charge. The absolute temperature cut-off is 60°C (140°F). A topping charge of 0.1C is added for about 30 minutes to maximize the charge. The continuous trickle charge that follows keeps the battery in full charge state.

Applying an initial fast charge of 1C works well. Cooling periods of a few minutes are added when certain voltage peaks are reached. The charge then continues at a lower current. When reaching the next charge threshold, the current steps down further. This process is repeated until the battery is fully charged.

Known as ‘step-differential charge’, this charge method works well with NiMH and Ni-Cad batteries. The charge current adjusts to the SoC, allowing high current at the beginning and more moderate current towards the end of charge. This avoids excessive temperature build-up towards the end of the charge cycle when the battery is less capable of accepting charge.

NiMH batteries should be rapid charged rather than slow charged. The amount of trickle charge applied to maintain full charge is especially critical. Because NiMH does not absorb overcharge well, the trickle charge must be set lower than that of the Ni-Cad. The recommended trickle charge for the NiMH battery is a low 0.05C. This is why the original Ni-Cad charger cannot be used to charge NiMH batteries. The lower trickle charge rate is acceptable for the Ni-Cad.

It is difficult, if not impossible, to slow-charge a NiMH battery. At a C-rate of 0.1C and 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must depend on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged on a charger with a fixed timer. The same occurs if the battery has lost charge acceptance due to age and can only hold 50 percent of charge. A fixed timer that delivers a 100 percent charge each time without regard to the battery condition would ultimately apply too much charge. Overcharge could occur even though the NiMH battery feels cool to the touch.

Some lower-priced chargers may not apply a fully saturated charge. On these economy chargers, the full-charge detection may occur immediately after a given voltage peak is reached or a temperature threshold is detected. These chargers are commonly promoted on the merit of short charge time and moderate price.

Figure 4-2 summarizes the characteristics of the slow charger, quick charger and fast charger. A higher charge current allows better full-charge detection.

	Charge C-rate	Typical charge time	Maximum permissible charge temperatures	Charge termination method
Slow Charger	0.1C	14h	0°C to 45°C (32°F to 113°F)	Fixed timer. Subject to overcharge. Remove battery when charged.
Quick Charger	0.3-0.5C	4h	10°C to 45°C (50°F to 113°F)	NDV set to 10mV/cell, uses voltage plateau, absolute temperature and time-out-timer. (At 0.3C, dT/dt fails to raise the temperature sufficiently to terminate the charge.)
Fast Charger	1C	1h+	10°C to 45°C (50°F to 113°F)	NDV responds to higher settings; uses dT/dt, voltage plateau absolute temperature and time-out-timer

Figure 4-2: Characteristics of various charger types.
These values also apply to NiMH and Ni-Cad cells.

Charging the Lithium Ion Battery

The Li-ion charger is a voltage-limiting device similar to the lead acid battery charger. The difference lies in a higher voltage per cell; tighter voltage tolerance and the absence of trickle or float charge when full charge is reached.

While the lead acid battery offers some flexibility in terms of voltage cut-off, manufacturers of Li-ion cells are very strict on setting the correct voltage. When the Li- ion was first introduced, the graphite system demanded a charge voltage limit of 4.10V/cell. Although higher voltages deliver increased energy densities, cell oxidation severely limited the service life in the early graphite cells that were charged above the 4.10V/cell threshold. This effect has been solved with chemical additives. Most commercial Li-ion cells can now be charged to 4.20V. The tolerance on all Li-ion batteries is a tight +/-0.05V/cell.

Industrial and military Li-ion batteries designed for maximum cycle life use an end-of-charge voltage threshold of about 3.90V/cell. These batteries are rated lower on the watt-hour-per-kilogram scale, but longevity takes precedence over high energy density and small size.

The charge time of all Li-ion batteries, when charged at a 1C initial current, is about 3 hours. The battery remains cool during charge. Full charge is attained after the voltage has reached the upper voltage threshold and the current has dropped and leveled off at about 3 percent of the nominal charge current.

Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer. Figure 4-5 shows the voltage and current signature of a charger as the Li-ion cell passes through stage one and two.

Some chargers claim to fast-charge a Li-ion battery in one hour or less. Such a charger eliminates stage 2 and goes directly to ‘ready’ once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70 percent. The topping charge typically takes twice as long as the initial charge.

No trickle charge is applied because the Li-ion is unable to absorb overcharge. Trickle charge could cause plating of metallic lithium, a condition that renders the cell unstable. Instead, a brief topping charge is applied to compensate for the small amount of self-discharge the battery and its protective circuit consume.

Depending on the charger and the self-discharge of the battery, a topping charge may be implemented once every 500 hours or 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off when it reaches 4.20V/cell again.

Figure 4-5:    Charge stages of a Li-ion battery.
Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer.

What if a battery is inadvertently overcharged? Li-ion batteries are designed to operate safely within their normal operating voltage but become increasingly unstable if charged to higher voltages. On a charge voltage above 4.30V, the cell causes lithium metal plating on the anode. In addition, the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up.

Much attention has been placed on the safety of the Li-ion battery. Commercial Li-ion battery packs contain a protection circuit that prevents the cell voltage from going too high while charging. The typical safety threshold is set to 4.30V/cell. In addition, temperature sensing disconnects the charge if the internal temperature approaches 90°C (194°F). Most cells feature a mechanical pressure switch that permanently interrupts the current path if a safe pressure threshold is exceeded. Internal voltage control circuits cut off the battery at low and high voltage points.

Important: In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.

Ultra-fast Chargers

Some charger manufacturers claim amazingly short charge times of 30 minutes or less. With well-balanced cells and operating at moderate room temperatures, Ni-Cad batteries designed for fast charging can indeed be charged in a very short time. This is done by simply dumping in a high charge current during the first 70 percent of the charge cycle. Some Ni-Cad batteries can take as much a 10C, or ten times the rated current. Precise SoC detection and temperature monitoring are essential.

The high charge current must be reduced to lower levels in the second phase of the charge cycle because the efficiency to absorb charge is progressively reduced as the battery moves to a higher SoC. If the charge current remains too high in the later part of the charge cycle, the excess energy turns into heat and pressure. Eventually venting occurs, releasing hydrogen gas. Not only do the escaping gases deplete the electrolyte, they are also highly flammable!

Several manufacturers offer chargers that claim to fully charge Ni-Cad batteries in half the time of conventional chargers. Based on pulse charge technology, these chargers intersperse one or several brief discharge pulses between each charge pulse. This promotes the recombination of oxygen and hydrogen gases, resulting in reduced pressure buildup and a lower cell temperature. Ultra-fast-chargers based on this principle can charge a nickel-based battery in a shorter time than regular chargers, but only to about a 90 percent SoC. A trickle charge is needed to top the charge to 100 percent.

Pulse chargers are known to reduce the crystalline formation (memory) of nickel-based batteries. By using these chargers, some improvement in battery performance can be realized, especially if the battery is affected by memory. The pulse charge method does not replace a periodic full discharge. For more severe crystalline formation on nickel-based batteries, a full discharge or recondition cycle is recommended to restore the battery.

Ultra-fast charging can only be applied to healthy batteries and those designed for fast charging. Some cells are simply not built to carry high current and the conductive path heats up. The battery contacts also take a beating if the current handling of the spring-loaded plunger contacts is underrated. Pressing against a flat metal surface, these contacts may work well at first, and then wear out prematurely. Often, a fine and almost invisible crater appears on the tip of the contact, which causes a high resistive path or forms an isolator. The heat generated by a bad contact can melt the plastic.

Another problem with ultra-fast charging is servicing aged batteries that commonly have high internal resistance. Poor conductivity turns into heat, which further deteriorates the cells. Battery packs with mismatched cells pose another challenge. The weak cells holding less capacity are charged before those with higher capacity and start to heat up. This process makes them vulnerable to further damage.

Many of today’s fast chargers are designed for the ideal battery. Charging less than perfect specimens can create such a heat buildup that the plastic housing starts to distort. Provisions must be made to accept special needs batteries, albeit at lower charging speeds. Temperature sensing is a prerequisite.

The ideal ultra-fast charger first checks the battery type, measures its SoH and then applies a tolerable charge current. Ultra-high capacity batteries and those that have aged are identified, and the charge time is prolonged because of higher internal resistance. Such a charger would provide due respect to those batteries that still perform satisfactorily but are no longer ‘spring chickens’.

The charger must prevent excessive temperature build-up. Sluggish heat detection, especially when charging takes place at a very rapid pace, makes it easy to overcharge a battery before the charge is terminated. This is especially true for chargers that control fast charge using temperature sensing alone. If the temperature rise is measured right on the skin of the cell, reasonably accurate SoC detection is possible. If done on the outside surface of the battery pack, further delays occur. Any prolonged exposure to a temperature of 45°C (113°F) harms the battery.

New charger concepts are being studied which regulate the charge current according to the battery's charge acceptance. On the initial charge of an empty battery when the charge acceptance is high and little gas is generated, a very high charge current can be applied. Towards the end of a charge, the current is tapered down.

Get the Most from your Batteries

A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service. Although fully charged, the battery eventually regresses to a point where the available energy is less than half of its original capacity, resulting in unexpected downtime.

Downtime almost always occurs at critical moments. This is especially true in the public safety sector where portable equipment runs as part of a fleet operation and the battery is charged in a pool setting, often with minimal care and attention. Under normal conditions, the battery will hold enough power to last the day. During heavy activities and longer than expected duties, a marginal battery cannot provide the extra power needed and the equipment fails.

Rechargeable batteries are known to cause more concern, grief and frustration than any other part of a portable device. Given its relatively short life span, the battery is the most expensive and least reliable component of a portable device.

In many ways, a rechargeable battery exhibits human-like characteristics: it needs good nutrition, it prefers moderate room temperature and, in the case of the nickel-based system, requires regular exercise to prevent the phenomenon called ‘memory’. Each battery seems to develop a unique personality of its own.

Memory: myth or fact?

The word ‘memory’ was originally derived from ‘cyclic memory’, meaning that a Ni-Cad battery can remember how much discharge was required on previous discharges. Improvements in battery technology have virtually eliminated this phenomenon. Tests performed at a Black & Decker lab, for example, showed that the effects of cyclic memory on the modern Ni-Cad were so small that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be noticed.

The problem with the nickel-based battery is not the cyclic memory but the effects of crystalline formation. There are other factors involved that cause degeneration of a battery. For clarity and simplicity, we use the word ‘memory’ to address capacity loss on nickel-based batteries that are reversible.

The active cadmium material of a Ni-Cad battery is present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. When the memory phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a voltage depression, which leads to a loss of capacity. In advanced stages, the sharp edges of the crystals may grow through the separator, causing high self-discharge or an electrical short.

Another form of memory that occurs on some Ni-Cad cells is the formation of an inter-metallic compound of nickel and cadmium, which ties up some of the needed cadmium and creates extra resistance in the cell. Reconditioning by deep discharge helps to break up this compound and reverses the capacity loss.

The memory phenomenon can be explained in layman’s terms as expressed by Duracell: “The voltage drop occurs because only a portion of the active materials in the cells is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. Subsequent full discharge/charge cycling will restore the active materials to their original state.”

When NiMH was first introduced there was much publicity about its memory-free status. Today, it is known that this chemistry also suffers from memory but to a lesser extent than the Ni-Cad. The positive nickel plate, a metal that is shared by both chemistries, is responsible for the crystalline formation.

In addition to the crystal-forming activity on the positive plate, the Ni-Cad also develops crystals on the negative cadmium plate. Because both plates are affected by crystalline formation, the Ni-Cad requires more frequent discharge cycles than the NiMH. This is a non-scientific explanation of why the Ni-Cad is more prone to memory than the NiMH.

The stages of crystalline formation of a Ni-Cad battery are illustrated in Figure 10-1. The enlargements show the negative cadmium plate in normal crystal structure of a new cell, crystalline formation after use (or abuse) and restoration.

Lithium and lead-based batteries are not affected by memory, but these chemistries have their own peculiarities. Current inhibiting pacifier layers affect both batteries – plate oxidation on the lithium and sulfation and corrosion on the lead acid systems. These degenerative effects are non-correctible on the lithium-based system and only partially reversible on the lead acid.

How to Restore and Prolong Nickel-based Batteries

The effects of crystalline formation are most pronounced if a nickel-based battery is left in the charger for days, or if repeatedly recharged without a periodic full discharge. Since most applications do not use up all energy before recharge, a periodic discharge to 1V/cell (known as exercise) is essential to prevent the buildup of crystalline formation on the cell plates. This maintenance is most critical for the Ni-Cad battery.

All Ni-Cad batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed. The battery can be used with any desired user pattern without the concern of memory.

The Ni-MH battery is affected by memory also, but to a lesser degree. No scientific research is available that compares NiMH with Ni-Cad in terms of memory degradation. Neither is information on hand that suggests the optimal amount of maintenance required to obtain maximum battery life. Applying a full discharge once every three months appears right. Because of the NiMH battery’s shorter cycle life, over-exercising is not recommended.

A hand towel must be cleaned periodically. However, if it were washed after each use, its fabric would wear out very quickly. In the same way, it is neither necessary nor advisable to discharge a rechargeable battery before each charge – excessive cycling puts extra strain on the battery.

Exercise and Recondition – Research has shown that if no exercise is applied to a Ni-Cad for three months or more, the crystals ingrain themselves, making them more difficult to break up. In such a case, exercise is no longer effective in restoring a battery and reconditioning is required. Recondition is a slow, deep discharge that removes the remaining battery energy by draining the cells to a voltage threshold below 1V/cell.

Figure 10-3:  Effects of exercise and recondition.
Battery A improved capacity on exercise alone; batteries B and C required recondition. A new battery with excellent readings improved further with recondition.

Battery A responded well to exercise alone and no recondition was required. This result is typical of a battery that has been in service for only a few months or has received periodic exercise cycles. Batteries B and C, on the other hand, required recondition (dotted line) to restore their performance. Without the recondition function, these two batteries would need to be replaced.

After service, the restored batteries were returned to full use. When examined after six months of field use, no noticeable degradation in the restored performance was visible. The regained capacity was permanent with no evidence of falling back to the previous state. Obviously, the batteries would need to be serviced on a regular basis to maintain the performance.

Applying the recondition cycle on a new battery (top line on chart) resulted in a slight capacity increase. This capacity gain is not fully understood, other than to assume that the battery improved by additional formatting. Another explanation is the presence of early memory. Since new batteries are stored with some charge, the self-discharge that occurs during storage contributes to a certain amount of crystalline formation. Exercising and reconditioning reverse this effect. This is why the manufacturers recommend storing rechargeable batteries at about 40 percent charge.

The importance of exercising and reconditioning Ni-Cad batteries is emphasized further by a study carried out by GTE Government Systems in Virginia, USA for the US Navy. To determine the percentage of batteries needing replacement within the first year of use, one group of batteries received charge only, another group was exercised and a third group received recondition. The batteries studied were used for two-way radios on the aircraft carriers USS Eisenhower with 1500 batteries and USS George Washington with 600 batteries, and the destroyer USS Ponce with 500 batteries.

With charge only (charge-and-use), the annual percentage of battery failure on the USS Eisenhower was 45 percent (see Figure 10-4). When applying exercise, the failure rate was reduced to 15 percent. By far the best results were achieved with recondition. The failure rate dropped to 5 percent. Identical results were attained from the USS George Washington and the USS Ponce.

Maintenance Method	Annual Percentage of Batteries Requiring Replacement
Charge only (charge-and-use)	45%
Exercise only (discharge to 1V/cell)	15%
Reconditioning (secondary deep discharge)	5%

Another study involving Ni-Cad batteries for defense applications was performed by the Dutch Army. This involved battery packs that had been in service for 2 to 3 years during the Balkan War. The Dutch Army was aware that the batteries were used under the worst possible conditions. Rather than a good daily workout, the packs were used for patrol duties lasting 2 to 3 hours per day. The rest of the time the batteries remained in the chargers for operational readiness.

After the war, the batteries were sent to the Dutch Military Headquarters and were tested with Cadex 7000 Series battery analyzers. The test technician found that the capacity of some packs had dropped to as low as 30 percent. With the recondition function, 90 percent of the batteries restored themselves to full field use. The Dutch Army set the target capacity threshold for field acceptability to 80 percent. This setting is the pass/fail acceptance level for their batteries.

Based on the successful reconditioning results, the Dutch Army now assigns the battery maintenance duty to individual battalions. The program calls for a service once every two months. Under this regime, the Army reports reduced battery failure and prolonged service life. The performance of each battery is known at any time and any under-performing battery is removed before it causes a problem.

Ni-Cad batteries remain the preferred chemistry for mobile communications, both in civil and defense applications. The main reason for its continued use is dependable and enduring service under difficult conditions. Other chemistries have been tested and found problematic in long-term use.

During the later part of the 1990s, the US Army switched from mainly non-rechargeable to the NiMH battery. The choice of chemistry was based on the benefit of higher energy densities as compared to Ni-Cad. The army soon discovered that the NiMH did not live up to the expected cycle life. Their reasoning, however, is that the 100 cycles attained from a NiMH pack is still more economical than using a non-rechargeable equivalent. The army’s focus is now on the Li-ion Polymer, a system that is more predictable than NiMH and requires little or no maintenance. The aging issue will likely cause some logistic concerns, especially if long-term storage is required.

Simple Guidelines

Do not leave a nickel-based battery in a charger for more than a day after full charge is reached.

• Apply a monthly full discharge cycle. Running the battery down in the equipment may do this also.
• Do not discharge the battery before each recharge. This would put undue stress on the battery.
• Avoid elevated temperature. A charger should only raise the battery temperature for a short time at full charge, and then the battery should cool off.
• Use quality chargers to charge batteries.
•  

How to Prolong Lithium-based Batteries

Today’s battery research is heavily focused on lithium chemistries, so much so that one could assume that all future batteries will be lithium systems. Lithium-based batteries offer many advantages over nickel and lead-based systems. Although maintenance free, no external service is known that can restore the battery’s performance once degraded.

In many respects, Li-ion provides a superior service to other chemistries, but its performance is limited to a defined lifespan. The Li-ion battery has a time clock that starts ticking as soon as the battery leaves the factory. The electrolyte slowly ‘eats up’ the positive plate and the electrolyte decays. This chemical change causes the internal resistance to increase. In time, the cell resistance raises to a point where the battery can no longer deliver the energy, although it may still be retained in the battery. Equipment requiring high current bursts is affected most by the increase of internal resistance.

Battery wear-down on lithium-based batteries is caused by two activities: actual usage or cycling, and aging. The wear-down effects by usage and aging apply to all batteries but this is more pronounced on lithium-based systems.

The Li-ion batteries prefer a shallow discharge. Partial discharges produce less wear than a full discharge and the capacity loss per cycle is reduced. A periodic full discharge is not required because the lithium-based battery has no memory. A full cycle constitutes a discharge to 3V/cell. When specifying the number of cycles a lithium-based battery can endure, manufacturers commonly use an 80 percent depth of discharge. This method resembles a reasonably accurate field simulation. It also achieves a higher cycle count than doing full discharges.

In addition to cycling, the battery ages even if not used. The amount of permanent capacity loss the battery suffers during storage is governed by the SoC and temperature. For best results, keep the battery cool. In addition, store the battery at a 40 percent charge level. Never fully charge or discharge the battery before storage. The 40 percent charge assures a stable condition even if self-discharge robs some of the battery’s energy. Most battery manufacturers store Li-ion batteries at 15°C (59°F) and at 40 percent charge.

Simple Guidelines

• Charge the Li-ion often, except before a long storage. Avoid repeated deep discharges.
• Keep the Li-ion battery cool. Prevent storage in a hot car. Never freeze a battery.
• If your laptop is capable of running without a battery and fixed power is used most of the time, remove the battery and store it in a cool place.
• Avoid purchasing spare Li-ion batteries for later use. Observe manufacturing date when purchasing. Do not buy old stock, even if sold at clearance prices.