how to discharge and charge your battery

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Charging Bateries Generally, batteries have to be charged initially (as in, after first taking it out of the box) about 11 to 12 hours. After that, between four to six hours charge after using. As a rule of thumb, charge until the battery is warm, not hot.

Do I have to discharge my battery everytime before I recharge?

Yes and no. Discharging the battery is supposed to counter the "memory" effect of NiCads and some NiHm batteries. Thus, some degree of discharging is required. However, the battery should not be totally drained as this will effect the battery's charging/discharging performance in the future. As a rule of thumb, when the rate of fire (ROF) dramatically is reduced, but the gun still shoots, time to stop and have the battery recharged.

Also, see the related thread:

How to charge/discharge batteries
acmgames.proboards66.com/index.cgi?board=ask&action=display&thread=1108618431

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additional info for your battery guys....check this forum

acmgames.proboards66.com/index.cgi?board=madscience&action=display&thread=1108628036

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i found this very interesting, you can read this at this site....happy reading...

www.buchmann.ca/chap2-page2.asp

Chemistry Comparison
Let's 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 NiCd remains a standard against which other batteries are compared, let’s evaluate alternative chemistries against this classic battery type.

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density. The NiCd 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 NiCd contains toxic metals and is not environmentally friendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd 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.

Lithium Ion Polymer (Li-ion polymer) — a potentially lower cost version of the Li-ion. This chemistry is similar to the Li-ion in terms of energy density. It enables very slim geometry and allows simplified packaging. Main applications are mobile 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.

Figure 2-1 compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy density, cycle life, exercise requirements and cost. The figures are based on average ratings of commercially available batteries at the time of publication. Exotic batteries with above average ratings are not included.





NiCd 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



Figure 2-1: 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.

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.
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.
Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
The discharge is highest immediately after charge, then tapers off. The NiCd capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
Internal protection circuits typically consume 3% of the stored energy per month.
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.
Capable of high current pulses.
Applies to discharge only; charge temperature range is more confined.
Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
Cost of battery for commercially available portable devices.
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 NiCd 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.

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The Nickel Cadmium (NiCd) Battery
Alkaline nickel battery technology originated in 1899, when Waldmar Jungner invented the NiCd battery. The materials were expensive compared to other battery types available at the time and its use was limited to special applications. In 1932, the active materials were deposited inside a porous nickel-plated electrode and in 1947, research began on a sealed NiCd battery, which recombined the internal gases generated during charge rather than venting them. These advances led to the modern sealed NiCd battery, which is in use today.

The NiCd prefers fast charge to slow charge and pulse charge to DC charge. All other chemistries prefer a shallow discharge and moderate load currents. The NiCd is a strong and silent worker; hard labor poses no problem. In fact, the NiCd 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 NiCd will gradually lose its performance.

Among rechargeable batteries, NiCd 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 NiCd. However, the introduction of batteries with higher energy densities and less toxic metals is causing a diversion from NiCd to newer technologies.





Advantages and Limitations of NiCd Batteries



Advantages
Fast and simple charge — even after prolonged storage.

High number of charge/discharge cycles — if properly maintained, the NiCd provides over 1000 charge/discharge cycles.

Good load performance — the NiCd allows recharging at low temperatures.

Long shelf life – in any state-of-charge.

Simple storage and transportation — most airfreight companies accept the NiCd without special conditions.

Good low temperature performance.

Forgiving if abused — the NiCd is one of the most rugged rechargeable batteries.

Economically priced — the NiCd is the lowest cost battery in terms of cost per cycle.

Available in a wide range of sizes and performance options — most NiCd cells are cylindrical.

Limitations
Relatively low energy density — compared with newer systems.

Memory effect — the NiCd must periodically be exercised to prevent memory.

Environmentally unfriendly — the NiCd contains toxic metals. Some countries are limiting the use of the NiCd battery.

Has relatively high self-discharge — needs recharging after storage.




Figure 2-2: Advantages and limitations of NiCd batteries.

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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

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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 NiCd batteries only. With the need to service both NiCd 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 NiCd 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 NiCd 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.

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Simple Guidelines

A charger designed to service NiMH batteries can also accommodate NiCd’s, but not the other way around. A charger only made for the NiCd 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. The safety vent on a NiCd cell opens at 1034 to 1379 kPa (150 to 200 psi). In comparison, the pressure of a car tire is typically 240 kPa (35 psi). 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 NiCd 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.

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More advanced NiCd 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.

More precise full charge detection of nickel-based batteries can be achieved with the use of a micro controller that monitors the battery voltage and terminates the charge when a certain voltage signature occurs. A drop in voltage signifies that the battery has reached full charge. This is known as Negative Delta V (NDV).

NDV is the recommended full-charge detection method for ‘open-lead’ NiCd chargers because it offers a quick response time. The NDV charge detection also works well with a partially or fully charged battery. If a fully charged battery is inserted, the terminal voltage raises quickly, then drops sharply, triggering the ready state. Such a charge lasts only a few minutes and the cells remain cool. NiCd chargers based on the NDV full charge detection typically respond to a voltage drop of 10 to 30mV per cell. Chargers that respond to a very small voltage decrease are preferred over those that require a larger drop.

To obtain a sufficient voltage drop, the charge rate must be 0.5C and higher. Lower than 0.5C charge rates produce a very shallow voltage decrease that is often difficult to measure, especially if the cells are slightly mismatched. In a battery pack that has mismatched cells, each cell reaches the full charge at a different time and the curve gets distorted. Failing to achieve a sufficient negative slope allows the fast-charge to continue, causing excessive heat buildup due to overcharge. Chargers using the NDV must include other charge-termination methods to provide safe charging under all conditions. Most chargers also observe the battery temperature.

The charge efficiency factor of a standard NiCd 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 NiCd 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 NiCd is about 14 hours, which relates to the charge efficiency of 1.4.

During the first 70 percent of the charge cycle, the charge efficiency of a NiCd battery is close to 100 percent. Almost all of the energy is absorbed and the battery remains cool. Currents of several times the C-rating can be applied to a NiCd battery designed for fast charging without causing heat build-up. Ultra-fast chargers use this unique phenomenon and charge a battery to the 70 percent charge level within a few minutes. The charge continues at a lower rate until the battery is fully charged.

Once the 70 percent charge threshold is passed, the battery gradually loses ability to accept charge. The cells start to generate gases, the pressure rises and the temperature increases. The charge acceptance drops further as the battery reaches 80 and 90 percent SoC. Once full charge is reached, the battery goes into overcharge. In an attempt to gain a few extra capacity points, some chargers allow a measured amount of overcharge. Figure 4-1 illustrates the relationship of cell voltage, pressure and temperature while a NiCd is being charged.

Ultra-high capacity NiCd batteries tend to heat up more than the standard NiCd if charged at 1C and higher. This is partly due to the higher internal resistance of the ultra-high capacity battery. Optimum charge performance can be achieved by applying higher current at the initial charge stage, then tapering it to a lower rate as the charge acceptance decreases. This avoids excess temperature rise and yet assures fully charged batteries.



Figure 4-1: Charge characteristics of a NiCd cell.
These cell voltage, pressure and temperature characteristics are similar in a NiMH cell.

Interspersing discharge pulses between charge pulses improves the charge acceptance of nickel-based batteries. Commonly referred to as ‘burp’ or ‘reverse load’ charge, this charge method promotes high surface area on the electrodes, resulting in enhanced performance and increased service life. Reverse load also improves fast charging because it helps to recombine the gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers.

Charging with the reverse load method minimizes crystalline formation. The US Army Electronics Command in Fort Monmouth, NJ, USA, had done extensive research in this field and has published the results. (See Figure 10-1, Crystalline formation on NiCd cell). Research conducted in Germany has shown that the reverse load method adds 15 percent to the life of the NiCd battery.

After full charge, the NiCd battery is maintained with a trickle charge to compensate for the self-discharge. The trickle charge for a NiCd 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.

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Charging the Nickel-Metal Hydride Battery
Chargers for NiMH batteries are very similar to those of the NiCd 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 NiCd 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 NiCd 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 NiCd. The recommended trickle charge for the NiMH battery is a low 0.05C. This is why the original NiCd charger cannot be used to charge NiMH batteries. The lower trickle charge rate is acceptable for the NiCd.

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.