The performance and longevity of rechargeable batteries depend, to a large extent, on the quality of their chargers. Battery life is frequently measured in the number of charge/discharge cycles obtained. In typical field use, the cycle-count specified by the manufacturer is often difficult to achieve and the batteries fall short of their expected performance.

Battery chargers are commonly given the least priority when purchasing battery powered equipment. Considering the high cost of replacement and the frustration that poorly performing batteries create, choosing a quality charger makes common sense. In most cases, the extra money invested in a quality charger is quickly paid back by being able to keep the batteries longer, not to mention the increased battery reliability the better charger provides.

Some lower priced chargers allow uncontrolled over-charge, causing damaging heat build-up in the battery. Excessive temperatures during charge and standby mode is one of the most common killers of batteries. Over-charging occurs when the charger keeps the battery at a temperature that is warm to touch (body temperature) after it is fully charged.

Some temperature rise during charge cannot be avoided, especially in a NiMH battery. If the temperature does not drop after full charge is reached, the charger is not performing correctly and the battery should be removed immediately after it reaches full charge. It is much better to store the battery on the shelf and apply a topping-charge before use rather than leave it in the charger for days.

When purchasing a battery operated device, the buyer is often given a choice of chargers in various price classes:

Slow-Charger - The lowest price buys a slow-charger which applies a fixed charge rate of 0.1C (one tenth of the rated capacity) for as long as the battery is connected. Typical charge time is 14 to 16 hours. No full-charge detection occurs to switch the battery to a lower charge rate at the end of the charge cycle.

If the charge current is set correctly, a battery in a slow-charger remains only lukewarm to the touch when fully charged. In this the case, the battery does not need to be removed immediately when ready but should not stay in the charger more than a few days.

A problem arises if a smaller battery is placed on a charger designed for a larger battery. Such a situation is common with cellular phone batteries that are available in various sizes, capacities and chemistries with the same footprint. Because of the higher charge current in relation to the battery rating, heat-damaging over-charge condition can occur once the battery is fully charged. If no alternative charger is available, the user is advised to carefully observe the temperature of the battery while charging and to disconnect it when warm to the touch.

The opposite may also occur where a larger battery is charged on a charger designed for a smaller battery. In such a case, a full charge can never be reached. The battery remains cold during charge and will not perform as expected. A battery that is continuously undercharged will eventually loose its ability to accept a full charge due to memory. A periodic full discharge is needed to prevent such premature capacity loss.

Fast-Charger - The fast-charger offers a number of advantages over the slow-charger, the obvious one is shorter charge times. The time to fully charge a battery with a fast-charger depends on the state of charge, capacity and chemistry of the battery, as well as the charge rate. At a 0.5C charge rate, an empty NiCd typically charges in a little more than two hours; at 1C, the charge time is about one hour.

When fully charged, the charger automatically switches to trickle charge to compensate for the self-discharge of the battery. Although more forgiving than the slow-charger in terms of accommodating different battery sizes, there is a limit as to the types and variety of batteries a fast-charger can charge.

Because of the fast-chargerís higher charge current and the need to monitor the battery during charge, it is important to charge only those batteries which the charger manufacturer has specified. Modern fast-chargers commonly accommodate both NiCd and NiMH batteries. SLA and Li-ion chemistries are charged with very different algorithms and are not compatible with nickel-based batteries as far as charging is concerned, unless the charger is designed for these chemistries as well. Some battery manufacturers electrically code the batteries to identify their chemistry and rating. When connected to a designated charger, the correct charge current and algorithm is applied automatically.

Generally, it is better to fast charge rather than slow charge nickel-based batteries. Slow charge is known to build-up a formation of large crystals, a phenomenon that lowers the battery performance and shortens its service life. However, if the temperature rise during charge cannot be reasonably controlled with a fast-charger, it is better to use a slow-rate charger.

A battery left on trickle charge should only be slightly lukewarm. If warm to touch, the trickle charge is set too high and the battery will eventually sustain irreversible damage due to overcharge. It is not recommended to leave the battery in the charger for weeks, even with a correctly set trickle charge current. If a battery needs to remain in a charger for operational readiness, an exercise cycle should be applied once every month.

In this section, we examine various charging methods and investigate why certain chargers perform better than others. Since fast-charging rather than slow-charging is the norm today, well designed closed loop chargers that communicate with the battery and terminate the fast charge when full charge is reached are considered best for battery life and performance.

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