ABC OF RECHARGEABLE BATTERIES （3）

Published:2011/8/9 1:53:00 Author:Phyllis From:SeekIC

By Gert Helles

Charging nickel-metal-hydride cells

Though similar to NiCd chargers, a NiMH charger employs the AT/dt method, which is by far the best method for charging NiMH cells. The end-of-charge voltage decrease for NiMH batteries is smaller, and for small charge rates (below 0.5 C, depending on temperature) there may be no voltage decrease at all.

New NiMH batteries can show false peaks early in the charge cycle, causing the charger to terminate prematurely. Moreover, an end-of-charge termination by -AV detection alone almost certainly ensures an overcharge, which in turn limits the number of charge/discharge cycles possible before the battery fails.

It seems there is no available -dV/dt algorithm that works well for charging NiMH batteries under all conditions: new or old, hot or cold, and fully or partly discharged. For that reason, don’t charge a NiMH battery with a NiCad charger unless it utilizes the dT/dt method for end-of-charge termination. And because NiMH cells do not absorb overcharge well, the trickle charge must be lower (about 0.05 C) than that recommended for NiCd cells.

Slow-charging a NiMH battery is difficult if not impossible, because the voltage and temperature profiles associated with a C-rate of 0.1 C to 0.3 C do not provide a sufficiently accurate and unambiguous indication of the full-charge state. The slow charger must therefore rely on a timer to indicate when the charge cycle should be terminated. Thus, to fully charge a NiMH battery you should apply a rapid charge of approximately 1 C (or a rate specified by the battery manufacturer), while monitoring both voltage (AV=0) and temperature (dT/dt) to determine when the charge should be terminated.

Charging lithium-ion and lithium-polymer cells

Whereas chargers for nickel-based batteries are current-limiting devices, chargers for lithium-ion batteries limit both voltage and current. The first lithium-ion cells called for a charge-voltage limit of 4.10 V/cell. Higher voltage means greater capacity, and cell voltages as high as 4.2 V have been achieved by adding chemical additives. Modern Li-ion cells are typically charged to 4.20 V with a tolerance of ±0.05 V/cell.

Full charge is achieved after the terminal voltage has reached the voltage threshold and the charging current has dropped below 0.03 C, which is approximately 3% of lcharge (Figure 6). The time for most chargers to achieve a full charge is about three hours, though some linear chargers claim to charge a Li+ battery in about one hour. Such chargers usually terminate the charge when the battery’s terminal voltage reaches 4.2 V. That kind of charge determination, however, charges the battery only to 70% of its capacity.




A higher charging current does not shorten the charge time by much. Higher current lets you reach the voltage peak earlier, but then the topping charge takes longer. As a rule of thumb, the topping charge will take twice as long as the initial charge.

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