ABC OF RECHARGEABLE BATTERIES （5）

Published:2011/8/9 2:07:00 Author:Phyllis From:SeekIC

By Gert Helles

Charging at high and low temperatures.

Efforts should be made to charge at room temperature. Nickel-based batteries should only be fast-charged between 10°C to 30°C (50°F to 86=F). Below 5°C (4TF) and above 45=C (113=F), the charge acceptance of nickel-based batteries is drastically reduced. Li-ion batteries offer reasonably good charge performance through-out the temperature range, but below 5°C (41°F) the charge rate should be less than 1 C.

Conclusion

NiMH chargers can accommodate NiCd batteries, but not the other way around. Chargers dedicated to NiCd batteries will overcharge a NiMH battery. The cycle life and performance of nickel-based batteries are enhanced by fast charging because it reduces the memory effect due to formation of internal crystals. Nickel- and lithium-based batteries call for different charge algorithms. Li+ batteries need protection circuitry to monitor and protect against overcurrent, short circuits, over- and under-voltage, and excessive temperature. Remember to remove a battery from its charger when the battery is not used regularly, and apply a topping-charge before use.

Lithiumion safety- meet the DS2720

Because overcharging (or overdischarging) a Li-ion cell can cause it to explode and injure people, safety is a major concern in handling this type of storage cell. As a result, commercial Li-ion battery packs contain a protection circuit such as the DS2720 (Figure A), which provides all electronic safety functions required for applications involving rechargeable Li+ batteries: protecting the battery during charge, protecting the circuit against excess current flow, and maximizing battery life by limiting the level of cell depletion.





DS2720 ICs control the conduction paths for charge and discharge currents with external switching devices such as low-cost n-channel power MOSFETs. The ICs internal 9-V charge pump provides high-side drive to the external n-channel MOSFETs, yielding lower on-resistances than do the same FETs operating in a more common low-side protection circuit. FET on-resistance actually decreases as the battery discharges (Figure B).





The DS2720 lets you control the external FETs from the data interface or from a dedicated input, thereby eliminating the redundant power-switch controls otherwise required in a rechargeable Li+ battery system. Through its 1 -wire interface, a DS2720 provides the host system with read/write access to the status and control registers, instrumentation registers, and general-purpose data storage. A factory-programmed 64-bit net address allows each device to be individually addressed by the host system.

The DS2720 provides two types of user memory for battery-information storage, EEPROM and lockable EEPROM. EEPROM is a true non-volatile (NV) memory whose contents (important battery data) remain unaffected by severe battery depletion, accidental shorts, or ESD events. When locked, a lockable EEPROM becomes a read-only memory (ROM) that provides additional security for unchanging battery data.

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