Power Electronics

NiMH Charger Incorporates Protection Features

Dallas Semiconductor's DS2714 is a loose-cell NiMH charger that detects and avoids charging primary alkaline cells, lithium primary cells, and defective NiMH or NiCd cells. Designed to work with a regulated charge source, the DS2714 can charge from one to four NiMH or NiCd battery cells, including the AA or AAA sizes typically found in many consumer electronics devices.

The unit's charge cycle begins with a precharge qualification test that prevents the fast charging of deeply depleted cells or damaged cells. Deeply depleted cells are charged at a reduced rate to protect them from damage and extend the life of the battery. When each cell reaches a safe threshold, fast charging begins.

During the fast-charge phase, the unit tests for inappropriate or damaged cells that should not be charged. Fast charging terminates with the negative-delta-V or flat-V method, and is followed by a top-off phase that completely charges the cells. Finally, a maintenance phase continues indefinitely to keep the cells fully charged.

Besides testing for nonrechargeable or damaged cells, the DS2714 has other features that ensure safe and effective charge operation. Throughout charging, the DS2714 continuously monitors cell voltages and temperature to assure that they remain within proper ranges.

Whenever the measured values are outside preset thresholds, charging is discontinued and a fault condition is issued. The DS2714 includes an adjustable timer for increased safety and secondary termination. The device is available in a 20-pin TSSOP package. Prices start at $1.99 for 1000-unit quantities.

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