NiCad and NiMH Battery Chargers

When power for the battery is used in parallel with power for the system various problems can develop as the system power is cycled on and off under normal usage patterns. For instance when the system power is off, the battery may overcharge due to excess charge current. Other problems can occur when the system power is cycled on, a Delta V Charger may prematurely switch into the trickle charge mode, this could cause the battery to undercharge. Additionally, the battery would then be supplying power to the system, rather than the charger.

Several solutions exist to these problems. Solutions can be categorized as 2 wire or 3 wire solutions. The 2 wire solution uses the basic wires supplying power to the battery from the charger to deal with the various issues, the 3 wire solution uses an extra wire which can provide additional functionality.

2 WIRE SOLUTION:

Solution A involves the use of a dual current level regulated current source. When the system and the battery are both drawing power, the higher charge current level is activated. When only the battery is drawing power the lower charge current is activated. A voltage sensor at 1.38V/cell is used to transition beteen the 2 charge current levels. This solution is normally applied to C/10 type regulated chargers.

3 WIRE SOLUTIONS

Solution B uses a feedback line from the system power circuitry in order to inform the charger that the system is consuming power. This information is then used to adjust the current limit on the power supply / battery charger between two levels. This solution is normally applied to C/10 type regulated chargers.

Solution C requires separate outputs, one for the battery and one for the system power. At the battery system end, then it becomes necessary to automatically hook the two power points together should battery power become necessary. This is normally done with an OR-ing diode. Generally a 1N5822 series schottky diode is used, for it's low forward voltage drop and high efficiency. Although it is conceptually simpler for the system design engineer to contemplate the power supply output as a regulated voltage source, in many instances a smaller and less costly power supply / charger can be designed and manufactured where the power supply output is allowed to deviate over a wider range. Jerome Industries power supply engineers will help explain the various intricacies involved when pursuing this option.

Diagrams showing the charger, battery, system power load, and the interactions between them, can be supplied by the Jerome Engineering dept, to help clarify the system implementation.