Because 22-cell nickel-cadmium/nickel-metal-hydride (NiCd/NiMH) batteries are ubiquitous, the need for a compatible battery charger that is inexpensive and simple to use and takes up little space is also common. You can implement such a charger by making simple modifications to the standard application circuit for a 16-cell NiCd/NiMH fast-charge controller (Fig. 1).

The MAX712 (or its companion, the MAX713) charges one to 16 NiCd/NiMH cells while maintaining power to the battery's load. The number of cells is limited by the maximum voltage allowed across battery terminals BATT+ and BATT2, which is 62 V times the programmed number of cells. But you can fool the IC into sensing fewer cells than are actually present by reducing the voltage between BATT+ and BATT2. That's accomplished by placing a zener diode in series with the BATT+ terminal.

Thus, you can accommodate a cell number in excess of the maximum (16) by choosing a zener diode with the correct voltage. For a 22-cell battery, configure the IC for 16 cells by connecting PGM1 and PGM2 to BATT2. Reduce the voltage at BATT+ by (6 3 1.65 V) with a 10-V zener diode (D2) as shown. In general, the zener voltage V_Z should equal the number of desired cells (22 cells in this case) minus 16 cells, times 1.65 V:

V_Z = (22 cells 2 16 cells) 3 1.65 V

The supply voltage must be at least 1.5 V above the maximum battery voltage. For a 22-cell battery with a limit of 1.65 V per internal cell, the supply voltage must be 22 31.65 + 1.5 = 37.8 V or greater. A medium-power pnp pass transistor (TIP32) feeds a fast-charge current into the battery and load, and a 2N3904 transistor prevents excessive power dissipation in the IC by maintaining the DRV voltage within specifications. Power dissipation in the pass transistor is:

P = (V_IN 2 V_BATMIN) 3 16 3 I_FAST

For fast currents greater than 250 mA, the pass transistor should include a suitable heatsink.

R1 limits current into the internal shunt regulator (V+ terminal). Select an R1 value that passes at least 5 mA at the minimum dc input voltage and an R2 value that limits the current into DRV to less than 100 mA. The graph illustrates the system's operation (Fig. 2). It shows a 22-cell NiMH battery being charged in fast-charge mode with a charging current of 0.5 A. As the battery reaches full charge, the charger goes into trickle-charge mode. Load voltage is maintained at 36.3 V (22 3 1.65 V) throughout the charge cycle.

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