Fast-Charge Controller Handles 22-Cell NiCD/NiMH Battery

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...
July 5, 2004
3 min read

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 VZ should equal the number of desired cells (22 cells in this case) minus 16 cells, times 1.65 V:

VZ = (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 = (VIN 2 VBATMIN) 3 16 3 IFAST

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.

About the Author

Sign up for Electronic Design Newsletters
Get the latest news and updates.

Voice Your Opinion!

To join the conversation, and become an exclusive member of Electronic Design, create an account today!