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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Reader Comments I am trying to build a NiMH battery charger that matches these specifications: 24V, 4.4Ah. I have some questions regarding this circuit: 1) I plan on charging 2 24V - 2200mAh NiMH batteries (20 cells each) in parallel so they can be charged as 24V instead of 48V - 4400mAh. Can this circuit support 4400mAh? 2) What is the charging current of this circuit? 3) Can you give me tips on how to choose a resistor for RSense? Thank you and have a nice day EE student -September 11, 2008 Sir ,i need to a circuit diagram,design for a 12v battery charger sanju -August 26, 2008 sir, i need the complete circuit of a solar powered security light. Adegboye olaoluwa -May 19, 2008 sir, i need the complete circuit of a solar powered security light. Adegboye olaoluwa -May 19, 2008 Sir ,i need to a circuit diagram,design for a 9.6v(8x1.2v) battery charger using either nicad/nimh. Battery Specification: - 8 cells - 9.6Volts - 1700mAh - NiMi Thank in ADVANCE electronic student -February 13, 2008 sir, can anyone send me a circuit diagram/design for a 9.6v battery charger using either nicad/nimh. Battery Specification: - 8 cells - 9.6Volts - 1700 mah - Ni CD thnx! rhel rivera -January 18, 2008 thanks endris mulu -January 04, 2008 Thank you for your circuit but i need charger for only one until four cell 2500mAh NiCD/NiMh, can you give it, thanks again. Farid Ruliawan -October 02, 2006 thanks for the circuit diagram, but i'm wonder is the batteries placed in series?. Which this circuit, can it charging one single battery? not 22 batteries. ndeng -September 03, 2006 i need a circuit 12volt 1300mAh ni-cd battry charger with backup means i want oprate a opratous 12volt smps supply if power fail then atometicaly find current by battry. k p singh -August 05, 2006   (Article Rating: ) Sirs...can anyone show me how to make or build a NiCd charger...Thank you. Battery Specification: - 8 cells - 9.6Volts - 1700 mah - Ni CD GiL -March 04, 2006   (Article Rating: ) not what i am looking for.SKC 6VRE 0.6AA 600mAh 7.2V Alvin J.RETZLAFF -September 19, 2004 "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." I think you might want to fix the typo... 62 V times 16 cells would be 992 V. Mark Olson -July 09, 2004
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