NiHM Battery Charger with Maxim712

Discussion in 'The Projects Forum' started by Macabra, Oct 31, 2010.

  1. Macabra

    Thread Starter Active Member

    May 31, 2008
    Hi guys,

    I've been working with this for a while now, and somehow I don't seem to have it working right. Please see attachment for the schematic of what I want to do. What I have is a 4.8V (4 x AA batteries - rated at 2600mAh each from Gold Peak, I've also attached the data sheet on these). The batteries seem to charge fine but when I use them again, the batteries don't last long at all, at least not as long as if they were brand new..I reckon I must have messed up the batteries from doing different trials.

    Based on the equations provided on the data sheet, I calculated my "Ifast" current flowing to the battery to charge them at 867mA (This is 2600mAh/3 hrs = 867mA).

    My R1 = ((minimum wallcube Voltage - 5V)/5mA) = 1.4kΩ. So, I'm using a power supply I got and so I don't know what they mean by minimum wall cube voltage but I just assume it's the voltage that is just applied to it, in this case, 12V. I'm applying 12V because I've got a wall adapter I want to use for this application, but don't want to cut the cable and put it in my breadboard, hence the power supply.

    Rsense = .25V/867mA = .30Ω

    The PGM# pins are based upon tables provided:
    PGM0 = V+ (Using 4 AA battery cells)
    PGM1 = BATT- (Using 4AA battery cells)
    PGM2 = BATT- (264minute timeout and voltage drop termination enabled)
    PGM3 = BATT- (C/2 trickle charge)

    I'd really appreciate any help on this matter and thanks in advance!


    Forgot to add data sheet of MAX712..
    Last edited: Oct 31, 2010
  2. wayneh


    Sep 9, 2010
    Have you actually measured what it's doing during the charge cycle? It's pretty easy to overcharge a NiMH battery, especially if you're only using voltage or dV/dt to estimate SOC. How does it know what your starting SOC is?
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    Did you set a VLIMIT for the 4 Cells? The datasheet limits are between 1.25V - 2.5V. VLIMIT should not exceed 2.5V/cell. A 12V Adapter and o externally set limit could produce up to 3V/cell. Accordint to the datasheet VLIMIT should be set to between 1.9V and 2.5V per cell, although I don't understand the high lower limit. If I hade to set it I'd set them at 1.5V -1.6V per cell. About 1.45V is typical for many NiMH batteries when fully charged. Anything over 1.50V would be overcharged in my book, for whatever that's worth.

    Perhaps the batteries have never yet received a charge, especially if you did not really charge them in your circuit. This, of course, would give you pretty mediocure performance the a device you are powering. Measure the battery voltages individually and record their voltage. You also need to verify that you are getting a charge voltage and current and note what they are.
  4. Macabra

    Thread Starter Active Member

    May 31, 2008
    Thanks for the reply. I measured Vlimit and it measured to be 1.9V, so it is at least within the limit that the datasheet states. I'm confused on some of the stuff from data sheet like the A/D input range equation...but anyhow I've been re-reading and trying to make some sense of it.

    I just tried again and I still get 1.9V on Vlimit but noticed that I was getting 6.1V on the Batt+ pin, that definitely something wrong in there I'd reckon..especially since that's where the positive side of the battery is connected to in order to be charged.

    The mosfet gets pretty hot as well, I've got a heatsink mounted on it.