NiMH charger circuit

Discussion in 'The Projects Forum' started by gopalyajur, Aug 5, 2010.

  1. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    Hello all,

    I am trying to build NimH charger based on Max712 chip to charge four 2500mAh sony batteries. I am planning to use wall power adapter capable of giving an output of 2 A at 12 V based on the circuit discussed in the link below

    Now, I have problem in understanding in choosing the time necessary for the charging. In the maxim datasheet its mentioned that one can choose the charging by selecting suitable programmable pins. But, in the link I provided the time required for charging is calculated based on the relationship

    T_charge= C_cell/I_charge x 1.2, where C_cell is the cell capacity

    For my case the calculated value of T_charge is 1.5 hours.

    The question is, if I want my batteries charged in 22 mins (lowest possible time), do I have to use higher current??

    I am bit confused, can someone help me out?

    Thanks in advance.

  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
  3. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    Thx Bernard. I decided to charge the batteries in 90 mins.
  4. Darren Holdstock

    Active Member

    Feb 10, 2009
    You absolutely need to monitor the temperature of your NiMH cells is you're giving them any sort of fast charge, which at the end of the day is the only type of charge they like. If not, you'll just destroy batteries if you're lucky, start a fire if you're not. A maximum temperature of 50 deg. C or a rate of temperature rise of 1 deg. C/min (whichever comes first) is the recommended criteria for terminating a fast charge. You can apply a top-up charge (about 25% of the fast charge current) for a couple of hours after that if you like. Don't trickle charge for too long (a day or two at most), they don't like it.

    I'd throttle back on the fast charge current a bit; anything more than 0.5C is a bit racy, and won't give the best charge or lifetime, but it will make everything much more critical.

    I'll second Bernard's fine suggestion to visit BatteryUniversity, it's a very fine site.