Maxim712 based charger

Discussion in 'The Projects Forum' started by gopalyajur, Sep 6, 2011.

  1. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    93
    12
    Hello everyone,

    I built a battery circuit based on Maxim712 chip. I have attached my circuit and the board layout. I have set the charge time to 90 mins at 2 amps charging current. I have used BD 132 for fast charging.

    I have serious problem with heat. Both my transistor and batteries become extremely hot. The transistor is connected to a decent heat sink. Data sheet show that max Ic for BD132 is 2 A. I measure 1.8 A flowing into the battery and around 1.4 V across each cell. I am bit worried that I might damage my batteries. I am using sony 2100mAh (four of them) batteries to charge.

    Is my transistor getting hot bcoz I am close to its rating. Is is some other way to solve the heating issue. The data sheet shows the use of thermistors although its not very clear to me.

    Can someone show some pointers?

    Thanks in advance.
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    The voltage across each cell, was that measured while charging or at rest, in other words out of circuit for a while?

    There seems to be no thermal shutdown in the charger, or at least it is not working.
    If one of the four batteries is bad you will mess up all the rest in a flash, especially in a series configuration and with such a high charge current.

    One thing is for certain, I would not continue to attempt to charge the batteries with the circuit as it stands as battery damage will occur.
     
  3. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    93
    12
    Thanks for the reply.

    The voltages I mentioned were measured during charging/in circuit.
    The batteries are actually old, probably they are damaged. I wanted to check the circuit first before using the good batteries.

    Can I use two PNP transistors in parallel to reduce the amt of heat generated/wasted in each transistor? How can I reduce the heat generated in the batteries?
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I'm not sure you want to try to reduce the HEAT in the circuit arbitrarily as this is an indication that something is wrong with the circuit or batteries.

    Measure the batteries you had used in the circuit after they have been sitting for a day and see what they read. As long as the transistor is good and has the specs listed in the datasheet I wouldn't try something out of the ordinary. I'm not a big fan of charging batteries in series and at such high currents either.
     
  5. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    93
    12
    Thanks once again IONic for the reply.

    I actually checked the batteries by inserting them in a commercial micro controller based charger. It showed no errors and indicated 3 of them to be charged to 75 % and one to 80 % from the previous charging with my circuit. May be the batteries are ok.

    Also, the commercial charger completed the remaining charging of all the four batteries around 10 mins. Its specification mentions it can charge 6 batteries at 1 A. There was no heat no fan and was not heavy too. How do these commercial fast chargers work. I have seen 20 mins chargers, some of them have fans in them to cool the batteries and probably the internal circuitry too. Are there not so heavy transformers which can deliver 2 A current.

    I will check the circuit again and in the mean time I also tried to discharge the batteries by connecting it to 6V, 20 W bulb. Would you then suggest I charge batteries in parallel after checking my circuit?
     
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