lead acid battery charger

Discussion in 'The Projects Forum' started by 016chris, Nov 20, 2007.

  1. 016chris

    Thread Starter New Member

    Oct 30, 2007
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    I am currently designing and building a 12V lead acid battery charger for a college project but I'm having difficulty finding certain information and help. I aim to build a two stage smart charger which will both boost charge, and float charge. In particular i need details on the charging currents fed from the charger. Any other links or info on diagrams or relevant issues would be greatly appreciated.
    Many thanks
     
  2. GS3

    Senior Member

    Sep 21, 2007
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    35
    A two stage charger is very simple and, in fact, any lab power supply will do it because all you have is a power supply limited in both voltage and current. At first, when the battery charge is very low, the current limiter is operative, then, as the battery is charged and the voltage rises, the voltage regulator is operative. A voltage-regulated power supply with current limitation is what you need.
     
  3. 016chris

    Thread Starter New Member

    Oct 30, 2007
    5
    0
    Thanks. But do you have any numbers of the actual bulk charging currents, and how they change when the charger switches to float? Also do you have any web sites i can gain further info?
    Thanks
     
  4. GS3

    Senior Member

    Sep 21, 2007
    408
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    Well, you can charge a battery at say, 1/10 of C (its capacity in Ah) which, if the battery were ideal would mean it would be charged in 10 hrs but in reality it would take like 14 hrs due to the inefficiency of the charge process. Once the float (voltage regulation) phase begins the current will slowly decrease until (if the float voltage is ideal) it will be a tiny maintenance current. All those those parameters can be adjusted to suit your specific battery and needs.

    This is quite simple in principle and can get as complex as you want to go deep into it.

    Have you tried Google? Wikipedia? If not, why not. There must be plenty of information out there. Like http://jgdarden.com/batteryfaq/carfaq9.htm

    If you just want a simple charger then adjust a regulated power supply to limit at 2.3 V/cell and 0.1C Amps and you have what you are looking for.
     
  5. Spoggles

    Well-Known Member

    Dec 2, 2005
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    016chris:

    Also check out http://www.batteryuniversity.com/.

    You may want to use three numbers. 12.7, 13.7 and 14.7. 13.7 should be the 'float' charge voltage, 14.7 is the equalization charge voltage, and 12.7 is the point where you would want to recharge.
    In charging systems that I know about, the sequence of events is as follows.

    The battery is brought up to equalization at 0.1C then charged at its "float charge" VOLTAGE.

    If the float charge voltage goes below 12.7, the equalization voltage is reapplied.
    ...this process is repeated as necessary. A good battery will eventually settle down to the 12.7 volt point.

    In order to do this a hysteresis loop is incorporated in the circuit.

    Spoggles
     
  6. GS3

    Senior Member

    Sep 21, 2007
    408
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    That's three phases and he specifically said he wanted two but, yes, that is how three phase chargers work. Another aspect to take into account is that the voltages change with temperature so, for better results a temperature sensor and compensating circuit should be used. Notice how all cell phone batteries have temperature sensors.

    The problem with the two phase charger I described is that it takes a long time to top off the battery. This is undesireable in some situations like a boat where you want to top off the batteries as fast as possible because you don't want to be running the motor to trickle charge the battery.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    For a 2 stage charger you could do as follows:

    For a 35AH 12V Battery initial Bulk charge current max at .1(35AH)=3.5A with 14.7V. When the battery reaches 13.7V you would make the switch to a trickle charge, .01(35AH)=350mA max at 13.7V. This trickle or "float charge" ought to be able to stay on indefinitely.
     
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