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Charge bateries with regulated power supply

  1. Aug 28, 2011 #1
    I have a 230V to 13.8V / 20A DC regulated power supply and two 12V / 50A car batteries.
    What circuit must I use between them to charge the batteries and how long it will last the cycle?
    Thank you
  2. jcsd
  3. Aug 28, 2011 #2


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    You would be able to partly charge the batteries, but maybe not fully charge them.

    The power supply voltage really needs to be another volt or two higher to give the batteries a full charge.

    Your batteries would be 50 Amp-hour batteries, so if you fully charged them at, say, 10 amps each, they would take about 5 hours to charge. 10 amps times 5 hours = 50 amp-hours.
    If they were charged at 5 amps, they would charge in 10 hours. This is only a guide and the actual time may be a bit longer or shorter.

    You would need some current limiting if your power supply does not already have it. The simplest is probably a car head lamp, but any suitable high powered resistor of about 2 ohms would be OK.
    You just place this in series with the battery.
    It will get hot, though. A flat battery being charged from 13.8 volts through a 2 ohm resistor would cause about 95 watts to be dissipated in the resistor.

    The realistic answer to this, though, is to go and buy a battery charger.
  4. Aug 30, 2011 #3
    Dear vk6kro,
    Thank you for your immediate response.
    I wonder if there is any circuit to convert the power supply to a charger.
  5. Aug 30, 2011 #4
    Good afternoon, nricacos and welcome to Physics Forums.

    Your power supply will charge your car batteries just fine, though not both at once.

    A pretty flat (but otherwise good) nominal 12 volt battery will have have a terminal voltage of about 10.5 volts.

    Connecting it to your power supply will probably initially draw 8 - 12 amps. This current will reduce as the battery voltage rises towards 13.8 volts.

    I doubt you need an intervening resistor, just stout wiring and an ammeter if the power supply does not already have one.

    By comparison a car alternator is set to around 14.5 volts so will charge faster (in the 20 - 30 amp range). This is not a problem, your supply will just take longer to do the job.

    Connect the battery, and monitor the current on the ammeter.

    Initially it will be several amps
    As time proceeds the current will fall
    Until it has reached a fairly low steady value (less than 0.5 amps)
    Your battery is then charged.

    Don't forget to do this in a ventilated space, check the battery fluid levels and open the battery caps if appropriate.

    go well
  6. Aug 30, 2011 #5


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    Yes, there are ways.

    One would be to open up your regulated power supply and get access to the higher voltage before the regulator. This may be something like 20 volts and you would need to limit the current.

    A battery needs about 14.4 volts to fully charge it.
    Wikipedia: Typical (daily) charging: 14.2 V to 14.5 V (depending on manufacturer's recommendation)
    There are voltage booster modules on Internet and you could use one of these.
    I saw some recently that would give enough boost to charge the batteries properly and they cost only a few dollars.

    This one may be suitable:
    You would still need to add current limiting, though.
    They claim you could even charge a 24 volt battery from a 12 volt one.
    I haven't tried these, but they seem promising.

    However 12 V battery chargers are very cheap and well worth considering.
    Last edited by a moderator: Apr 26, 2017
  7. Aug 30, 2011 #6
    I have no knowledge of battery charging. Do you have to worry about over charging? I know NiCad will be destroy if left in charger without shut off upon fully charged( temperature sensor). Or is Lead Acid battery is different?
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