Battery charger repair

Discussion in 'General Electronics Chat' started by Bordbrown, Dec 9, 2008.

  1. Bordbrown

    Thread Starter New Member

    Dec 9, 2008
    Please keep in mind that this is from an amateur and that it may be cheaper to replace the charger. This is for the educational benefit.

    With no load on the charger, on the 12 v. setting, it holds a steady 15.2 v. DC indefinitely. When I attempt to charge a battery, the ammeter starts to pulse rapidly, 0 to 6.5amps after 90 seconds. The transformer reads 15.5 v. AC.

    I am assuming that the two squares on the back panel are diodes(?). Are the diodes the likely problem? Can I replace them with a 25 amp bridge rectifier that I have left over? Thanks for the help.

    Also, what is the downside of using high amp bridge rectifiers? They seem cheap enough that a high amperage would be safer.
  2. awright

    Well-Known Member

    Jul 5, 2006
    Your description of what is happening is a little cryptic. Can you explain more clearly what you see and the time periods when you attempt to charge a battery? What capacity and condition of battery are you trying to charge?

    That kind of cycling does not sound like a diode problem. More likely excessive current is being drawn by the battery and the circuit breaker is cycling off, cooling, and resetting itself. Measure the voltage across the circuit breaker (the black object mounted to the upper corner of the diode heat sink) as the charger cycles. If the voltage across the breaker terminals goes from about zero to about 15 VDC when the charger cycles off, the breaker is cycling.

    You could try charging a battery that is in better condition and doesn't draw as much current to see if the problem goes away. You could try loading the charger up with a power resistor that draws less than 6.5 amps to see if the charger tolerates that. You would need a power resistor of (13V/5amps)=2.6 ohms with a power rating of (13*5)=70 watts or (preferably) more - like 100 watts. Or just put a lower power resistor in a bucket of water.

    If excessive current seems to be the source of your problem you might insert a dropping resistor in series with the output terminal to limit charging current to 5 or 6 amps even when charging a dead battery. The regulation on the transformer should have limited the current to less than 6 amps, but apparently it is not doing that. The series resistor would lengthen battery charging time (but perhaps not as much as having the charger cycle off or fail) but not final charging voltage because the voltage drop across the resistor would be proportional to charging current and charging current decreases to a very low value as the battery reaches full charge. I don't know the proper resistance - you'd have to experiment. But I suspect about 0.2 ohms would be about right. Power rating at least ((6.5^2)*0.2) or about 10 watts minimum.

    Be aware that this type of charger should not be left connected to the battery indefinitely, as it will ultimately try to charge it up to 15.2 volts which will damage the battery.
  3. Bordbrown

    Thread Starter New Member

    Dec 9, 2008
    The latest battery I tried it on was a conventional fork truck battery. It was at 11.2 volts. Since then we recharged this battery with another charger and it appears to be fine.

    When I attach the charger to the battery the ammeter goes to something over 6.5 amps and holds for about 90 seconds. Then the ammeter starts pulsing, approx. 6.5 amp to 0 to 6.5 amp, roughly 1 pulse per second. I was thinking that that this was due to 60 cycle frequency.

    I noticed that our shop outlets are almost 141 volts AC. Is this causing the high DC voltage?
  4. SgtWookie


    Jul 17, 2007
    Fork truck batteries are usually VERY high capacity, and at 11.2v would be considered completely discharged. A 6A charger would be woefully inadequate for a battery that large. It would take days to recharge it.

    The high outlet voltage would certainly cause problems!

    You really need to get an electrican in there to check the situation out, and fix it.

    It may be something simple. But right now, your shop is in danger of burning up equipment plugged into the outlets. That can get expensive in a hurry.
  5. leftyretro

    Active Member

    Nov 25, 2008
    I would first recommend you have your meter validated or measure the AC voltage with a different meter. That would be a very unusually situation for your utilites power to be that high of a voltages, not impossible, but I would suspect your meter first.