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Auto Lead Acid Battery Charger

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  • clearchris
    Badcaps Veteran
    • Dec 2013
    • 664
    • United States

    Auto Lead Acid Battery Charger


    I have a super basic schumacher 120v to 6/12v 2/5a auto battery charger. It's a metal box with a meter, switch that goes to the taps on the transformer, a rectifier attached to a sink and a thermal cutout switch. No electronics PCBs, or other circuitry.

    When I attach the clamps to the battery, it charges properly for maybe 30 seconds, then I hear what I believe is the thermal fuse tripping, and it goes off. In a minute or so, it turns back on and the cycle repeats. This happens on 2a mode and 5a mode.

    With no battery attached, I measure 14v DC at the clamps. At the secondary taps on the transformer, I measure 14v AC.

    The thermal fuse is sealed in plastic, I can't check the points without destroying it. I don't see much rust in the unit, but it's not pristine.

    Does anyone have a suggestions on what to check next? Could a bad battery cause this (suspected) overheating?

  • petehall347
    Badcaps Legend
    • Jan 2015
    • 3995
    • United Kingdom

    Re: Auto Lead Acid Battery Charger

    could be the battery .. try measuring voltage whilst its charging .


    • eccerr0r
      Solder Sloth
      • Nov 2012
      • 8273
      • USA

      Re: Auto Lead Acid Battery Charger

      Is that a automatic resetting circuit breaker or PTC resistor thermal fuse?

      Is the ammeter getting pegged?

      Agreed, a shorted battery could also cause this behavior and is the more likely cause. How many volts is the battery? Call it suspicious if it's less than 10 volts.


      • redwire
        Badcaps Veteran
        • Dec 2010
        • 3413
        • Canada

        Re: Auto Lead Acid Battery Charger

        What model #?
        The thermal circuit breaker is usually glass so post a pic of what you've got.
        I think the charger is legit and likely being overloaded by a bad battery?
        The only other thing that causes the CB to trip is if the charger is outputting AC due to a shorted rectifier diode. Some can still sort of regulate even with a bad diode and fool a multimeter.
        I would try run it with a i.e .#1157 tail light load and see what the output looks like if you think it's the charger and not the battery.

        Schumacher Electric SE 5212A battery charger, it uses an SCR and a TL431, few transistors for control. Schematic.
        Weakness is the pellet rectifier diodes overheat esp. if someone was cranking the engine with it connected.