old battery safety/using a car battery temporarily

leafleaf Posts: 37Solar Expert ✭✭✭
I have two 6V batteries in a 12V system. They're pretty defunct (won't hold a charge), and I've been keeping the system going with a car battery charger attached to mains power. The batteries are a bit smelly and have bulged a little bit. I won't be able to replace them for another month. The batteries are connected to a solar panel, but don't appear to be charging from that (we don't know why yet and that predates the issue with the batteries).

Are there any safety issues I should be aware of?

The car charger is currently disconnected. The only reason I want to keep it connected is so the water pump works. Or does anyone have a work around to run a 12V pump off mains power eg can I put a car battery in the system in the meantime? (disconnect the solar, and use the car charger to a car battery to run the pump).


  • BB.BB. Posts: 27,776Super Moderators admin
    Your old batteries run from problematic to outright dangerous.

    They can split the case and dump acid out. They are generating hydrogen gas and could explode and pop the cap off or spray acid in somebody's face. Or they could overheat and actually catch fire.

    Highly recommend you disconnect the batteries and the battery charger and recycle them.

    A pair of 6 volt at 200 amp hour batteries AKA golf cart batteries would be a nice replacement.

    Battery chargers tend to run kind of high voltage maybe upwards of 17 volts of bumpy sine waves from rectified ac transformer/mains power. May or may not damage the pump. If simple brush/universal motor, probably fine if does not turn too fast.

    If electronic commutation, may be damaged.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • leafleaf Posts: 37Solar Expert ✭✭✭
    yes, the batteries are disconnected currently, thanks. I'll remove them.

    I'm not looking for a replacement right now (the whole system is going to get revamped early next year, and I don't know if the panel/regulator is working). I need a way of running the pump over the next month or so. Golf cart batteries aren't easily available here.

    Did you mean that charging a car battery and running the pump off that battery might damage the pump? What if the battery was charging over night then the charger is disconnected during the day when the pump is in use? It's not going to be a lot of draw (basically a kitchen sink).
  • littleharbor2littleharbor2 Posts: 1,009Solar Expert ✭✭✭✭
    Out of curiosity, have you checked the water/electrolyte level in them? I'm visualizing looking in and seeing dry lead plates.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • leafleaf Posts: 37Solar Expert ✭✭✭
    water has been low in the past but was topped up.
  • BB.BB. Posts: 27,776Super Moderators admin
    Running the pump from a battery charger without a battery may damage the pump.

    Of course, the new&fancy battery chargers may not turn on without a battery (use battery voltage to make sure polarity is correct).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Posts: 2,760Registered Users ✭✭✭✭
    > @leaf said:
    > water has been low in the past but was topped up.

    6v batteries are probably literally toast. If you have grid power and an AC charger, you can likely use a car battery to run the pump until you get things figured out. Check the type of car battery vs output of the charger.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Posts: 27,776Super Moderators admin
    Sure, why not. Assuming it is not a large pump and the charger has a float setting that holds the battery voltage around 13.4 to 13.8 volts on average, should be fine.

    If the battery is over 14.2 volts or so, that will use a lot of water. And the battery may get warm/hot.

    If you only have to add water to the battery once a month or less, it should last for many months.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • leafleaf Posts: 37Solar Expert ✭✭✭
    Thanks Bill. I'll check the charger, but I can keep an eye on the battery voltage and turn the charger off periodically. I found a sparky who will sell me a battery very cheap so it's good interim measure until I can get the system sorted properly.
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