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Thread: Can not charge batteries using +ve terminal under hood

  1. #1
    Join Date
    Feb 2005
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    e34 535i e31 850i

    Can not charge batteries using +ve terminal under hood

    Hey folks,

    1992 v12, swapped in new batteries. After sitting in the garage for few days the batteries were dead.. car wouldn't start. Charged batteries from post under hood with 2amps until full.. few days later they are dead again


    I have tried 3 different chargers and they all won't charge the batteries.

    The other day I heard a noise coming from the relay box while the a charger was on for few hours.

    As I'm writing this it has dawned upon me that I may not have a charging issue so much as maybe there is something draining the batteries. Not sure where to start with this.. any guidance would be appreciated.

    Thanks
    Last edited by Incantation; 10-24-2017 at 03:02 AM.

  2. #2
    Join Date
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    Start by disconnecting the negative lead on both batteries, charge (say) the left-hand one fully (a 2A charger will take around 2 days for a full charge). Disconnect the charger. Use an Ammeter (set to the 10A range) to connect between the negative battery cable and the negative post of the left-hand battery and watch the current demand (doors closed, key out of the ignition, lights off etc). Even with the trunk open you should see the current demand drop to 50mA or so after 16 minutes as long as you don't fiddle with anything.

    Anything over 50mA indicates that the car has not entered sleep mode - this test can only be done with a fully charged battery (you can ignore the right-hand battery for these tests as long as it remains disconnected).
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  3. #3
    Join Date
    May 2014
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    They let me out again!

    The noise from the relay box was your consumer cutoff relays coming alive during charging once your batteries have been charged to an acceptable point. The clicking relays points to your General Module having faulty caps and requires a rebuild, thats whats draining your new batteries. Are you also having an issue with your interior lights and/or your windows acting up? Timms test will confirm this as it wont enter sleep mode.

  4. #4
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  5. #5
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    Thanks for all the input, much appreciated

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