Battery Totally Dead- Drain Cause?

  • Hey all,


    After leaving my car sitting for 4 days, went to start her this morning and nothing. Not even a splutter so it seems the batteries are shot. I have had the car for almost 6 years and never had a single issue (that does mean my batteries are at least this old though).


    Car used to sit for a week or two quite happily. Pretty sure no interior lights etc. were left on to cause this so although it could well be batteries past their lives, I am a bit surprised how flat they must have gotten. Once I opened the car (electronically with the key) I couldn't use the key anymore so didn't even have the power to look/open the car after the first go.


    When I opened the boot using the key, the viewing window on the left side battery was black (meaning insufficient charge but not red which would mean battery is done) and there was a constant ticking coming from the fuse/relay box sitting just above the battery.


    I have gone and bought two new ones as it is time they were changed but I am bit concerned that after I fit them there is still an issue with something causing a drain. Any likely candidates here that could cause complete drain?


    :)

  • Hello,


    As the batteries are in parallel, if one develops a bad cell, it will cause the other battery to discharge.


    If you have a multi-meter:


    Try charging the batteries and then measure their voltage after leaving them standing (disconnected from the car) for an hour. You should read 12.6v or there abouts if the battery is good (voltage varies a little with ambient temperature- you can find voltage tables for lead-acid batteries through google)


    If that appears all OK, you can use a multimeter to measure the current drain. You need only connect one battery (leave the other battery disconnected- at the negative terminal ) and make the circuit on the remaining battery through the meter to read amps or miliamps - again safest on the negatibve terminal. Close the doors so that the interior lights go out and place some tape on the pin-switch to turn off the boot light. Then leave the car for 15 -20 mins, during which time the unloader relay should disconnect all loads, except alarm and you should see the current drop from around 300mA to around 20mA, if all is well.

  • Thanks for that. I changed out the batteries last night to 74Ah Bosch Silver 5007's. All seems fine now. The batteries in place were BMW branded and one still had green showing in the viewing glass whilst the other was black. Could have been a bad cell in one of them I guess. As long as the charge holds should be an easy fix :)


    Fraser

  • I used to experience the same issues...I eventually bought a CTEK MSX 10 3 weeks ago and have been very happy with the outcome.

    Richard
    840 Ci Individual
    1995 factory CSi sport kit
    Ferrari 599 GT Red
    Hong Kong

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