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Whats causing my battery problems?

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From 2008 to 2010 my 996/2001/C4 was laid up, battery disconnected with battery condition charger attached. I charged up the battery, connected it up ok and car started first time! Got MOT and insurance and all hunky dory. Two weeks later the alternator went suddenly. Got new alternator and new battery. Car has been running fine for 8 months until now. Last week when driving along the interior light came on, and both windows went down and doors unlocked. After about a minute back to normal. This happened about 6 times that day. Then at 2.00am alarm went off. I disconnected battery and charged up for 2 days. Connected up again. Next day windows going down on their own and alarm went off again and battery flat. Took battery to supplier who carried out a check and said battery ok just needs charged up. So charged it up again and then connected up. Noticed last night that interior lights were on even though all doors were shut properly and car locked. Very early this morning alarm went off again. The battery was so flat that turning ignition on before disconnecting battery did not silence alarm so had to connect up a second battery to disconnect main battery, if you see what I mean. Incidentally the windows lowered and back wing raised all on their own during this process. Now battery disconnected and on charge.

What on earth do I do next. I don't want to waste money on a new battery and the problem reoccur. Or is it just that the low voltage is playing havoc with the control system and activating windows/door locks/back wing/interior lights which of course drains the battery making it worse. In which case a new battery will sort the problem.

Please some advice needed.

Notes: When fully charged battery voltage reads 12V, when engine started voltage is 14V or so. So alternator working. With doors shut, car locked but bonnet up drain on battery is 0.2amps. The second battery I mentioned is the old battery from 2010 which I kept in case I have to open the bonnet, so would not be any better fitting that.

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Is the 0.2 amps with the front compartment light on? The drain should be around 0.04 amps with everything off, but if the light is on that would account for the difference. You can get the battery load tested at a good tyre & battery place, and if it checks out then I would agree with Loren and start with the ignition switch. Also check that it is dry under the passenger seat (RHD car) as that is where the alarm control unit is fixed.

Edit: You can stop the siren going off when you disconnect the battery by switching on the ignition and leaving the key in that position.

Edited by Richard Hamilton

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Check the alternator and its set-up. When my alternator failed, I noticed the interior light would not turn off.

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A fully charged battery in good health should read 12.6 v min. at the terminals - 12 v is too low and may indicate a low charge or a faulty plate.

The most cost effective is to replace the battery with a "borrowed" known good one and see if the fault re-occurs.

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Thanks for your rapid responses. Following your ignition switch comment I have looked at internet and this is a common fault. So have ordered new switch. Will come back when fitted and report results.

It is dry under passenger seat, I've had this problem before on previous 911s, so its one of the first places I look.

The 0.2amps was with the bonnet open but the bonnet light is not working so was off. I will look at this again with the new switch.

Good idea to get the battery load tested.

New alternator has been working fine for 8 months so will check its set-up if new ignition switch does not fix it.

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I have now fitted a new ignition switch but unfortunately the problems are the same. As soon as I connected the fully charged battery the windows went down themselves. With car locked but bonnet up draw is now 1.8 amps, more than before!!! It has now started raining and looks like it will rain for the next few days so have not done any further tests, that's Scotland for you. Once the weather improves I'll do some more tests. Any idea what further tests I can do? or do I have to take the car to a specialist to get a computer diagnosis check.

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The static current draw of 1.8 amps is a concern. For the test below, make sure the ignition is switched off and the key removed (car unlocked) when you checked the draw, both doors & boots closed etc !!

For a starter, with the multi-meter connected, start removing each fuse one by one and the faulty (?) circuit will be isolated when you see the draw reduced or dissapear. The fuse you remove should give you an indication of what piece of equipment is pulling the exessive load.

By the way, is an aftermarket stereo, lights, alarm or tracker fitted - current draw is sometimes the case of an el-cheapo installation....

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Problem fixed!!

The only aftermarket kit is the Tracker. I tried your (Steve) test and pulled out each fuse. The only fuse that made any difference was SI C3 that is for: CU CLS ALARM, POWER WINDOWS, SUN ROOF, CU POWER TOP, INSIDE LIGHT. What happened when the fuse was taken out was the alarm started and the horn started. Put fuse back and got alarm and horn silenced. Then I locked door with remote on key about a dozen times and it "clicked" each time. Then suddenly on 13th time or so it "clunked" instead of "clicking" if you see what I mean. Another dozen times it "clunked". It seems that door is now locking properly. Amp drain after about 40 minutes is now 0.08 amps. That was 3 days ago and battery has not gone down at all. So problem could have been a combination of the ignition switch and sticky lock. Anyway glad to have my car back.

Thanks for all your help and suggestions.

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