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Flat battery, v. high current drain - Fuse No. 8 under bonnet. Water pump continuous...

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Flat battery, v. high current drain - Fuse No. 8 under bonnet. Water pump continuous...

Post by Newbers » Thu Nov 02, 2017 1:47 am

Where do I start looking for a current drain?

Long story short: Car has been fine (200 mile round trip Thursday). Tuesday morning battery flatter than a pancake (not a flicker). Left it on charge all day (4Amp charger on max) still flat in the evening (lights + locking but not enough to start). Disconnected battery and charged overnight.
Next day big spark connecting battery back up. Started fine but dead again when I tried a few hours later. I've been intending to change the 10 year old battery so bit the bullet and bought a decent Yuasa earlier. Car started noticeably livelier than usual however 6 hours later battery too flat to start :x

It's an 80Ah battery. Allowing for it having been standing in the shop for a while lets say it started off good for 70Ah. I know its not scientific or strictly accurate but lets say it should still crank the car at a third charge (26Ah). On that basis its lost over 44Ah in 6 hours. i.e. Average of >7 Amps current draw (consistent perhaps with the big spark reconnecting the terminals). That's 88 watts that are going somewhere.

This seems to me to be quite a high current when no lights on and I can't hear anything. I guess either something is moving very quietly (not typical of a Z4!) or more something is getting hot.

Any thoughts about where I might start looking for the drain?
Last edited by Newbers on Mon Nov 06, 2017 10:19 pm, edited 4 times in total.
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Flat battery, v. current high drain - where to start?

Post by Ducklakeview » Thu Nov 02, 2017 1:52 am

Process of elimination I'm afraid. You need a good ammeter, preferably a clamp on type as the current draw is high, as most digital multi meters only go to about 10a max. Then I'd pull a few of the bigger fuses and see if that reduces the draw.

Where abouts are you? Happy to have a look.

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Flat battery, v. current high drain - where to start?

Post by Newbers » Thu Nov 02, 2017 2:06 am

Thanks Mike
I'm in St Albans - a good few miles from the view of the duck lake....
I only have a multimeter unfortunately.
Very irritating! Perhaps being that high a load it will be easier to identify by pulling fuses - although I can probably expect a pretty substantial spark pulling at least one of them....
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Flat battery, v. current high drain - where to start?

Post by Newbers » Thu Nov 02, 2017 3:06 am

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Flat battery, v. current high drain - where to start?

Post by Stuartt » Thu Nov 02, 2017 7:26 am

If you are in the AA or RAC call them out to check it over for you.

The alternator can sometimes go dead short and drain the battery or if you have electric seats check them for full operation as I've seen the switches stick on.

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Flat battery, v. current high drain - where to start?

Post by Ewazix » Thu Nov 02, 2017 8:18 am

In addition to the above feel the starter motor, I had one go 'open' on one car and it very nearly caught fire! They warm up if they are going open circuit and grounding.
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Flat battery, v. current high drain - where to start?

Post by bladeowner » Thu Nov 02, 2017 9:12 am

If I unlock my car, remove the boot lamp and wait a few minutes for the car to go back to sleep it draws about 70mA.
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Flat battery, v. current high drain - where to start?

Post by Newbers » Thu Nov 02, 2017 11:36 am

Thanks those are good points to check - I must confess I was only planning to go after the fused circuits so I will check the starter and alternator. Both starter and alternator appear to work OK (i.e. engine cranks and starts and I see the voltage increase when I start and then rev the engine). Is it still possible to have the faults referred to above?
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Flat battery, v. current high drain - where to start?

Post by ph001 » Thu Nov 02, 2017 11:50 am

If you have a basic ammeter you can of course just connect it in series with the battery and see what the actual current is. Ideally you'd want 20A capability on the ammeter but 10A will probably suffice as long as you don't lock the doors with it connected (it will take a current pulse that will probably blow the fuse in your ammeter).

If the current draw is >10A it's unlikely to be any of the fused accessories (things like the amp / cd player / aftermarket roof modules have all been known to cause problems in the past) and more likely to be starter / alternator related. If it's around 5-10A I suppose it could be an accessory but that current will still get the fuse pretty hot even if it doesn't cause it to blow so have a good feel across your fusebox after 15 mins or so and see if you can detect any heat anywhere.
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Flat battery, v. current high drain - where to start?

Post by Newbers » Thu Nov 02, 2017 12:27 pm

Can anyone suggest a reasonably priced ammeter capable of 20A (I don't want it to cost more than a rip to my local independent!)
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Flat battery, v. current high drain - where to start?

Post by ph001 » Thu Nov 02, 2017 12:43 pm

What can your existing multimeter do?

A lot of the clamp ammeters don't do DC. Just looking now.

How about this... https://www.amazon.co.uk/Signstek-Digit ... mp+ammeter
Last edited by ph001 on Thu Nov 02, 2017 6:50 pm, edited 2 times in total.
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Flat battery, v. current high drain - where to start?

Post by dans6490 » Thu Nov 02, 2017 12:48 pm

I feel your pain!!! Although I have the same issue, mine is a smaller drain. Changed battery etc.

I haven’t got round to finding the drain yet although I have checked boot light, cubby light etc. The general consensus seems to be cd changer, gap tech for my fault.

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Flat battery, v. current high drain - where to start?

Post by ph001 » Thu Nov 02, 2017 1:05 pm

Or this is about a quid cheaper, not quite as accurate on DC current but has a few extra bells and whistles such as true rms and a niffty little NVC function (non-contact voltage) which is useful for around your house if you want to know if a wire is live or not.... https://www.amazon.co.uk/Multimeter-Tac ... JEHQG793MJ
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Flat battery, v. current high drain - where to start?

Post by Newbers » Thu Nov 02, 2017 5:23 pm

ph001 wrote: Thu Nov 02, 2017 1:05 pm Or this is about a quid cheaper, not quite as accurate on DC current but has a few extra bells and whistles such as true rms and a niffty little NVC function (non-contact voltage) which is useful for around your house if you want to know if a wire is live or not.... https://www.amazon.co.uk/Multimeter-Tac ... JEHQG793MJ
Good shout, thanks for putting the time in to researching. I've just ordered one - £20 with Prime.

It may be a while before I need to measure true RMS but I am in the process of re-wiring my house so some of the other capabilities may come in handy :)
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Flat battery, v. current high drain - where to start?

Post by ph001 » Thu Nov 02, 2017 5:51 pm

Wow..well done you got the lightening flash deal thing. £20 is a steal for what it does.
2007 E85 Z4 3.0Si manual :driving:
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