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  #1 (permalink)  
Old Dec 17th, 2011, 01:29 PM
jmb
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Mini Cooper S 2003: Battery drain

Hello,

I have a curious battery drain problem and am unable to locate the fault. I have a low mileage Mini Cooper S (2003).

The original BMW battery died (went flat) and didn't want to hold charge, so I bought another. The new battery subsequently failed so I exchanged it for another and that fails. I have therefore concluded that the battery probably isn't at fault.

I've used some of the super information on this forum to locate the fuse boxes in the passenger foot well and engine bay to carry out a drain test. I measured the amps flowing from the battery whilst pulling out each fuse. The following was noted:

1. With the interior lights on, 1.6A is measured.
2. When the interior lights dim to off, the current reduces to 0.6A. Removing the lighting fuse at this point does not reduce the current any further.
3. After a while, there's a drop to 0.2A.
4. Any activity that caused the car to 'think', opening a door, caused an increase to 0.6A for a minute, before dropping back to 0.2A.
5. The voltage across the battery is 12V, and 15v whilst charging (engine running).
6. No fuse was located in the foot well or under bonnet box that caused the 0.2A to disappear.

I suspect there is a third fuse box which is apparently in the boot, but I can't find it.

Is there anyone with some information on what may be causing the 0.2A constant drain and why I couldn't find the fuse on that circuit?

Thanks


John

Last edited by jmb; Dec 19th, 2011 at 09:36 PM. Reason: Correction to voltage.
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  #2 (permalink)  
Old Dec 17th, 2011, 11:04 PM
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The normal draw is 10mA base model to 18mA fully equipped model. Anything over 40mA is a problem. Your biggest problem is measuring this when the car is fully locked i.e. in the state it is overnight. I had a faulty remote unlocking sensor module and it took the stealer several attempts to track that down.

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Old Dec 18th, 2011, 08:25 AM
jmb
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Steve

Do you have any idea why I can't get the 0.2A to drop down to 0, or close to zero? I seem unable to find the fuse that has any effect on this figure.


John
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Old Dec 19th, 2011, 05:42 PM
jmb
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I've read that the immobiliser/alarm may be at fault, and I assume it's not fused - or the fuse is elsewhere. Does anyone know how I can disable this to take it out of the equation?


John
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Old Dec 19th, 2011, 07:16 PM
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You state a couple of points that need clarification...
Quote: Originally Posted by jmb (original)
5. The voltage across the battery is 12V, and 15v whilst charging.
6. When the engine is running, the voltage across the battery increases to 18V, which indicates the alternator is running and the battery charging.

Point 5/ - readings show near enough what the reading should be
BUT....
Point 6/ - ... the voltage across the battery reads 18v when the engine is running??

So how are you measuring the {different} 15v & 18v charging voltages ?
They should be one & the same - both measured directly across the battery terminals whilst engine running without anything being disconnected.

If you have a reading of 18v across the battery, then this indicates that the voltage regulator inside the alternator has gone & you are 'cooking' the battery - hence its early failure.

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Old Dec 19th, 2011, 09:38 PM
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Hello

Thanks for your reply. Sorry, I copied the figures incorrectly and have corrected the post. To confirm, the voltage across the battery is 12v when the engine is off, and 15v when it's on.


John
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Old Dec 19th, 2011, 10:22 PM
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Ok - so its not the alternator then

I had a faulty cd/radio on my 2002 MCS unit that could drain the battery overnight.
Although yours, with a drain rate of around 200ma would take over a week to drain a fully charged 60Ah stock battery.

Had a bit of a search around & there was a recall for batteries in 2003/4
There was also a problem with the electric power steering pump which could cause a drain with ignition off.

Keep looking - seems like your going about it in a logical way - maybe disconnecting the steering pump might be a good next step?

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Old Dec 20th, 2011, 07:32 PM
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I'm still struggling with the process I've gone through; removing all the fuses and not eliminating the 120mA. What else could it be?
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Old Dec 20th, 2011, 09:23 PM
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Quote: Originally Posted by jmb (original)
I'm still struggling with the process I've gone through; removing all the fuses and not eliminating the 120mA. What else could it be?

Well, 120mA isnt too bad a drain as the alarm system & clock will be taking a small amount - but if the info 'SteveM3' has of a normal 18mA drain is correct, then i'd try disconnecting the alternator as a 'leaking' diode or faulty regulator could easily cause this type of drain.

One other point - How long are you leaving it after switching everything off when making the measurments?
When I had LED sidelights fitted, these would be fed with a low voltage feed to make them 'just glow' for around 15 minutes after the car was locked, so might be an idea to take some measurements say 30mins after shutdown to see if its still at fault.

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Old Dec 21st, 2011, 03:37 PM
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The alternator looks a complete pain to access. Are there some "easy access" steps somewhere?

I'm leaving it about 5 minutes but I take onboard your point. I will run a 30 minute test when I return to the problem in daylight. It's not the best time of year to be trying to find time for this task :-)

Update from today's half hour investigation: I've pulled every fuse out of the interior fuse board, and the three relays for good measure, and disconnected the ceiling alarm sensor, and it's still showing a 150mA drain (I've now moved to a digital not analogue meter to get some better readings).
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Old Dec 21st, 2011, 07:04 PM
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These diagrams may help
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Old Dec 22nd, 2011, 05:24 PM
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Thank you. So the alternator and immobiliser are on separate non-fused circuits. Do you happen to have any pictures or instructions on the easiest way to disconnect them? Are there three wires going from the luggage compartment? (ie one to the fuse boxes, one to the alternator and one to the immobiliser?)
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Old Dec 22nd, 2011, 07:22 PM
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To isolate the Alternator - Just disconnect the Thick Red wire {Rt} from either the battery terminal or the Alternator -
MAKE SURE IT DOSN'T TOUCH ANY METAL EARTH POINTS WHEN DISCONNECTED

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Old Dec 22nd, 2011, 08:02 PM
jmb
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I hadn't noticed there were two or more wires attached to the + terminal on the battery.

Would you mind explaining the reason for your warning regarding touching earth?
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Old Dec 22nd, 2011, 08:26 PM
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Quote: Originally Posted by jmb (original)
I hadn't noticed there were two or more wires attached to the + terminal on the battery.
Would you mind explaining the reason for your warning regarding touching earth?

I'm trying to recall details from memory so cant be 100% sure exactly, but the alternator cable may go to the large covered 'Jump point' next to the air box in the Cooper S, before it heads back to the boot.

The warning just applies if you removed the red cable from the alternator end, as it will still be an unfused live & would therefore cause a bit of a flash if shorted out to earth {negative}

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