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Discussion Starter #1
Hi there all

I'm still trying to get to the bottom of why my battery is discharging overnight, its the only problem ive got left with my car, thank god:)

Ive gone across the battery with a multimeter and 'in standby' the car is pulling 0.4A, ive pulled all the relays/fuses and the fuse that supplies the headlamp washwipes pulls 100ma, the interior light fuse pulls 100ma and the check control/onboard computer fuse pulls 200ma, is 0.4A ok or does my car have an earth fault.

The battery that is currently in the car is rated at 75 ampere hours, ive been through 2 battery spec books and cant find the correct battery, i have bought a battery according to bosch's book and it is huge and rated at 88ah. does anyone know the correct battery spec/battery code for a 92 3.8

Many thanks for reading and looking forward to any response

Andy
 

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Hi Andy , I suspect that tnhe current battery may be to blame for your woes as I believe that 85ah is required and your new Bosch acquisition will be up for the job .

D
 

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Does your alarm/radio pull any trickle current?
 

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Discussion Starter #5
Many thanks for the swift replies gents :)

David, thanks for the clarification on the battery size, its looking like the battery that has been in the car just isnt up to the job, ive always replaced it with the same size battery that has been in the vehicle before, so im guessing the owner before me put the smaller battery in because it was a cheaper unit.

Mmm-five, ive turned the BMW alarm off at the key under the bonnet and my brother has ripped all the old stereo cables and sorted the bodged wires up, looks like a butcher had had a go at rewiring the car before me:grrrrr:

Thanks again for the replies, i'l let you know how i get on this evening when i install the new battery.

Best regards Andy
 

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Hi Andy,
Have to say that if you have a 75 ah battery instead of an 88ah one,this will not be the reason why it is going flat over night.
A good 75ah battery will power a 75 amp device for 1hr, or a 7.5 amp device for 10hrs or a .75 amp device for 100hrs ...etc..etc.
So although i think your current drain of .40 amps is a little high, my calculator says your fully charged 75 ah battery should last 187.5 hours or nearly EIGHT DAYS!
That is not to say that you still dont have the correct battery,just that your 75 ah one is more likely faulty.Have you had it tested?
What about the charge rate at the battery (engine running)?

Regards
Colin.
 

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Discussion Starter #7
Hi Colin, how are you doing:)

Many thanks for the detailed reply, ive learnt a lot about car electrics over the past few days especailly from this great board, ive left most of it to my brother who swapped the battery over to the bigger one last nite, which seems to fit in the mountings under the back seat a lot better too.

He traced another fault to the rear left number plate light having a tiny bit of current going through the wire at all times just enough to glow the bulb filament orange. This must have been on all the time, helping my battery go flat.

And a strange almost inaudible hum/buzzing coming through fuse 20 (check control unit/heater controls/onboard comp) that is always on even when there is no key in the ignition, it seems to be coming from behind the heater fan speed control, any ideas what this could be would be greatly appreciated.

Hes pulled the fuse 20 and im going to see if the battery has discharged when i get home tonight.

Thanks for reading and all your responses

Andy

PS Colin are you going down to santa pod this year to the ultimate BM show, if yes it;d be great to meet up again
 

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Hi Andy,
Hope to be at the pod again this year,be great to crack a 13.2.
Like to see the fresh paint on yours-
hey we could compare panel gaps! :hihi:

Colin.
 

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Discussion Starter #9
Hiya Colin, that'l be excellent if you can make it:), im going to drive down with my brother and hopefully Kevin (sharkfan) with his touring and another M5 owner from lancashire is going to make it too.

Lookin forward to seeing your car do some good runs again:thumbsup:, comparing panel gaps will be fun, my bonnet still looks bloody terrible lolouich

All the best

Andy
 

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Discussion Starter #11
Colin that'd be great, (providing my car hasnt failed on me again, damned exhaust)

could see your car eating mine for breakfast though:M5launch:

See you in September:)

All the best Andy and James
 
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