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Matt, Mechanical Engineer
Category: Audi
Satisfied Customers: 22734
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Why does battery keep draining after not being driven

Customer Question

Why does battery keep draining after not being driven for 24 hours
Submitted: 3 years ago.
Category: Audi
Expert:  Matt replied 3 years ago.
I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine / gearbox.Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.

if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement

And one last thing is – how old is the battery? – As even good quality ones needreplacing every 5 years andfaulty battery just won’t hold a charge

Customer: replied 3 years ago.
The battery is just a few months old . I bought a new one whe it started to drain. Same thing happened . The mechanic checked the altinator and its fine. The Mimi system in the car does not work at all so the radio and backup camera and screen does not come on . I'm not sure if the battery drainage and this have anything in common. Can u tell me more so that I can get the mechanic to check wat u said earlier ruling out the battery being bad and the altinator being bad. Is there anything else u can add.
Expert:  Matt replied 3 years ago.

if the MMI system is faulty then yes its quite possible this is the reason for the battery drain

if you use a current meter as I've descirbed whilst pulling the fuses 1 by 1

then this will show what circuit is at fault
Expert:  Matt replied 3 years ago.

do you still need help with this?