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Matt, Mechanical Engineer
Category: UK Car
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Experience:  BEng hons Mech engineering, in auto industry 22 years
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audi a3: My car battery keeps draining...alternator

Resolved Question:

My car battery keeps draining. I have had 3 batteries in less than 12 months and just changed alternator. Audi A2 2002.
Submitted: 2 years ago via AskTheMechanic.
Category: UK Car
Expert:  Matt replied 2 years ago.
Hello

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



Customer: replied 2 years ago.
Matt,

Thanks for this, it's really useful. The car has been in the garage twice for this same issue and the mechanic can't find the drain. Is there a known fault with A2's. Also, does it require any special diagnostic equipment that would be specific to Audi?
Expert:  Matt replied 2 years ago.
Hello

there's Audi diagnostic kit but this works with all models so nothing specialist required for the A2

the only possibility with the A2 is the battery well is getting wet / filled with water as this will shorten life dramatically and also check that the vent hose from the battery isn't blocked as they produce hydrogen gas all the time
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 16622
Experience: BEng hons Mech engineering, in auto industry 22 years
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