Audi TT mk1. Battery won’t hold it’s charge .

[Rich Ham]

The battery is new , the alternator works fine , nothing obvious left on..... any ideas what could be draining the battery , it’s flat after 2-3 days when not used ? 🤔🤔

[RazerSauber]

Alarm maybe? No idea. Take to an auto electrician. Few quid on diagnosis is worth not spending hours crawling about your car.

[Penelope Stopit]

Do you have an ammeter?

[deggles]

^^^ This. You can put a multimeter in line with the battery to see if current is being drawn with everything ‘off’, then start pulling fuses to track down the cause. Not always straightforward though, and you might need to ‘trick’ the car into full sleep mode by closing bonnet switches etc. Whatever you do don’t crank the engine while you have the meter in line

Lots of stuff online, Youtube etc., search for 'parasitic battery drain test' or similar.

On my missus' Honda Civic it turned out to be a faulty hands-free bluetooth module. Now unplugged and no more battery problems.

Edit to add: Might be worth getting a fault code reader on it (VCDS or VagCom if you have access) - might show up any faulty central locking units and such like which can often be the problem.

Edited by deggles on Monday 8th July 16:23

[GreenV8S]

deggles said:

^^^ This. You can put a multimeter in line with the battery to see if current is being drawn with everything ‘off’, then start pulling fuses to track down the cause. Not always straightforward though, and you might need to ‘trick’ the car into full sleep mode by closing bonnet switches etc. Whatever you do don’t crank the engine while you have the meter in line

These days volt meters are sensitive enough and accurate enough for you to estimate the current from the voltage drop across a fuse. It's far easier and less intrusive this way.

[alexmonkey]

If it's just started happening and nothing of note has changed with the car's electrics, try unplugging/unbolting the alternator's live terminal (with the engine off) and connect your ammeter between the alternator and its live terminal - a dead voltage regulator in the alternator can drain the battery with the engine off, even though the alternator will still charge the battery back up once the engine's running.

[SlimJim16v]

Something like this, just make sure you get one for correct the fuse size - https://www.ebay.co.uk/itm/Automotive-Current-Test...