Brand new battery - draining inexplicably?

[Scraff27]

Browsed Pistonheads a lot but never had occasion to post here - hoping I can get some advice here.

I have a 2007 Audi A5 3.0 TDI. A few weeks ago in the cold snap the battery died, so I replaced it with a spare one I had to hand (95Ah, 750CCA). It's struggled to start a few mornings, and then I replaced it yesterday with a brand new 100AH 890CCA battery. This morning I went to start the car and it was completely dead. Took the battery out, recharged it and put it back in, but still wouldn't start. In hindsight I think my battery charger is a bit too naff to give it a full charge, so that's probably the problem there.

Anyway, I'm suspecting a parasitic drain but I wanted a second opinion. RAC came out, jumped the car off their van (started up fine), tested the health of the battery (came back good) and then did a drain test.

When the car is immediately turned off and locked, it's draining about 7A. This lasts for about 30 seconds, then it drops to 2.5A or so. After a minute or so, according to his drain tester, it dropped to 0.4A. He was happy with that, although 400mA of drain seems a lot to me. I've left it alone for about an hour and a half and the battery's dropped to 12.0V.

I'm going to set up a multimeter with a camera and test the drain myself - I think it's possible the car takes a while to properly 'go to sleep' so it's possible it draws 400mA for a while and then drops to a more reasonable number.

My question is, am I on the right track here? I don't like auto electrics at all (much prefer the mechanical side) but in my mind, a healthy, brand new battery which is being charged properly by a working alternator that dies overnight and loses 0.4V in less than two hours has to be a parasitic drain of some sort, right?

[Scrump]

Sounds about right to me.
I had similar one one of my cars recently. With the car locked and the doors and bonnet open but with latches flipped to the closed position I fooled the car into going into shutdown mode. Then spent a bit of time with the multimeter and found the cause of the parasitic drain.

[Panamax]

Scraff27 said:

I replaced it with a spare one I had to hand (95Ah, 750CCA). It's struggled to start a few mornings, and then I replaced it yesterday with a brand new 100AH 890CCA battery. This morning I went to start the car and it was completely dead.

Took the battery out, recharged it and put it back in, but still wouldn't start. In hindsight I think my battery charger is a bit too naff to give it a full charge, so that's probably the problem there.

I think you need to get to the bottom of the actual battery condition and whether your charger works before setting off on a long and complicated mission in search of trickier electrical problems.

First up, how old was the battery in the car before you started changing anything? What was its specification?
Ignore the first battery change, if it was an old battery you had hanging around it was probably dead. And was it the right specification?
Does the car have start/stop? Should it have an AGM battery? If so, is the one you've fitted AGM? Is you car one where "coding" for a new battery is recommended?
The brand new battery - where did you buy it? If it wasn't from a place that has a fast throughput of sales it might have been standing around for months or years.

From where you're sitting the first step IMO is to get the new battery indoors for appropriate ambient temperature and give it a proper full charge. Then leave it to stand for a few hours, ideally overnight. then check the voltage at the terminals with a decent voltmeter. You should see a minimum of 12.6v if the battery is indecent condition; ideally 12.7 or even 12.8. Anything less than these numbers and somebody's sold you a duff battery. If you haven't got a decent voltmeter take the charged battery to your local battery shop and get them to check it with a high current drain test.

If the battery's good then the next significant question is around start/stop, coding etc. It's not impossible for a modern charging system to get confused when faced with a battery change, especially if the specifications aren't identical - as has been the case through your chopping and changing.

I wouldn't touch anything else until you've completely eliminated the battery itself and whether the charging system understands what it's being asked to do (i.e. coded correctly) if that's the way your car works.

[Belle427]

Have you also verified its charging on the car?

[E-bmw]

You either have something draining the battery, but more likely you have a loose connection between the battery/fusebox/alternator/chassis.