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V50 New battery yesterday. Flat today.

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Old Apr 8th, 2021, 14:50   #1
Barberskum
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Default V50 New battery yesterday. Flat today.

I jump started my battery the other day but the following morning it was flat again. I swapped it out for a brand new battery and it started fine. This morning, dead again.
So it's not the battery but something drawing current. Everything is 'off' and there's no chargers or the like plugged in.
Any ideas?
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Old Apr 8th, 2021, 15:38   #2
JohnM 855 T5R
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With your battery charged up, if you can lay your hands on a multimeter (a cheap one is only 10 or so), do the following:

Make sure everything in the car is switched off, doors closed etc.

Disconnect the positive lead from the battery. Select an amp range on the meter, attach one multimeter probe to the positive battery terminal, the other to the cars positive battery lead (I use a couple of large clips I have for the purpose).. You are now reading what current the car is drawing "at rest". It should be somewhere between 25 and 40 milliamps.

With what you are describing, it would suggest you have a drain of 2 or 3 amps or more going on.

Now, one at a time, pull your fuses until you find the offending circuit. Now you can start to investigate each item on that circuit.

Usual culprits are often things like door marker lamps staying on when the doors are shut, glove box lamps remaining on, boot lamps remaining on , all of which aren't visible. Interior lamps staying on is another but you should be able to see that. However, your current drain sounds like it is something heavier duty than that.

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Old Yesterday, 17:16   #3
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Hi John.
Thanks for your advice.
I've had the new battery disconnected now for a couple of days.
Before I tried working on it today, I tested the battery and it's good.

I connected the positive head to see would it start today but it's still the same. There was a clicking sound from or near the fuse box to the right of the batter and no alarm sounding.
There wasn't even any lights going on in the dash (clock etc.)
I disconnected the positive again and I purchased a multimeter.

I Checked the current as you described and it's showing that it's drawing 13Amps at rest (no key in ignition - nothing in the car 'on').
I checked every fuse under the bonnet & under glove box - all are good.
13Amps to me seems massive.
I am at a loss as to what is the culprit.

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Old Yesterday, 17:58   #4
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From a response I gave in a similar thread.
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Originally Posted by cheshired5 View Post
To save many hours of time, leave every fuse in place and probe the two exposed fuse contacts with a multimeter set to the lowest DC volts setting.
You're basically doing a voltage drop test across the fuse and a 0.000 reading means there's no current flowing through that fuse so it can be eliminated.

If you get a reading, there is current through that fuse but still leave it in place for now and make a note of the fuse.

Test every available fuse this way and you'll be left with a much easier to manage 4 or 5 active fuses/circuits by the end and you can see the impact of their removal on the current drain.

Don't discount the alternator diodes either. If you have a failure there, you'll have a drain which won't be spotted by any amount of fuse checking or pulling.
The quickest way to get an idea of alternator performance is to measure the DC output directly from the alternator b+ post not the battery on a running engine.
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Old Yesterday, 18:44   #5
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Quote:
Originally Posted by cheshired5 View Post
The quickest way to get an idea of alternator performance is to measure the DC output directly from the alternator b+ post not the battery on a running engine.
Thanks!
I think you are on to something. It could be faulty diode on the alternator draining power.
I'm not testing on a running engine as it won't start. I assume you mean to jump start the car then do the test?
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Old Yesterday, 19:24   #6
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Quote:
Originally Posted by Barberskum View Post
I assume you mean to jump start the car then do the test?
Yes ideally and a failed diode would show a hugely reduced alternator output <11v DC.

Alternatively, with the engine off you can disconnect the negative battery lead then the alternator live lead then use a multimeter to do a diode test both ways between the alternator b+ post and alternator casing.
On the diode test, you're looking for around 500-800 with red meter lead on casing and black meter lead on b+ and open circuit/ol/no reading with red meter lead on b+ and black meter lead on the casing.
If you get a reading both ways on the diode test, you have a failed diode.
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