Battery Help Please!

[SteveB VXR8]

I recently replaced the Alternator on my VXR8 & all checked out fine.

I mainly do short runs 10 -20 miles and after this battery typically shows 75% charge 13v on Eng Mode.

The next morning I often get a message Battery Saving Mode & on checking Eng Mode charge is 55% & 11.8 volts.

The battery is a Bosch S 5008 only 2 years old.

Have only experienced this since the new alternator fitted, have checked boot light not on etc.

Any ideas?

[ARAF]

Maybe the new alternator is draining the battery over night. It's not unheard of, and as the problem has only been since the new one was fitted, it could be your culprit.

However, if the new alternator was fitted because of the problem, then I guess you haven't cured it, and something else is draining current.

[vxr2010]

I don't know how much I trust the on board diagnostics , mine has started as low as 20 per cent if it means any thing I don't know , a trickle charger may be , or solar panel charger ?

[vxr2010]

A test for a battery drain is disconnect the battery and put a car bulb between battery and the battery lead and see how bright it is , if it's a dull glow then all is fine , if it's like normal brightness then some thing is draining it , trackers and car alarms do tend to drain quite a bit , my battery drops voltage when not used and shows what appears to be bad news voltage wise and per cent battery life but appears ok when it comes to starting , is it garaged or on the street ?

[SteveB VXR8]

I fitted a new alternator as I had an Alternator Fault & Battery Light as on; garage tested it & confirmed no output from the old alternator.

Have had no previous fault like this; left the car for a week in June without any problems.

When I had my first battery problem a couple of years ago the old battery wouldn't hold charge more than 40% & went down as low as 24% before I changed it so I know the Eng Mode is a pretty good indicator.

Thanks for the comments.

[FishmanUK]

I would try
1) drive car around as usual
2) unplug battery - record voltage with multimeter
3) couple of hours later - record voltage with multimeter
4) next morning record voltage with multimeter

Repeat but leaving battery connected

If the connected battery drops way down then you know you have a short/something draining. If they both drop loads from 3-4 then the battery is knackered.

Edited by FishmanUK on Monday 10th August 22:54

[SteveB VXR8]

Final part of the story.

Replaced the battery as the old alterator had fried the last one.

All now fine.

[SOULCREW]

alternators dont last long on the vxr8 it seems....i have a new one waiting to be fitted next week