XSeries-forum

I had a flat battery again yesterday. That's the second in two years. I admit I mainly only do short journeys of a few miles.
Ive just had my scanner plugged in and the voltage was hovering around 13.6 - 13.9 volts. Is that about right?

Engine on or off?

Battery type (as in what chemistry is it - AGM, Calcium, wet)?

AGM on charge should be 14.4V and off charge (and left for at least 4 hours, car still asleep) 12.8V fully charged and ‘good’ battery.

Wet batteries charge at 13.8V and should be just slightly less than an AGM when fully charged and ‘good’. I’m guessing that with that voltage you may well have a wet battery?

12V or less after charging and leaving (4 hours) and it’s dying. If it’s holding it’s charge somewhere between 12V and 12.8 then you could try a reconditioning charge - if you have that type of charger? Some of the CTEK range (other brands are available) include an 8-stage reconditioning cycle and that might recover the state of the battery.

It's a wet battery and that reading was from my C110 which just says 'voltage'. I have just put my meter across the terminals in the back and it's charging at bang on 14v.
The bmw battery has been on a slow charge all night but the little window on it is black not green. The voltage on that battery now reads 12.73v.
The battery on the car at the moment is the old one off the Range Rover that I didn't throw away when I got a new one for it. That has a window on it too and that is green. It's been OK today so far.

It it doesn’t go green that the battery is definitely not happy unfortunately. You can only really get a good reading at least four hours after any charging has completed. Earlier than that leaves ‘stray charge’ on the plates which throw the readings too high. If the reading holds, 12.7V is a good battery though.

So.. I charged the X5 battery overnight and it read 12.68v this morning. I took it back to where I bought it and the chap put a stabbed on it and it showed that it was perfectly ok. Obviously short journeys were slowly killing it..
Anyway, I’ve put it back in the car as it’s a 98AH battery compared to the one off the Range Rover that is only 82AH.
But.. I put the 82AH battery on charge until the charger said it was full and the voltage read 13v. That’s .3 of a volt higher than the one that is now in the car. As far as I am aware,a loss of 1v is something silly like the battery is only at 70% or whatever
So, do I use the 13v 82AH battery or the 12.68v 98AH battery? The suspense is killing me..

12.62V after full charge is a 90% fit battery - so still good. 12.73V would be if it were brand new, Both figures are open circuit, four hours after last charge. 13V sounds like it still has stray charge to be absorbed.

It’s more about what your engine needs to crank it and the electrical load from the on board systems compared to what the alternator can deliver in the winter. The smaller battery usually cannot deliver a sustained current for as long, and means the alternator has to work harder as there’s less ‘reserve capacity’ in the battery. On my X6 for example it draws more than 60A, ignition on only (no engine). It’s fitted with a 95Ah battery giving useful reserve for when things like lights, heated windows and bum warmers are on. It doesn’t matter so much in the summer.