Join Date: Oct 2007
My Motorhome: Swift Kontiki 615
Thanked 777 Times in 693 Posts
Have a look at the picture on the link I posted earlier, 12.7v IS fully charged if you are getting a reading of 12.9v was that obtained after the charging system, whatever it was - the alternator or the battery charger, has been switched off for AT LEAST 30 minutes ?
Your gauge saying the % may well be where the misinformation comes from, 12.7v is fully charged.
The ONLY way that I know of to reliably check whether the alternator is charging, assuming that you have checked the operation of anysplit charge relay, is to partially discharge using a light for instance, so that the voltage drops to say 12.3v and THEN try driving to test the charging, once again waiting at least 30 minutes after stopping any potential charging, then test the voltage which should now be higher than the 12.3 earlier.
If it gas gone up, the charging system is working correctly, if not there MAY be a fault. BUT, it takes a long drive to raise the charge - typically several hours to restore full charge. The reason for the slowness of the charging is due to characteristics of the control system for the alternator which primarily recharges the vehicle battery until it is fully charged, it then reduces the output from the alternator which is what slowly charges the leisure battery.
It is possible to fit a larger alternator or modify the charging control system, but suchbthings are beyond me although they have been discussed on here many years ago I think.
30 - 40 minutes charge will make little difference to the leisure battery charge as I have said, during that time the charge drained from the vehicle battery for starting is being replaced only then will current trickle into the leisure battery.
The leisure battery should not change on starting the vehicle,nthat is why there are two batteries.
Any typo that I make means the errorists win.
Any comments above are only MY OPINION
and should be read as that.