Changing one leisure battery only

Matthew Ringer · Aug 17, 2020

After 10 years of using my van, i'm having battery issues. I tested both leisure batteries, one under the seat is 4V and one in the cupboard is 11.7V. Is it OK to just replace one of the batteries?

Velma's Dad · Aug 17, 2020

I believe the received wisdom is that you should change them as a pair, but someone will come on here and explain the electro-chemistry of why that is.

Wesel · Aug 17, 2020

Did you measure the batteries in place without removing the connections, or disconnecting them?

GSX1300R · Aug 17, 2020

From your measurements you can say that the battery with 4 volts is totally and absolutely knackered..The battery with 11.7 volts is not good at all. I think if you are going to do the job you should replace them both then you know you are good.

Matthew Ringer · Aug 17, 2020

Wesel said:
Did you measure the batteries in place without removing the connections, or disconnecting them?

I measured the 4V battery in place before removing the connections. the 11.7V was measured after being removed.

Wesel · Aug 17, 2020

It is a shame not to have measured the 2 batteries under the same conditions. That is to say the 2 connected, and then the 2 disconnected.
However having such a difference in the two batteries, could be due to a burnt fuse which prevents the charging of one of the two batteries.
The two batteries must be replaced by two exactly identical new ones. And this fuse must be checked to avoid quickly killing your new battery.
Lots of information on this by searching this forum.
Look here for the fuse:
https://vwcaliforniaclub.com/threads/fuse-on-positive-leisure-battery-terminal.19122/post-219266

MattBW · Aug 17, 2020

If only one is replaced, you end up with the newer battery getting discharged into the bad one creating more problems.

Matthew Ringer · Aug 17, 2020

Wesel said:
It is a shame not to have measured the 2 batteries under the same conditions. That is to say the 2 connected, and then the 2 disconnected.
However having such a difference in the two batteries, could be due to a burnt fuse which prevents the charging of one of the two batteries.
The two batteries must be replaced by two exactly identical new ones. And this fuse must be checked to avoid quickly killing your new battery.
Lots of information on this by searching this forum.
Look here for the fuse:
https://vwcaliforniaclub.com/threads/fuse-on-positive-leisure-battery-terminal.19122/post-219266

I have just checked and the fuse is blown on the rear battery. I will have to source a new one from somewhere.
Thanks for you help.