Battery Tender Question

[MikeN]

Help. It is time to winterize my 1973.5 911T and I would like to hook up a battery tender. However, I just opened the hood and noticed that I have two batteries, assumedly hooked up in parallel. Does anyone know how to hook up a battery tender to this sort of hookup, or do I need two battery tenders?
Thanks.
Mike

[MConn]

I use 1 battery tender (Northern tool version) to charge the 2 batteries in the boat during the winter. Never had any problems.

[Plays with cars]

I just use one charger on my early 911. Hook it up to one battery and it will charge both through the battery cable harness; same way the alternator charges them.

This works fine unless you have one battery that isn't in as good of condition as the other. The weak battery will stop the good battery from charging properly. Ultimately I guess it's a self correcting problem because eventually the good battery will become a weak one.

[Canada Kev]

Actually, I think that the weak battery keeping the good one from charging is false. That DOES happen in a series wired situation; the weak one gets weaker and the stronger one boils off because it's over charged.

The situation you describe is only accurate if the batteries are not under charge. Then there will be current flowing from the stronger one to the weaker. On the charger (or alternator) this won't happen.

[E Sully]

I've always used a single tender for my 2 batteries in my 1973. As syzygy wrote, they are wired in parallel. I would make sure both are in good shape though, otherwise you may damage the tender from trying to charge a bad battery.