Boxerworks.com

Hi guys,

I've got one battery tender. And two batteries to tend, one is my BMW battery , lead, acid type and then there's the battery form my lawn tractor. Both will be out of use till springtime, let's say april if I'm lucky. Earlier years they've typically got every onther month tended and on their own, favorising one of course.
Now, would it be a good idea to connect them and cahrge them bth at the same time. They're both 12v but I guess a bit different amps.

Thanks!

Unless you have a tender with two outlets designed to charge two batteries at the same time, it's won't work unless the batteries have similar characteristics and are in a similar condition (Very unlikely). The 'weaker' battery will rob the charge from the stronger amd, at best, you'll end up with one battery less charged than it started. In extreme condition, you may ruin one or both batteries.

Rob

Thank you Rob! i'll get another tender.

Just switch back and forth every week.

Rob is right............. as usual.

I agree it would not be sensible to attempt to charge two different batteries in parallel from variable states of discharge.

On the other hand what if it's a regulated voltage float charger we are talking about and both batteries are of the same chemistry e.g. flooded wet cell and you start with both of them fully charged to the same rested open circuit voltage and neither are faulty.

How would there be a problem ?

You're generally never supposed to try to charge two batteries in parallel that are not identical in design and condition. I don't know what the results are likely to be if they are similar (rather than identical), or how far off identical becomes a problem. When they are significantly different, i.e. a gel and a standard wet cell, I've seen them destroy the gel.

From what I can find, batteries of identical design and condition, but different capacity, actually can be parallel charged together. This was news to me.

Major Softie wrote: From what I can find, batteries of identical design and condition, but different capacity, actually can be parallel charged together. This was news to me.

Agreed... but... The chances of any two random batteries being in the same condition (never mind design) are very slim. IMHO not woth taking the chance.

Of course you can (as has been said) simply swap the batteries at regular intervals but if you want a 'hands off' approach, 2 tenders is the way to go.

Rob