Dual battery problem

[dwh]

Coffee or no, you saved me a spit load of typing!

:wings:

It's allin de tumbs.

 

[dwh]

Keeping in mind, that apes show their teeth when they are pissed off and about to bite something...

 

[DaveInDenver]

Also critical if they are in series all the time.

(Ya, I know...picky picky. )

The principle still holds. Any battery is a battery because it's a collection of cells. How they work together depends on how you've wired them up. In either case, series or parallel, the rule that if they are connected full time they need to be matched always holds.

The problem in series is during charging. If you connect cells with different characteristics in series you will overcharge of the weakest one. In series under load the stronger cells are not asked to work any harder than the weakest one. If you could charge each 6V battery independently in a 12V connected series then you could mismatch them. This isn't unheard of with 24V and 48V systems, where there might be a 12V charger on each battery in the series string. This reduces the need to match batteries age just as long as they are all close in A-hr you only give up a little capacity under load without damaging one by chronic over- or undercharge.

It's the opposite problem of two 12V batteries in parallel. Differing batteries can be charged in parallel. Each battery will only consume the current it needs at the voltage you present. It's under load where the stronger battery is required to work harder than the weaker one.

We say you can parallel batteries under charge as long as they are not parallel under load but this does have it's limits. It's possible to overcharge a battery by putting too high a charging voltage on it to meet another type's requirements. You often mention gel type and you cannot parallel charge those with an AGM without damaging the gel or undercharging the AGM.

The rule, therefore, is parallel means voltage (generally AGM or flooded are similar enough) needs to match while series means current or capacity (generally A-hr or CCA) needs to match.

This series charging issue is an acute problem with lithium cells within batteries, which is why they provide the leads to monitor cells within the battery. Since the margin between fully charged and breakdown is small for lithium chemistries you want to equally charge and balance the cells to prevent your Samsung phone from catching fire.