Battery compatibility?

[allyncooper]

Planning on installing a dual battery setup per the " $50 dual battery setup" thread. I have a 42 ah Odyssey battery for starting and a few other low load items, and planning on putting in a 125 ah deep cycle battery to power a refer drawing 5.5amps. Someone posted on another site that the batteries in a dual setup need to be identical - first time I heard that. Is this true or not? TIA

 

[BobsCreek]

I think the basic view is, if they are always going to be separate (one for a fridge, one for cranking/etc") they don't need to be the same.

 

[DaveInDenver]

If they are intended to be parallel under load normally then you should consider matching type, age, condition up to the point of them being basically identical.

If they are only combined during charging or exceptional cases (e.g. self jump start) then you have flexibility in mismatched batteries. How much flexibility depends on a few things, mainly how you're charging them. If it's just using the alternator then you can even mix AGM and flooded. Alternator charging isn't ideal for AGM anyway so unless you modify your charging system to make it truly AGM-friendly you'll just be undercharging the AGM. IOW, it's not ideal but not damaging.

The problem arises in mixed batteries with shore charger capacity and solar since the proper charging profile isn't the same. In that case you can still mix potentially by using the lowest common profile but having the ability to isolate them for conditioning charges that are unique to each type would be good.

What you never want is one battery being strong enough that it's asked to stand in for the charging system. So small maintainer type chargers, solar, etc. should only see batteries that are similar in parallel if they can't hold up the voltage to the point that one battery starts sourcing (supplying) current while charging. This is irrespective of type, too. A weak battery of even the same type as a strong may cause issues.

 

[dwh]

Batteries tied into a full-time battery bank have to be as close to identical as possible. Same brand, model, size, age - hell, even the same manufacturing batch and date if you can swing it. If they aren't, one will do more work than the others and get worn out sooner. Then the next strongest will do the same. Rinse and repeat for a few years and you end up with a prematurely worn out battery bank. They live and die together.

For batteries tied only during charging, it doesn't matter. All that matters is that they require approximately the same charge voltage and that the charge source puts out approximately that same voltage.

Lead-acid batteries aren't precise electronic components. They are sloppy, goofy chemistry experiments in a plastic box. Most of the time, "close enough" is good enough.

Which is exactly what most voltage-regulated alternators do anyway.

 

[allyncooper]

Thanks for the great information. I only intend to have them tied together during charging from the vehicle alternator. I don't intend to use the second battery as a jump start either (I have a separate battery pack for a backup). Sounds like then I'm good to go on my intended use.

 

[rayra]

I'm getting 'close enough' out of my disparate pair of auto batteries, which are combined with the key on vi a 200A solenoid arrangement. My 'Aux' is a Grp 74, the Starter is a Grp 78. Both Interstate brand, manf dates a couple months apart. WHen I spot check with a handheld meter my aux is usually about .3 V higher. The Aux is daisy-chained off the positive terminal on the starter battery, rather than attached to the common connection point next to the alternator (little red box on GM / vortec / GMT800 setups).
Not sure if the internal regulator is being fooled somehow by the higher capacity Aux battery, or (more likely) that it's just a reflection of the parasitic drains I have from multiple aftermarket USB power ports with LEDs or the dashcam powered off the overhead courtesy circuit, which keeps running for 10mins after I turn the key off. Which are all on my factory wiring and thus draining the Starter battery.
I'm not driving the vehicle often enough or far / fast enough to keep the batteries topped off.
Individually I can charge them both up to 12.7-12.8, using a craftsman charger plugged into house power. But once back in the vehicle the disparity reappears.

I'll soon be adding a folding panel in a semi-permanent roof mounting and was intending to connect it to my cargo area power module / Aux circuit. But now I'm considering a way to make it switchable and leave it primarily feeding my Starter. Switchign it over to my Aux setup when I'm using that more heavily.