Which battery bank is better?

[Ethan]

I have 2 Trojan T105 batteries (flooded, 6v, 225ah, in series) and 2 EnerSys PowerSafe 12V170FS batteries (AGM, 12v, 170ah, in parallel). They will create two separate battery banks, one will be used for 12v and one will be used with a 2000-3000 watt inverter.

I imagine the 12v system will see more use than the inverter, but the inverter will draw lots of juice. My thought is to use the 340ah AGM bank for the 12v and the 225ah flooded bank for the inverter. Is there any reason to do the opposite? Flooded 6v in series vs. AGM 12v in parallel.

[turf]

charging time is your biggest difference that i see.

i'd keep it like it you say. that way i could run a 120v battery charger off the inverter to charge the agms.

the lead acid batteries charge slowly, the agm's charge pretty fast.

[CaptainInsaneo]

Look at the $/Ah at 12 v AND the total space required AND the over all cost. Make your decision from there. At the end of the day there won't be that much difference between the two. Both are lead acid technology.

[family wagon]

As I recall, AGM can deliver high current pretty well too. A 3 kW load on the inverter is about 290 amp draw on the battery (figuring 85% efficiency in the inverter). That's 1.28 C on the T105 bank vs 0.85 C on the AGM bank (*see footnote). I think the AGM would be better able to handle the peak draw from the inverter. However, again just winging it on memory.. it seems like AGM should not be allowed to reach the same depth of discharge that the T105 can tolerate. Whether the larger AH versus less discharge tolerance nets roughly equivalent available power, I don't know.

3 kW is a fair amount of power. It probably makes sense to have headroom in the inverter and wiring to support it, but it may be uncommon to actually draw that much for any length of time in practice. The AGM superiority at providing that much power may be irrelevant.

Why two separate 12 volt house battery banks..? I mean, I get that this is what you already have on hand and that it's not ideal to put that set of dissimilar batteries into a combined bank, but it kind of looks like a scenario where you'll create headaches for yourself by having separate banks with different battery types. If it's within the realm of possibility, maybe consider trading one pair so you can have a single bank of similar batteries.

footnote: "C" is battery terminology; it represents the AH capacity of the battery bank. In general, as the discharge rate in C goes down, the total power the battery can provide, seen as the length of time it'll power that load, goes up.

[Albatross]

I would not use either battery bank with an inverter of that size--I would go much larger if your going to run an inverter of that size. A 3KW inverter puts out enough current to power a 15-Amp 120-Volt circuit, and your 12-Volt current draw is going to be more than ten-times that. Either battery bank */should/* be able to handle the current draw on the 12V line, but your best bet would be to put them all together if you're going to be utilizing that kind of power. It is said that you shouldn't mix battery types, but the normal flooded lead-acid and AGM's go together better than any other kind; you just will probably want a little bit of space and some other sort of battery isolator device between the two banks.

The Trojan T-105's would be a good battery to use with a solar array system, if your intending to do something like that.