Combining Dissimilar Battery ah?

Gassing and out-gassing are two different things HTH John.
Gassing is normal part of lead acid battery recharging, On SEALED Valve Regulated Lead Acid Batteries there is chemistry inside the battery that tends to encourage the gasses to re-combine into water.. Thus the battery does not "OUT-GAS".

Out gassing (Where the battery VENTS those gasses to the outside is not normal on AGM or other maintenance free types. at least not in any quantity. Some out-gassing MAY occure but it should be minimal. Which is why you can use AGM's indoors safely I might add.. The path is convoluted enough that acid fumes are normally held in even if Oxygen and Hydrogen are not. And very little H2 and O2 escape as well.

Maintenance free are not quite as good at this containment, and thus should be installed outdoors away from delicate electronics

Flooded wet cells do not even attempt to contain it and thus MUST be outdoors out of contact with delicate electronics or in a box vented to the outside

Gassing is neither normal, required, nor necessary with AGMs. It is the improper application of excessive volts that force current and causing the unwanted gassing that may cause the problems.

I use temp-comp chargers with my AGMs because I prefer unattended charging with an actual cable-on-the-post RTS sensor. If the AGM battery is getting warm enough for gassing, the RTS tells the charger to roll back the Volts. This in turn, automatically lowers the current that is heating the battery. I believe this is optimal.

I have measured AGM temps from 6F to 110F and the Volts were automatically adjusted with the RTS cables.

HTH;
John

Same type and age of battery?

KUMBAYA

With batteries, more is better.
FWIW, I have three AGM telecom batteries in parallel, one 42AH and two 26 AH. They are treated similar to other batteries I'm using, solar does most of the charging.

If all the different size batteries being charged together have to be the same chemistry, how are thousands of MHs parallel charging a lead acid chassis batt, and 2 if not 4 AGMs, 6v or 12v on the house side, or, maybe vice versa ?

I you are combining AGM 12 volt batteries. Then they will form a perfect commune, that is each will produce power according to its ability and each will "Eat" according to its need..

There are folks who will yammer "Oh the batteries won't charge equally and when the smaller battery fills up it will signal the converter to go to float bla bla bla" And with batteries in PARALLEL such as you describe.. THEY ARE NUTS, is all I can say, that very simply can not happen.

Now with batteries in series yes, that is true, but you are not doing that so disregard this paragraph.

You may mix larger and smaller batteries no problem, Under most conditions, so long as they are the same in all other factors (AGM, DEEP CYCLE, AGM STARTING, AGM Marine/Deep cycle) I would mix starting and deep cycle though.

ONE warning... This is something I learned EARLIER today but frankly I should have figured it out on my own..... But I am still learning (Today is a good day, I learned).

AGM batteries gas just like flooded wet cells, the difference is the Oxygen and Hydrogen are re-combined inside the battery.. THIS,,, releases HEAT (I mean if you do it with enough of those two the boom may well be heard some distance away kind of heat, Inside the battery it's done a few molicules (3 to be precise 2 of H2, one of O2) at a time, so there is no BOOM, but there is heat.

And if it happens fast enough, said HEAT can cause a fire.

IF the smaller battery shorts a cell, the larger batter can push current in fast enough to cause problems.. (This is true of vented wet cells too but .. It takes a whole lot longer and a whole lot bigger difference in the batteries and poor ventilation but the BOOM is bigger)

Hi,

They all need to be the same chemistry. Ideally all the same size. 33 amp-hours is pretty minimal.

Are these batteries free or extremely cheap?

greenrvgreen wrote:
I know there are a few people here with real-world experience with this, so I hope they chime in!

QUESTION: With a small 12v AGM battery (33ah), I have the choice of paralleling a slightly larger battery (35ah), a much larger battery (42 ah), or a somewhat smaller battery (28 ah). All are 12v. Is there any reason to prefer one combo over the other? The charger appears to be ~6a.

Thanks!

The different size AH capacities don't matter for paralleling otherwise similar batteries. They will discharge and recharge at the same SOCs staying even with each other.

Other differences matter when paralleling batteries such as condition (old vs new, eg) and type (AGM vs Wet eg)

6a isn't much of a charger. You want about 30% so your combo of about 60AH would like a 20a charger. Lower amp size chargers will work, but take extra long generator time and more fuel.