Multiple Batteries in Parallel

The 250 8D took 63 AH to refill, where 68 plus some for heat was expected. This shows further that this test was "problematic" in exactness.

It did show that the smaller battery does not go flat, and that they stay "fairly even" to "good enough" standards (IMO, or at least in my case for how I do things)

I believe the issue with my test is mostly heat loss while recharging. Different in the big 8D from the little 27 and where they are also different brands and construction. Also Puekert is in there somewhere. Wiring too.

Members are invited to run their own little tests for this to see what they get.

There is mixing dis-similar batteries and then there is mixing same type batteries but of different AH sizes. You can do both and "get away with it" if you take care to recharge them to full by individual type.

You can bank them all for maximum Puekert (two 100AHs in parallel is worth more than two, one at a time) while camping. Big thing is separate them after and do their recharges to each type's specs. Then you can bank them again and Float them all at once.

time2roll here, says each will take what it needs from the float charger and keep up. I have no measurements of that, but IMO he knows his stuff, so it should be ok. Assumes each needs the same Float voltage in its specs.

In my case with two different brand /type AGMs, they have different charging specs--14.8ish for the 100s and 14.4ish for the 250. but they have the same Float spec of about 13.8.

So that's easy, I camp with them banked as 450, get home and separate them to recharge each to full, then banks again to float. A wrinkle with these AGMs is that the recharge must be at over 20% charging rate. So even if they all had say 14.8v charging spec, I need enough amps to do the 20%, which would be a 90 amp charger. Only have a 75 amper.

So I split the bank for two reasons: the 75 on the 250 for over 20% and the 14.4 vs 14.8 thing.

Point being you have to do some work. This is not a fire and forget deal.

In the marine world it is very common to combine battery banks & then control them through switching. The key is having same type & condition.

For example, in my boat I had 5 gel cell group 31 batterys. They were arranged into 3 battery banks. 2 banks of 2 in parallel & 1 single. Through the switches I could use any bank or combination of banks that I pleased. I also had a 3 bank charger selectable for wet cell, AGM or gel cell.

3 of the batterys were bought together. The other 2 were bought a couple years later.

Boon Docker wrote:
BFL13
There was 182 ah left in the 250 and 65 ah left in the 100 after your test.
So in theory if you continued draining them there would be 117 ah left in the 250 and 0 ah left in the 100. Yes?

No, because the current is not shared equally when discharging or charging. The voltage of the two batteries is equal (it has to be, they're connected in parallel, excepting for any tiny difference caused by bad balancing of the wiring), and the current varies however it will to maintain that.

If there are 0 Ah left in the smaller battery, it's voltage would be...umm, say maybe 10V, depending how you define capacity. The other would be at maybe 12.0V or something like that if it had around half its capacity left.

It also works the other way around, with loads rather than batteries. If you plug a lamp with a 100W bulb and a lamp with a 25W bulb into the same outlet, the 25W bulb does not consume as much current as the 100W bulb and burn out more quickly for that reason.

Boon Docker wrote:
BKF13
There was 182 ah left in the 250 and 65 ah left in the 100 after your test.
So in theory if you continued draining them there would be 117 ah left in the 250 and 0 ah left in the 100. Yes?

65/73 SOC = 8% diff in SOC (if it was that really) if maintained going down would be to 0% vs 8% I think. ??? Can't see that 117 left in the 250 which would be 47% SOC. Got me confused! :)

As the "experts" mentioned, the inverter low voltage shut down would occur long before things got that bad, so in my case the 100 would still be well up in SOC under the loaded voltage (which was about 11.9 when I stopped the 91 amp draw.)

On stopping the 91 amp draw, voltage bounces back to reflect the SOC in an hour or so after it settles. You can wait a whole day if you are fussy but it will be nearly the same.

Don't try to be too exact with my test numbers since it was a rough test, and an unknow margin of error from heat loss on recharge and my crazy wiring set-up.

BFL13
There was 182 ah left in the 250 and 65 ah left in the 100 after your test.
So in theory if you continued draining them there would be 117 ah left in the 250 and 0 ah left in the 100. Yes?

Thanks for the kind comment! Except now I will have to be really careful not to screw up.

BFL13 runs well thought out test. He keeps running test until his results make sense. I would believe his results over most of the experts on this forum.

Boon Docker wrote:
Here is a quote from "The Battery Experts"

The 100 AH battery will become fully charged long before the larger one. The combined voltage will rise, but by the time the controller turns off the charging sources, the 100 AH battery will be overcharged. Meanwhile, the 200 AH battery will not get fully charged. When the bank is being discharged, the 100 AH battery will go flat and its voltage will fall well before the 200 AH battery. The inverter will eventually cut out but not before the 100 AH battery is excessively drained.

So is the above not true?

I ran a test to see how it went with one of my 100AH 27 batts and my 250AH 8D. (all AGMs) The wiring was not properly balanced in my zoo of a set up and with one 100 missing, but just to get a general idea.

I ran the fully charged 350AH bank down with a 91 amp draw until the Trimetric AH counter said it was 103AH down.

So IMO the 100's share of that would be 100/350 x 103 = 29.43AH which would be 70% SOC approx.

I could not measure the SOC of each battery at that point, being AGMs and so no hydrometer.

I then recharged the 100 by itself and it appeared to take 35AH including some for heat despite the Tri's fixed fudge factor.

Using that 35 anyway, the 100 was down to 65% and the 250 was down 68 AH out of the 103, or at 73% of 250.

65/73 vs 70/70 may not mean much with the margin of error in this test. I believe from previous tests with better wiring balance it would have been more even, but whatever.

The 100 did not go flat! :)

How long will the lights be left on? Car batteries and you leave the car lights on and they get dim, you can recover the battery from that, but if you leave them on till the battery is dead maybe not.

OP says "without damaging them" so it is all about not leaving the lights on "too long". How long is "too long"?

Can you leave them on longer with the lawn mower battery in the mix? seems to be the real question.

If the lights get dim, can the bigger battery recover, but the little one can't?

If the lights are not to be left on very long at night and the battery will be recharged next day, do you even need a bunch of batteries or just the one bigger one?

Is relative CCA "size" an indicator of recovery ability whether lawn mower battery or car battery? Are they built the same to be able to "take a licking"?