Multiple Batteries in Parallel

OP here. I planned on charging them separate and disconnected from the trailer.

I vote with BFL.

Boon Docker wrote:
I think if you parallel a 100 ah and a 200 ah battery together. The 100 ah battery will be fully charged before the 200 ah battery is fully charged.
The same would happen when discharging. The 100 ah will discharge fully before the 200 ah battery will.

Not so. If they are properly balanced for load and charging, they "equalize" with each other and maintain the same SOC throughout, at the same voltages. BTDT. They get down to 50% together at the same time and recharge together to full at the same time.

I think if you parallel a 100 ah and a 200 ah battery together. The 100 ah battery will be fully charged before the 200 ah battery is fully charged.
The same would happen when discharging. The 100 ah will discharge fully before the 200 ah battery will.

dons2346 wrote:
Your battery pack will be as good as your smallest battery

FALSE

FactL the batteries will work it out in a perfect commune. each according to its ability (Whebn delivering power) each according to it's need.

I will use a 2 battery system as an example a 130 amp hour Group 31 with a 70 amp hour Group 24 in parallel

IF you are drawing say 10 amps. the smaller battery will be putting out 3.5 amps and the larger one 6.5 (Total 10)

If you are charging at 10 amps.. The smaller one gets 3.5 and the larger one 6.5)

No matter the courrent that 35/65 ratio will apply

Once they have cycled ONE TIME they will be in perfect lock step state of charge wise. The instant one is "FUll up" so is the other.

NOW. THe danger....

With batteries in SERIES (Does not apply here) a shorted cell in one can damage the other (over charge)

With batteries in PARALLEL and for this it matters not if they are the same size or different sizes with one minor exeception. A battery with a shorted cell may "Explode" (not that big a deal more of a pop than a boom)

The danger is greatest if the smallest battery has the shorted cell.

But it is there even if the batteries are twins.

Quote
Don't know about CCAs! That could be a different story for how much each will do towards cranking.
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I don't know how paralleling CC amps could be any different than paralleling AHs. Just add them !!

I use four Gp 27 in series for 48 volts yet store them in parallel with a fifth battery for a trolling motor on a battery tender with a timer for 7 months of the year. Their ages are 2008, 2009, 2012, 2015, 2016. They are diverse and accepting of all and care deeply for each other.

I expect the 2008 to be finished next spring so I'll be introducing a gp 24 to the group.

swimmer_spe wrote:

BFL13 wrote:
Yes, but the main thing is they be in similar "condition", which means past usage and care of each one. Not good to bank a battery in poor condition with a good one, or you soon have two poor ones.

You can parallel different "size" batteries of similar condition, no problem. Eg an 80 AH with a 100 AH to get a 180 AH bank. They will draw down together (if balanced) so that at 50% one will be 40AH and the other 50 AH. They will recharge together and get to "full" at the same time also.

It is hard to balance several batteries in parallel. One way is to group them in smaller banks and then bank two groups. More wires but so what? With four batts you can make two pairs and then parallel the pairs. Then put your positive load/charge wire on one pair and the neg on the other pair.

That is what I was looking for. I have 2 batteries that are roughly in the same condition. However, their CCAs are about 100 and 200.

Don't know about CCAs! That could be a different story for how much each will do towards cranking. I was talking about Amp Hours and typical RV draws on house batteries, not for paralleling starting batteries.

Somebody who knows about that will have to jump in.

BFL13 wrote:
Yes, but the main thing is they be in similar "condition", which means past usage and care of each one. Not good to bank a battery in poor condition with a good one, or you soon have two poor ones.

You can parallel different "size" batteries of similar condition, no problem. Eg an 80 AH with a 100 AH to get a 180 AH bank. They will draw down together (if balanced) so that at 50% one will be 40AH and the other 50 AH. They will recharge together and get to "full" at the same time also.

It is hard to balance several batteries in parallel. One way is to group them in smaller banks and then bank two groups. More wires but so what? With four batts you can make two pairs and then parallel the pairs. Then put your positive load/charge wire on one pair and the neg on the other pair.

That is what I was looking for. I have 2 batteries that are roughly in the same condition. However, their CCAs are about 100 and 200.

Yes that will work fine while in-use assuming they are all in good shape.

If you store them without a float charge I recommend disconnecting the parallel connection after a full charge.