Multiple Batteries in Parallel

Not sure what would be right with those two. Normally you want a sustained draw (not a flash draw like starting the car) to be no more than about 25% of AH capacity. With these you could perhaps go by RC or CCA if no AH spec, to get the proportion each will do.

So say the little one is half the other so it gets 1/3 of the action. Now check the little one's specs and see if that 1/3 of whatever the action is, is more than 25% draw.

BFL13 wrote:

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

I missed whether they are starting batteries or house batteries. IMO it makes a difference whether you can do this with the high draw of a starting battery.

They are 2 types. One is a utility battery for a quad/something like that, but not that small. The other is a marine starting battery.

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

I missed whether they are starting batteries or house batteries. IMO it makes a difference whether you can do this with the high draw of a starting battery.

Thanks BFL13

The internal resistance of the battery increases as it discharges creating a regulating effect to keep them close to balanced. Likewise the applied voltage should be very close if the connections are adequate and further regulate the multiple batteries that are mismatched on capacity.

Not perfect but very close.

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?

Yes, it is not true. At the normal RV discharge and charging rates, the two batteries have plenty of time to stay even in SOC wrt the "combined voltage". My own battery bank is a pair of 100s for 200 in parallel with a single 250 (AGMs) I have checked for that.

Where you get uneven is with say three batteries in parallel where you have the load/discharge wires on the outside batteries so the middle battery is not worked as hard.

Or with both wires on one of a pair so the second is "downstream" and not worked as hard. Or if one battery is in poor condition.

I don't know, but perhaps it could get uneven if the discharge or recharge rate was crazy high, so the smaller battery could not keep up ( the chemical reaction needs enough time)

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

They seem to be saying that the smaller capacity battery will pull more than half the current of the other, and it won't taper the current draw as it reaches a higher SOC. I believe the answer depends of the construction and condition of each individual battery. I don't claim to be a battery expert, but I believe the matter-of-fact expert answer shows that it didn't come from an expert.

wa8yxm wrote:

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.

Where did you get this?

Link? I don't buy it.

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?