Connecting backup batteries of differing capacity's

**Andrew Leigh** · 2019/11/14, 02:40 PM

Hi,

Is there any efficient / effective way to use batteries on different Ah ratings in the same circuit.

I have 6 ea 12V 60Ah batteries and 4 each 12V 100Ah batteries. Is there an effective method to connect these to make them usable with an inverter, be it 12, 24, 36 or 48V.

The first problem I can see is the difference in the recommended / ideal charging and float currents for the two different batteries.

Cheers

**bfreesani** · 2019/11/14, 03:23 PM

Any batteries you connect in series, keep the same Ah together. You can take a fair chance stacking them a parallel.

So you can use all your batteries in a 12 or 24v system, but you will have some left over, you can send to me, if you have a 36 or 48 system.

The experts will be along soon and shoot my Hillbilly approach down, it has worked for me before though.

**stephenplumb** · 2019/11/15, 06:17 PM

I have 150Ah 4x12V ->48V connected in parallel with 4 x 200Ah ->48V. Logic tells me that whichever bank pulls a lower voltage first will transfer the load to the higher voltage (200Ah in my case as they are newer) until that falls to the level of the lower bank and so it will go down until both banks are empty.

I have not had any problems with my setup to date, but Fluffy will have more experience and probably wiser input.

**Fluffy** · 2019/11/15, 07:16 PM

Originally Posted by bfreesani

Any batteries you connect in series, keep the same Ah together. You can take a fair chance stacking them a parallel.

So you can use all your batteries in a 12 or 24v system, but you will have some left over, you can send to me, if you have a 36 or 48 system.

The experts will be along soon and shoot my Hillbilly approach down, it has worked for me before though.

Spot on actually.

If any batteries are in series they MUST be the same AH or very similar.

Batteries in parallel, whether 12/24 or 48V series strings, can be a mix of AH. Batteries in parallel actually share quite nicely, both when charging and discharging.

Just one thing - make sure all batteries are FULLY charged before connecting parallel strings. If the batteries ever get out of sync, separate them and charge each string independently, then put them back in parallel.

**Andrew Leigh** · 2019/11/17, 11:57 AM

Hi guys,

So which of the following is more advisable?

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Cheers

**Prof** · 2019/11/17, 12:59 PM

Originally Posted by Andrew Leigh

Hi guys,

So which of the following is more advisable?

Cheers

Neither

**Andrew Leigh** · 2019/11/17, 01:08 PM

Thanks.

**Dave Faddel** · 2019/11/18, 07:37 PM

Originally Posted by bfreesani

Any batteries you connect in series, keep the same Ah together. You can take a fair chance stacking them a parallel.

So you can use all your batteries in a 12 or 24v system, but you will have some left over, you can send to me, if you have a 36 or 48 system.

The experts will be along soon and shoot my Hillbilly approach down, it has worked for me before though.

shoot my Hillbilly approach down .... spot on actually, why shoot it down ?

The only thing wrong is you want the left over batteries, would do much better in my system

On a serious note though, try not to put more than three sets in parallel, you start picking up problems with charging. The more series you have the better. The series string containing the lowest A/H battery will determine your A/H rating for that bank.