Hi Guys,

A friend of a friend asked me for some advice to replace their old AGM batteries on a 24V PV system.

I found it interesting how the installer wired the battery bank. Attached a picture of the setup.

There are 6 x FIAMM 12FLB350 cells @ 95Ah ea. 5 of them are wired in parallel and the parallel circuit wired in series with a single cell.

The obvious reason to do this is to increase the Ah rating of the system, maintaining 24V. Is this something generally done?

In my mind that 6th cell is going to be overworked and will last much shorter than the rest of the bank.

Comment / insight appreciate please.

 

Really not a good idea. It should be 2s3p connection or 3ps2.

 

The installer hasn't a clue what he's doing and should be avoided.

Not really gaining anything with that setup. 24v yes but no gain in Ah seeing that the 6th cell is the weak link and will overcharge and overdischarge, dropping the voltage on that battery up to where the inverter will switch off. Not a very bright idea....

I'd actually go back to the installer and insist that he replaces the batteries. At best only 1/3 of the capacity of the batteries potential will have been available and I bet it's the single battery in series that has had it.

If no joy from the installer then I would wire the batteries up as 2P2S and see if they are ok. You'd have one battery out of the 5 parallel not used and the 1 series battery dumped. That would give twice the available capacity that he previously had been using!!

Edited July 13, 20224 yr by Tinbum

Well, after 5 years of service that wiring configuration lasted all of 20 minutes when grid fails.

Went there last night and re-connected the cells in a 3Sx3S parallel configuration after testing each cell's voltage. As expected, the single cell was lower than the others but still alive. Cable lugs not crimped. Insulation tape wound around battery connections instead of sealed heat shrink. Fuses bypassed in fuse box.

Load shedding started shortly after I finished and the system switched over to battery power nicely. It provided the house power throughout the 2 hours of load shedding. Not sure how much battery power was left when grid returned - but it seems to be working far better than before.

Will have to go back some time to fix those cable lugs.

Some people....