Question about parallel/series hookup

[AKTroy]

Hi folks:

So I currently have a 24 volt set up using eight 304 AH Eve cells. I'm using an 8S current connect BMS. I had some problems initially with my vendor from China, and long story short, I ended up with an extra four batteries, so I now have 12. I don't want to get into replacing the BMS, and would end up with an oddball voltage (36V) if I did anything in series. i.e. 2 groups of six batteries. So I'm thinking if I combined banks of two cells in parallel, and then combined those 4 mini banks of parallel into series with the other 4 batteries combined in series that would still give me two 12 volt banks I could combine to make 24. One bank would just have greater amp hour capacity. And because I'm combining two cells in parallel, I'm assuming the BMS is only going to monitor both two cells banks as one and not the individual cells. I don't think there's any way to monitor all 12 cells individually?

I could use some help with the wiring. I'm going to sketch something up here of what I think it should look like, but please correct me before I do something wrong if it looks funky. I've never done anything like this before, so thanks in advance.

 

[sunshine_eggo]

No. you will have series elements with different capacities. A 12V 304Ah battery in series with a 12V 608ah battery is a 304Ah shit show waiting to happen.

The only options you for the four spare cells is as spares or as a separate 12V battery.

 

[AKTroy]

It looks like I screwed up that wiring diagram, but sounds like two 12 volt banks with different amp hour ratings is a disaster waiting to happen.

 

[AKTroy]

Would like to somehow use these extra four batteries. Any ideas?

 

[AKTroy]

And I'm guessing it wouldn't be a disaster it's just that one one of the banks reached low voltage that would be the weakest link and shut off the whole 24 V bank, so there would be no advantage to adding the extra 4 in parallel.

 

[AKTroy]

I guess I'll just keep these extra four as spares or wire up something for home use. I don't think I want to shell out another $500 plus for four more batteries, and I'd be into several hundred pounds at that point. Of course that would give me a 1.2 KW AH setup. I can't imagine needing that much power though unless I hook up an air conditioning or something. This setup is going in my class B motorhome.

 

[42OhmsPA]

Would like to somehow use these extra four batteries. Any ideas?

Use the extra 4 cells to make a 12v battery.
Order a JBD or JK BMS from one of the ali's. Use it to power 12v led lights, small pumps, diy security system, a small fan, device charger, etc. You could even add a 12v inverter to have additional AC power if you wanted to buy a separate inverter.
Then when you decide you're ready for 48v order 4 more cells and put everything you currently have on the spare parts shelf.

 

[AKTroy]

Probably not real practical as I have a 24 volt MPP solar, so I"d end up having to use a step down regulator to charge. That's kind of what I'm doing right now with my DC to DC Victron, expect the other way>>12>24. For solar and shore power I can output 24 volts directly, as the current bank is 24 volts. This is just a case of dumb luck here ending up with four extra batteries, and trying to figure out what to do with them. Those things are about $120 a piece plus shipping from China.

 

[HRTKD]

Sell the extra four cells or keep them as spares.