Multiple Battery Capacities - What to do?

[jjaj823]

Hi all!

I find myself with a fairly nice problem for a change. I recently purchased 32 - 320 Ah LifePo4 battery cells. Which gives me 24,576 Wh of usable capacity, once adjusted for battery longevity.

My current battery bank is 32 - 120 Ah cells, for a capacity of 10,368 Wh.

IIRC, you shouldn't mix capacities in your system...which means I have to figure out what to do with the current 10k wh of capacity I have now.

My current setup is split phase 240 via a pair of 3k inverters. The 240v is for my dual zone mini split. Otherwise, I don't have any 240v loads, so I'm just using it to split up my kitchen stuff so I can run a couple of kitchen gizmos (conduction burner, microwave, oven, etc.) at the same time.

Anyone have any ideas on how I can set this up so that I don't "waste" the batteries I already have? They're still relatively new with less than 300 cycles on 3 of them and under 500 on the other (I messed up the cabling, so one was getting drained faster than the others).

This is certainly a nice problem to have, so any ideas are welcome!

:Thanx:

JB

 

[JasonHurst.studio]

Hello,
Are you running any sort of BMS on tour cells? Any sort of monitor or balancing?

 

[jjaj823]

Hello,
Are you running any sort of BMS on tour cells? Any sort of monitor or balancing?

Yes to both. I’m using BMSs from overkill solar. They have built in Bluetooth monitoring. I can see everything from the individual cell voltage and condition to overall state of the battery. There is also a temp sensor that will prevent charging when the cells are too cold. No onboard balancing though.

 

[JasonHurst.studio]

While this is not a best practice... You CAN parallel multiple battery banks of different capacities provided that you have a BMS on each bank to protect those cells. I would highly recommend looking into a balancer or switching to a balancing BMS.
Also you need to correct your wiring so that each of your current batteries is seeing the as close to the same resistance as possible and will cycle as close together as possible. One more thing, when connecting your new batteries to your existing battery make damn sure that the voltage is as close as you can get it. You will see a surge of current flow from one the other to being them in to parallel balance. The voltage differential will dictate the size of this current... and it can be a LARGE surge so be careful.
I have not covered this topic yet on my channel but here is come links to the Off grid garage, Andy is very knowledgeable and test all his hypothesis.
It is doable but this will just add another level of complexity to your setup and another possible point of failure so you just have to plan for it, protect against it and just be ready.

https://youtu.be/fQz7vZEYLfY?si=cPuRBrLVy4GcO_1o
https://youtu.be/bhCSccNsQOw?si=m-DT2KmkJfs9ECHG
https://youtu.be/iV1O4K4-F2A?si=FeIBzaT6taGWnQN9

 

[jjaj823]

While this is not a best practice... You CAN parallel multiple battery banks of different capacities provided that you have a BMS on each bank to protect those cells.

I know you can, I just always thought it was a really really BAD idea to do so. I've never done it professionally (as a boat electrician), so I never really thought about if you even COULD.

Seems like you'd run into a situation where the lower AH rated batteries would drain first, which could fool the controller. Also, amp draw would be limited to the upper limit of the lower Ah rated battery bank as well (so, if the smaller rated bank had a max draw of 100A, and the larger capacity a max draw of 200 you'd only be able to pull a max of 100A).

I would highly recommend looking into a balancer or switching to a balancing BMS.

I was wrong...the Overkill Solar BMS has balancing, so I have that.

Also you need to correct your wiring so that each of your current batteries is seeing the as close to the same resistance as possible and will cycle as close together as possible.

Yeah...I had cut one shorted when making an emergency repair on the road, and forgot about it at the conclusion of my trip.

So much for my monthly checklist...

I have not covered this topic yet on my channel but here is come links to the Off grid garage...

I'll check out the links. Thanks! and yeah, I've seen a few things on that channel before.

I think what I might do is take the lower capacity batteries and turn them into a "critical systems" bank...my fridge is 12v and so is most of my lighting and heaters, etc. This way, they are separated and able to be used no matter what.

Overkill? Probably, but cool anyway.

Appreciate the comments!

JB