Mixed Ah Batteries in Van Build?

[Corbury]

I think I know the answer here. But I’d like to confirm my understanding.

My plan was to build two 12v 280Ah batteries for my van - each with 120A Overkill solar BMS or a 4S2P 12v battery bank. But I’m going middle-aged blind and I mistakenly ordered 1 set of 4 x 280Ah cells and then a few weeks later another set of 4 x 200Ah cells (when I intended to order 280Ah). I know - not my smartest move!

So i guess I can ebay the 200Ah cells and order some replacement 280Ah cells.

BUT - how bad an idea would it be to build the 2 batteries and hook them up in parallel? 1 x 280Ah and 1 x 200Ah - each with 120A Overkill solar BMS?
- They probably won’t charge or discharge evenly?
- One battery will end up wearing out before the other?
Thoughts?

thanks, C

 

[gecko]

So i guess I can ebay the 200Ah cells and order some replacement 280Ah cells.

If it's not too late you may be able to just cancel the 4 x 200Ah cell order. I recently cancelled an AliExpress order for cells and the monies were credited back to my account within four days.

 

[Steve_S]

You can build a 280AH and a 200AH pack both as 4S and set them in Parallel. 80AH difference is not huge and workable. Once you get beyond 100AH difference between packs things get kinda squirley for charge & discharge when reaching the Top during charge & bottom during discharge. The 200AH pack will obviously hit empty & full before the 280AH at which point the BMS will trigger an LVD/HVD (Lo / Hi Volt Disconnect) and unless both batteries actually hit full, the odds are that the 200AH being full while the 280 is not, it will trickle charge over to the 280, decreasing the 200AH stored capacity. I have this issue with my 175AH packs sitting next to the 280AH packs. I'm now having to rethink the 175's and will likely have to convert the two 175's into one 2p8s 350AH pack because 70AH difference is better than 105AH which is just too much.

As Gecko suggested DO TRY to cancel the order and place a replacement order. It's worth reaching out to the vendor, they really would prefer a ha[ppier customer when possible and if they haven't shipped it out the door yet, it is quite possible.

 

[sduser]

Unless you can control the amount of current flow to each bank proportional to its capacity (or better control the power proportional to its energy), you will be limited by the smallest bank. So in your case you will effectively have two 200Ah banks. One will see the full SoC range, the other partial.

There may be ways to manage the mismatch somewhat, but the easiest would be to cancel the order or sell the cells and get matched capacity.

 

[Corbury]

If it's not too late you may be able to just cancel the 4 x 200Ah cell order. I recently cancelled an AliExpress order for cells and the monies were credited back to my account within four days.

I just received them - sadly. But I can probably sell them.

 

[Corbury]

You can build a 280AH and a 200AH pack both as 4S and set them in Parallel. 80AH difference is not huge and workable. Once you get beyond 100AH difference between packs things get kinda squirley for charge & discharge when reaching the Top during charge & bottom during discharge. The 200AH pack will obviously hit empty & full before the 280AH at which point the BMS will trigger an LVD/HVD (Lo / Hi Volt Disconnect) and unless both batteries actually hit full, the odds are that the 200AH being full while the 280 is not, it will trickle charge over to the 280, decreasing the 200AH stored capacity. I have this issue with my 175AH packs sitting next to the 280AH packs. I'm now having to rethink the 175's and will likely have to convert the two 175's into one 2p8s 350AH pack because 70AH difference is better than 105AH which is just too much.

As Gecko suggested DO TRY to cancel the order and place a replacement order. It's worth reaching out to the vendor, they really would prefer a ha[ppier customer when possible and if they haven't shipped it out the door yet, it is quite possible.

I will reach out to them. It’s clearly my mistake. But I might try to just work with it. You gave me some hope here. Thanks!

 

[PF075]

Could you stick an isolator in between one or both of the banks to stop the current from flowing between them? Then separate out any charging on to different controllers?

I considered doing something similar on a Lead Acid / LiFePO4 experiment, but was advised it wasn't required.

 

[Dhopewell]

I don’t see a problem here. Just run 2 independent bms and when the 200ah cuts out the 280ah will still be on line. I have parallel banks (same size) and run each w it’s own bms set to slightly different voltages and there hasn’t been an issue. Seems like a good analogy to this.

It can be handy because you have an early warning when the 200ah shuts down, esp on discharge

 

[Corbury]

Well now i have 2x280 + 1x 200 - i may just do that. As you say - bms will cut off the smaller bank when it’s done - the others will continue to run. Thx! C