Ok to mix old and new cells in 6p16s bank without individual cell monitoring?

[rminkler]

Hello!

I currently have 32 calb ca180fi cells in a 2p16s configuration. I use a Batrium watchmon 4 and each cell has a blockmon. I set the bank up about 3 years ago. My charge controller is set to charge to 3.45 volts. The batrium bms balances starting at 3.4 volts. The bms turns my inverters off at 3.18 volts.

I have a 4kw solar array, 2x outback vfx3648 inverters and a flexmax 60 charge controller. I plan on setting up another 10-14kw of solar later this year.

Last week I ordered an additional 64 calb ca180fi cells. I'd like to set them up with the old cells in a 6p16s configuration. When they arrive, I plan to charge them individually with a lifepo4 charger at 20 amps to 3.65 volts, then use the same charger to top off my current 32 cells, then assemble the new 96 cell bank.

As I understand it, cell level monitoring in a parallel configuration does not actually work very well (at all?), so it seems like my best option is to remove 16 of my blockmons and set up the bms with only 16, one for each group of cells in the series.

I'm wondering about the ramifications of mixing old cells with new cells. Should I include 2 of the old cells in each 6p group? What's the likelyhood that this setup will be problematic? Would I be better off buying 32 more new cells and re-purposing the old cells?

Thank you for your perspectives!

 

[rminkler]

Anyone? Maybe the lack of feedback says my plan/assumptions are reasonable?

 

[ericfx1984]

So you can't actually monitor the individual cell voltages in parallel blocks

But here's the thing, any cells that are placed in a parallel block will automatically balance with each other...

 

[Craig]

Mixing new and old is fine as long as each pack of 6 cells has about the same amount of capacity. So yes i would make sure you mix the old and new evenly then use the batrium to make sure they all are close when fully charged and that you do not get any runners

 

[Ampster]

I have a 3P16S pack using LF280 cells. Previously my pack was Nissan Leaf Modules of various vintage and capacity. it was 7 modules in parallel which is actually 14 cells in parallel because the modules are 2P2S. Parallel cells are common in some EVs with 70 cells or more in parallel.

 

[rminkler]

Thanks so much! It’s good to know I’m not way off the mark.

 

[cinergi]

It will take a lot longer for the BMS to balance when full and it's much more likely you'll go out of balance. But as long as your charging voltages aren't aggressive and your BMS is up to the task, you'll probably be OK. Keep an eye on your voltage differences when full; you may need to get an active balancer.