Lots of small packs with individual BMS's working together

[qtturner1]

Hi All,

I'm a bit perplexed and am looking for advice. I have come across a handful of Li-Ion packs with their own BMS's, each is 4s3p.

All are the same age and capacity. I am thinking of putting a bunch (50ish) of them in parallel one with another, to make a large battery bank out of the individual packs. So the total will be 14.4 V with a large capacity. The seller says that this is just fine.

Here is where I'm confused. What happens if one cell's BMS trips for whatever reason. What will happen? What if the BMS opens the individual cell back up after the other cells have undergone a voltage change, will that cause a lot of current to flow back in/out of the cell re-tripping the BMS, or causing cell damage?

For example, cells are discharging and cell A BMS trips. Cells B-Z continue to discharge. Cell A BMS reactives and is no longer at the same state of charge as the others.

Is this a concern, or am I fabricating an unlikely scenario? Or am I misunderstanding how it may work?

I like the redundancy of multiple BMS's and obviously not having to purchase a larger BMS. However I don't want to cause a problem down the road. Am I better off stripping the BMS's off the cells and putting one large BMS in?

Thanks!!

 

[snoobler]

Hi All,

I'm a bit perplexed and am looking for advice. I have come across a handful of Li-Ion packs with their own BMS's, each is 4s3p.

Welcome to the forum.

Unfortunately 4S Li-ion of 3.7V nominal cells is not horribly useful for 12V. They can operate from 12-16.8V, and many inverters won't tolerate voltages over 15-16V, so much capacity is lost.

All are the same age and capacity. I am thinking of putting a bunch (50ish) of them in parallel one with another, to make a large battery bank out of the individual packs. So the total will be 14.4 V with a large capacity. The seller says that this is just fine.

The seller wants to sell you cells.

Sounds like a lot of complexity and work with relatively poor payoff vs. just getting larger cells.

Here is where I'm confused. What happens if one cell's BMS trips for whatever reason. What will happen?

That "battery" is taken out of the circuit.

What if the BMS opens the individual cell back up after the other cells have undergone a voltage change, will that cause a lot of current to flow back in/out of the cell re-tripping the BMS, or causing cell damage?

Very possibly.

For example, cells are discharging and cell A BMS trips. Cells B-Z continue to discharge. Cell A BMS reactives and is no longer at the same state of charge as the others.

Is this a concern, or am I fabricating an unlikely scenario? Or am I misunderstanding how it may work?

With that many used cells, the scenario is reasonably likely, and you are right to be concerned.

 

[qtturner1]

Makes sense, thank you! Something just didn't seem right and sounded risky. I'll forgo this plan and go for something more traditional.