Combine 2x 8S 24v Systems

[WillAK]

I have 2x 8S 24v 280ah LifePO4 battery packs which I want to combine to create 1x 24v pack.

I have 2 separate BMS which are 24v 8S.

What's the best way to connect these together? I don't have a 16S BMS so should I run two separate BMS 8S and connect in parallel?

Also, would I need to top balance 16 cells and after, separate into 2P8S?

Thanks.

 

[sunshine_eggo]

A 16S bms is for 48V. Two cells in parallel are a single cell.

Connect them in parallel as two separate 8S batteries each with their own BMS. Wait until they are about 0.2V apart before making the connection.

Top balancing is only needed if the individual 8S batteries are in need of top balance.

 

[WillAK]

A 16S bms is for 48V. Two cells in parallel are a single cell.

Yes of course you're right.
But what about if I put 2 cells in parallel and just use one BMS?

 

[sunshine_eggo]

Yes of course you're right.
But what about if I put 2 cells in parallel and just use one BMS?

1. You lose single cell monitoring.
2. You cut your current to the limit of ONE BMS rather than most of the two.
3. You lose redundancy - if one cell or BMS has a problem, you simply take that battery out of service and continue on the other. If you run 2P8S and there's a problem, the whole shebang is down while you troubleshoot which of the 2P cells are bad.

There is one good reason to parallel cells: when you have absolutely no other practical choice. I don't see that you're there.

 

[740GLE]

What BMS do you have?

If you already have 2 BMS there’s no real reason to only use 1.

People have had success using 2P8S and one BMS but it is kind of frowned upon.

I personally had a 2p4s before converting it to a 8s, and I had no issues with it as a 12v system.

Granted it was for a very very low usage, maybe max current of 20-30a or so.

When I do add another 8S to my current pack I’ll be using another complete BMs build wired into bus bars.

 

[WillAK]

I have 2x JK 200a BMS which ive been using for a while. I will just connect the 2 cells in parallel and continue.

Does anyone have a wiring diagram for 16S in 8S2P with 2x BMS?

 

[WillAK]

1. You lose single cell monitoring.
2. You cut your current to the limit of ONE BMS rather than most of the two.
3. You lose redundancy - if one cell or BMS has a problem, you simply take that battery out of service and continue on the other. If you run 2P8S and there's a problem, the whole shebang is down while you troubleshoot which of the 2P cells are bad.

There is one good reason to parallel cells: when you have absolutely no other practical choice. I don't see that you're there.

Well that's me convinced. Thanks very much!

 

[sunshine_eggo]

I have 2x JK 200a BMS which ive been using for a while. I will just connect the 2 cells in parallel and continue.

Huh? I hope you meant batteries?

Does anyone have a wiring diagram for 16S in 8S2P with 2x BMS?

Imagine you have two 12V batteries, and you want to parallel them. You simply connect (-) to (-) and (+) to (+). That's all you need to do here.

When you connect the parallel batteries to your system, you connect the system to the (-) of one battery and the (+) of the OTHER battery.

See link #6 in my sig, section 3. Several options on paralleling batteries.

 

[WillAK]

Yes I do mean batteries.

I don't have a problem putting them in parallel I'm just wondering how I would wire up the two BMS. Which would then be the main negative if I have two of them?

I guess Ill just shunt them both together the pos and the neg's.

 

[740GLE]

I don’t believe there will be an “main negative”, they will all be equal.

Each BMS will have its own negative off the BMS terminal, and each positive will be off the most positive cell terminal.

 

[sunshine_eggo]

Yes I do mean batteries.

I don't have a problem putting them in parallel I'm just wondering how I would wire up the two BMS. Which would then be the main negative if I have two of them?

I guess Ill just shunt them both together the pos and the neg's.

There will be no BMS interaction or wiring. You treat them as completely separate batteries connecting only at the (-) of each battery and at the (+) of each battery.

Please review link #6.