Okay to split battery with single BMS?

[alexcl]

Due to space constraints I need to split my battery cells into two different compartments. I have 24 cells in a 3p8s configuration with the #7-8 cell blocks in a different place than #1-6. They are connected via ~10ft of 4/0 awg copper cable. It's resistance (as measured by charging at 50A) is ~0.5 milliohm which is consistent with online calculators. Is this a problem?

I am getting some pretty wild voltage fluctuations (0.02-0.03V) on my #7 cell block as measured by my BMS even though the voltage is consistent when measured with a multimeter. I posted about the bms but am curious if splitting a battery like this is just bad form.

 

[LakeHouse]

Due to space constraints I need to split my battery cells into two different compartments. I have 24 cells in a 3p8s configuration with the #7-8 cell blocks in a different place than #1-6. They are connected via ~10ft of 4/0 awg copper cable. It's resistance (as measured by charging at 50A) is ~0.5 milliohm which is consistent with online calculators. Is this a problem?

I am getting some pretty wild voltage fluctuations (0.02-0.03V) on my #7 cell block as measured by my BMS even though the voltage is consistent when measured with a multimeter. I posted about the bms but am curious if splitting a battery like this is just bad form.

.5 milliohm at 50 Amps is .025V, so the voltage drop makes sense.
Why not split into two batteries: a 2p8s and a 1p8s and avoid having ten feet of balance cable?

 

[alexcl]

.5 milliohm at 50 Amps is .025V, so the voltage drop makes sense.
Why not split into two batteries: a 2p8s and a 1p8s and avoid having ten feet of balance cable?

I would understand 0.025V voltage drop, but the readings are actually fluctuating by 0.02-0.03V e.g. the 7th cell block reading bounces from 3.360V to 3.386V over the course of a couple seconds (as seen on the jkbms app).

Space is quite tight (this is going in a campervan) and I can't quite fit the 1p8s in the second compartment.

 

[Ampster]

Why not split into two batteries: a 2p8s and a 1p8s and avoid having ten feet of balance cable?

That is a good suggestion. That is often a necessity an RV application where there are space constraints like the OP may have. It also gives some redundancy.
An alternative would be a higher end BMS that can compensate for resistance of the balancing and battery interconnection leads.

 

[alexcl]

That is a good suggestion. That is often a necessity in a boat or RV application where there are space constraints. It also gives some redundancy.
An alternative would be a higher end BMS that can compensate for resistance of the balancing and battery interconnection leads.

Interesting - any suggestions for a bms?

I think the jkbms does compensate for resistance of balance leads but why does the resistance of the interconnection leads matter? Why would it cause voltage fluctuations?

 

[hwy17]

I think the Orion JR2 can theoretically handle this by putting 1-6 on cell group 1 and 7-8 on group 2.

It's not specifically described in the manual because they assume you're doing 8 cells in one bank but I believe it's possible.

 

[Ampster]

Interesting - any suggestions for a bms?

I have been using the Orion Jr BMS for six years and the post after my earlier post also suggested it.

I think the jkbms does compensate for resistance of balance leads but why does the resistance of the interconnection leads matter? Why would it cause voltage fluctuations?

I think the resistance would cause a compensation issue that would be consistant throughout the charge/discharge cycle. Fluctuations in voltage readings are typically a result of bad connections. I assume what you mean by fluctuation is changes voltage readings that differ from an independent reading from time to time.

 

[alexcl]

I have been using the Orion Jr BMS for six years and the post after my earlier post also suggested it.


I think the resistance would cause a compensation issue that would be consistant throughout the charge/discharge cycle. Fluctuations in voltage readings are typically a result of bad connections. I assume what you mean by fluctuation is changes voltage readings that differ from an independent reading from time to time.

Sounds good re. Orion

I mean that the actual voltage for cell 7 is swinging pretty wildly as measured by the bms leads. I'm unable to post a gif but here's a link to a 10 second clip:

https://imgur.com/a/9GXCjEn