Connect two packs in series and one BMS

[jai134]

I have to split my 14S18P battery pack in two due to limited space. It is two 7S18P and i wonder two things.

First is how to connect the BMS?
I don't think it has to be different from a 14S pack. B0 is Battery- which is connected after serial connection of the packs. B1 is first positive of the first pack and B2 is second positive of the first pack and so on. B8 will then be first positive of pack two and battery+ is after serial connection of both packs?

Second thing is the size of the connection which connects the two packs?
I guess it has to carry the whole packs capacity, 400 A after reducing it in the BMS settings. According to my "Battery Package Calculator" is 18P pack with Samsung 30T capable of 630A continuous discharge current and that will burn my motor.

 

[j bjork]

The connection between the two packs will have to be beefy enough to have minimal voltage drop at your max output.
Any voltage drop there will effect the voltage reading of the last cell of the first pack or the first cell of the second pack, depending of where you place your balance wires.
It can help to to put a balance wire both on the last cell on pack 1 and the first on pack 2, and y connect them close to the bms.

The capacity is in Ah, not A. Will your battery have a capacity of 400Ah, or have a max output of 400A?
The capacity dosent matter when sizing the connections, the output in A does.

It dosent matter if your battery has 14 cell groups in series (14s) or 18 in series (18s), it wont be capable of higher output in A. (In Watts though) What matters is what cells, and how many in parallel.

 

[jai134]

Thank You for great input! Yes, my mistake, I meant max output 400A, and 18S is a typo, it should be 18P. I will change my first post.
I will connect last cell, cell 7, of first pack and the first cell, cell 8, of the second pack with separate wires. Doesn't it confuse the bms if I connect them together?

 

[jai134]

Ah, I think I understand. You mean that I connect both cell 7 and 8 with separate wires and connect the wires together but still has two wires to the bms?

 

[999zip999]

400a what is this for ?
I like your finding on wiring on that BMS thread .

 

[jai134]

It's an old Husqvarna SM610 I am converting.
This is with Nissan Leaf cells. Now I'm replacing them with 21700 cells instead.

 

[j bjork]

When you connect your battery to the bms you have one wire from each positive side on your cell groups.
I suggest that you also connect a wire of the same size and length as the positive from the 7:th cell group to the negative on the 8:th cell group. You then connect these two wires together on the input for the 7:th cell group on the bms.

The idea is to even out the voltage drop in the longer series connection between those cell groups, that would otherwise look like a weaker 8:th cell group.
If your series connection is good enough it shouldn't really be a problem, but it wont hurt anyway.

 

[jai134]

Got it! I will try without that extra wire to start with and see if it is needed.
Another question is about a table I saw from evmadrid. It says that 1mm² copper wire is "good" for 19A. Is 2mm² "good" for 38A and 3mm² 57A?

 

[jai134]

Yes or no is ok.

 

[From-A-To-B]

Yes or no is ok.

Yes. Wire and copper bussing is simply additive like you suggested.

 

[jai134]

Thank You so much!