Question about Overkill Solar (4s120a) BMS unit and B- and C- (3) wire configuration.

[HRTKD]

That can't be right.

 

[Dzl]

I'm finishing up my battery pack build and purchased an Overkill Solar 4s120a BMS (with the bluetooth feature). It looks to be well built.

Is there a reason there are (3) wires for the blue (B-) and black (C-) instead of typically (2) that I see on some other BMS's? I assume all (3) are needed.

I think, the stock factory version of this BMS comes with two wires but solder pads for three, and Overkill orders or adds a third (which is great).

If you look at an ampacity table (like this one) 200*C 10 AWG, is good to 70A (so ~140A combined if current is evenly distributed). That is technically above the 120A rating of the BMS, so is technically sufficient, yet leaves little margin. Adding a third wire provides some safety overhead, and fault tolerance (if one connection is bad or goes bad, 2 can carry the full load).

 

[NCblueridge]

I think, the stock factory version of this BMS comes with two wires but solder pads for three, and Overkill orders or adds a third (which is great).

If you look at an ampacity table (like this one) 200*C 10 AWG, is good to 70A (so ~140A combined if current is evenly distributed). That is technically above the 120A rating of the BMS, so is technically sufficient, yet leaves little margin. Adding a third wire provides some safety overhead, and fault tolerance (if one connection is bad or goes bad, 2 can carry the full load).

That makes sense, I recently ordered another OK BSM for the 2nd system I am starting to build and upgraded the wiring to 8AWG wires in case I go with a larger inverter at some point on a 12 volt system.

 

[Nartmot]

The full manual on their website shows the 4th wire connected to the fourth cell negative (instead of 3rd cell positive), which is adding some additional confusion for me. Can someone more technical than me explain why they would do it this way?
View attachment 22939

Answering my own question to help out anyone else that is confused by this. I emailed overkill and he basically told me it doesnt matter which post you connect to (positive or negative) as long as it is essentially 1 wire per bus bar.

 

[Ampster]

it doesnt matter which post you connect to (positive or negative) as long as it is essentially 1 wire per bus bar.

And to further clarify since there are 5 wires and 3 buss bars on a typical 4 cell build. The first wire which is usually black is put on negative terminal. Wires 2, 3 and 4 are put on buss bars in sequence and wire 5 is put on positive terminal.