BMS Wire Gauge

Autocol

New Member
Joined
Oct 1, 2019
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11
I was wondering if anyone can tell me why it is okay for the wire connection to the negative terminal of a battery from a BMS to be significantly lighter than the gauge of the wire connected to a battery’s positive terminal?

For example, the Overkill Solar BMS documentation recommends 2 or 4 AWD for the positive but only requires dual 8 or 10 AWD to connect the BMS to the negative terminal

Thanks, Eric.
 

Zbugsii

Solar Enthusiast
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Aug 20, 2021
Messages
24
Well, realistically they should be the same. But actually it really depends on the maximum current that is expected to flow. So, as long as the smallest combined group of wires handles the load, then it really does not matter.
 

time2roll

Photon Sorcerer
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Mar 20, 2021
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3,580
#8 will go 80 amps compared to 160 for #4. Exactly 2:1. (105C insulation)
#10 will go 60 Amps so three are required to match #4.

These are short runs. #2 or better should be used depending on length and voltage drop. Either way the BMS (100 to 130 amps). will cut power before the wires get too hot.
 

Zil

Just another ass on the web
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May 27, 2020
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1,832
key word; dual. I don't agree using dual cables is proper, but too many BMS are doing this.
 

time2roll

Photon Sorcerer
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Mar 20, 2021
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3,580
I ordered mine with terminals. Cut a standard battery bus bar down and drilled a 6mm hole to go across the terminals. Single #4 wire with 6mm lug hole works perfectly.

If you choose wires I believe there is an ampacity issue in the circuit board where the connection needs to be spread over a larger surface area.
 

Autocol

New Member
Joined
Oct 1, 2019
Messages
11
Thanks for the input everyone. I am glad to hear that there is not some weird electrical rule about negative terminals or BMS’s that I did not know about.

I am going to see if I can get a bridged busbar connected to the BMS and run a single 2AWG wire. I will post a picture if I get it working.

This is a link to another forum thread where I am asking for input on my proposed design. I have updated it with the heavier BMS wires: https://diysolarforum.com/threads/powerwall-design-feedback.31256/
 
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