BMS wire gauge? Is 0.22mm2 enough?

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Alibro
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BMS wire gauge? Is 0.22mm2 enough?

Post by Alibro »

Hi guys
I'm at the stage where I need to sort BMS wiring to my battery box. I plan to bring all the BMS wires to a central point and leave them there for the time being until I sort everything else out. I have enough problems to sort without worrying about the BMS but I want to allow for it later.
I want to use a single or maybe two multicore cables to keep sealing the box as simple as possible and was wondering what the consensus is? I have 18 cells in my fuel tank battery box (four large and two small modules) so I believe I need 19 cores, one for neg and 18 for the pos of each module. Does that sound correct?
I have some 22 core multicore audio cable but it is only 0.22mm2 so I'm concerned it may not be heavy enough but having measured the BMS cables from the eGolf my pack came from they are very similar gauge.
The multicore is made up of 11 pairs of microphone cable so I could use 9 pairs for the Pos from each module and twist together the other 4 along with a single earth for the neg so the neg would have 5 x 0.22mm which should be enough, shouldn't it?

Does this make any sense or would I be better to use heavier gauge cable and make more entry points to the box? I'm trying to have as few holes in the box as I can to minimise the chances of water ingress.

Cheers guys.
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celeron55
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Re: BMS wire gauge? Is 0.22mm2 enough?

Post by celeron55 »

Small gauge is fine for monitoring. If you do balancing, it can create problems as the current messes up the voltage. It does that of course with heavier gauge too, and to some extent even without any cables. I guess just be aware what your planned BMS can deal with. I would think any BMS with a central unit would be designed to handle any size cables though.

9 cells in a single cable makes under 40 volts so that's fine, but if you'd be putting say 20, that's almost 100V and you'd need to be aware of the DC voltage rating of the cable.
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Re: BMS wire gauge? Is 0.22mm2 enough?

Post by Alibro »

I looked again at the 11 pair cable and the insulation is very thin and it is only single insulated so I don't think I would trust it with the high voltages.
I also dug out some 8 pair and 4 pair cable which is double insulated so for this box I could use two x 8 pair and one x 4 pair with each pair soldered together to give almost 0.5mm2 which is way more than the OEM's used. There is also a separate earth for each pair but I don't know if I should use it or not.
This is the cable I'm talking about.
IMG_20210816_160123720.jpg
IMG_20210816_155932443.jpg
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celeron55
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Re: BMS wire gauge? Is 0.22mm2 enough?

Post by celeron55 »

I'd say the cell cables coming from the battery case don't need to be internally double insulated, but externally definitely, and even then always add a cable protector as a third layer (and paint it orange). One should think about what happens if it fails: whether the car just doesn't work, or if you get battery voltage in the chassis or something else that people can touch. Of course the third thing is avoiding fire, and the fourth thing is avoiding expensive battery or other part replacements.

Personally, I'd use that cable, and specifically with the signal pairs connected together, as 8 cell taps, no earths or shields connected. It looks like a quality cable. Of course if you have any experience with those cables failing or being difficult to work with, I'd skip them and get a correctly rated quality cable from somewhere.
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Re: BMS wire gauge? Is 0.22mm2 enough?

Post by Alibro »

celeron55 wrote: Mon Aug 16, 2021 8:23 pm I'd say the cell cables coming from the battery case don't need to be internally double insulated, but externally definitely, and even then always add a cable protector as a third layer (and paint it orange). One should think about what happens if it fails: whether the car just doesn't work, or if you get battery voltage in the chassis or something else that people can touch. Of course the third thing is avoiding fire, and the fourth thing is avoiding expensive battery or other part replacements.

Personally, I'd use that cable, and specifically with the signal pairs connected together, as 8 cell taps, no earths or shields connected. It looks like a quality cable. Of course if you have any experience with those cables failing or being difficult to work with, I'd skip them and get a correctly rated quality cable from somewhere.
Thanks for getting back. I think you're right and am planning to go that way. I'll use car grade orange copex to protect the cable.
Each core is insulated, the pair is insulated and the external insulation is high quality so with the copex as well it should be OK.
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