Battery switch position ? positiv/negativ, before/behind bms

Sparfuchs

1 kW
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Nov 22, 2020
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305
Hello EV friends,
I'm about to build a high current 19s (because not enough space for 20s) battery with this 8-20s smart ant bms.(https://de.aliexpress.com/item/32997552090.html?spm=a2g0o.order_list.0.0.577a5c5fmX4joO&gatewayAdapt=glo2deu)

I want to reduce the risk of damaging the battery or the controller in case anything goes wrong with the bms or controller. So i thought of a 200A "fuse switch" (Not sure about the name). But there are different possible positions. Btw... it's a common port bms.

1) Positive wire. Would directly cut the contact to the controller (or the charger) without doing anything with the bms.
2) Negative wire between bms and controller. Same thing without any any advantages or disadvantages to the first option I guess?
3) Negative between battery and bms. This would probably not turn of the bms because the thin ground wire of the strings is still connected, but it would maybe also protect the battery and bms from to much current ?
4) Same than option 3, but with the thin ground string also behind the switch. This sounds quite interesting to me because it would also turn off the bms right ? And it protects the bms, the controller and the battery, right ?

What would you do ? And what are your thoughts about battery protection and is there anything I haven't considered yet ?

And a short off topic question: this is the fuse switch I wanna use:
https://de.aliexpress.com/item/32997552090.html?spm=a2g0o.order_list.0.0.577a5c5fmX4joO&gatewayAdapt=glo2deu
It says "working voltage: up to 50V". Do you think it will work with my max. 78V Battery ? I haven't found a better one yet and thought it shouldn't be a problem right ?

Thanks a lot for your help.

Best regards
Sparfuchs
 
I suggest either option 1 or 2. It won't matter which side you put it on.

Placing a switch between the BMS and battery may cause a condition where the voltages on the BMS channels exceed the ratings and blow something up.
 
fechter said:
I suggest either option 1 or 2. It won't matter which side you put it on.

Placing a switch between the BMS and battery may cause a condition where the voltages on the BMS channels exceed the ratings and blow something up.
Thanks a lot for your reply fechter,
you mean when switching on it might exceed right ?
 
A good BMS or controller should limit the current quicker than a fuse.
Here is my 40A BMS integral with my 13S 3P 21700 battery:21700Battery BMS Wiring.JPG
 
Very very few BMS limit current at all.

Many (most?) do not even offer OCP as a fuse does.

Usually it is the owner / system designer's job to just choose a BMS that tolerates a greater ampacity than will ever be allowed to flow through it, with a safety margin of say 15%.

Some BMS the current does not even flow through the FETs at all, only voltage related detection is used and external relays are used for the cutoffs.

Some controllers can be set with a highest current setpoint, that really is where that job is best performed.
 
john61ct said:
Very very few BMS limit current at all.

Many (most?) do not even offer OCP as a fuse does.

Usually it is the owner / system designer's job to just choose a BMS that tolerates a greater ampacity than will ever be allowed to flow through it, with a safety margin of say 15%.

Some BMS the current does not even flow through the FETs at all, only voltage related detection is used and external relays are used for the cutoffs.

Some controllers can be set with a highest current setpoint, that really is where that job is best performed.
thanks for your reply john61ct,
good point, but what does a bms do if it gets to "over current protection" ?
and would you agree with fechter that option 1 or 2 would be best ?
 
I have a security switch between bms and controller for safety. No2 in other words.
If you mean discharge overcurrent protection the bms will cut out and you have to restart it to continue.
 
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