# Fuses on BMS boards?



## Coulomb (Apr 22, 2009)

gdirwin said:


> Has anyone installed fuses on BMS cell boards?


Yes! (raises hand )



> I am in the pro-BMS group


(Keeps hand raised)



> but also feel that the additional wiring/boards increase exposure to fires due to failed BMS cell boards.


Yes, we've had a cell go very low volts (0.68 V from memory) due to bad software in a BMS. So we're looking at a "failsafe" curcuit, whereby even if commanded, a BMU (battery management unit, on a cell) can't discharge a cell below about 2.0 V, because of diode drops.



> If a fuse was installed on each BMS cell board, I think it would minimize the risks and give you the best of both worlds.


Well, it helps prevent catastrophes.



> Would a cell-board fuse affect voltage drop, shunting, communications etc...?


Yes, we found that our surface mount, quick acting fuses had rather embarrassing voltage drops. So we arranged for the cell monitor resistor to bypass the fuse. That way, the fuse voltage drop doesn't affect the voltage accuracy significantly. The cell and BMU are still protected from the presumably rare case of the bypass resistor failing shorted, or a short on the PCB. There can still be odd cases like the sense resistor failing shorted, but hopefully this still doesn't start a fire.

Our BMU schematic is here.


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## etischer (Jun 16, 2008)

I'm using mini BMS and didn't bother fusing the cell mounted circuit boards. The wires connecting the BMS to the battery are too short to short out, and if there was a short on the board itself, it would likely just burn the trace out.

I did fuse the centralized mini BMS boards since the wiring between the BMS and battery is more likely to start a fire if shorted. It also provides a convenient place to measure and charge individual cells.


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## dimitri (May 16, 2008)

gdirwin said:


> Has anyone installed fuses on BMS cell boards?


I guess you'd have to be more specific about your boards. For example, miniBMS boards already come fused, so extra fusing is not required.


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## roger (Jan 24, 2010)

Dimitri,
you´re right, but in centralized option as shown at the picture I would prefer additional small fuses near by the cells for every cable to prevent shortcut if they will be damaged..... Only to be at the safe side and in addition to your well designed cell modules. Fuse value I would choose a little higher than the balancing current of the cell modules, say 1Amp.

Roger


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## Yabert (Feb 7, 2010)

Hi dimitri

What do you think about add fuse for your 16 cells boards (centralised)? good idea? Necessary?


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## dimitri (May 16, 2008)

Yabert said:


> Hi dimitri
> 
> What do you think about add fuse for your 16 cells boards (centralised)? good idea? Necessary?


Only if you add fuses at the cells, not at the boards, as Roger mentioned, you would reduce the risk of shorting along the wire path, which is very unlikely if you do a good job at wiring looms. But then again, it would not hurt anything to have extra safety check. Adding fuses at the board is useless since boards are already fused.


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