Fuses behind a BMS: what's the point ?

qwerkus said:

I keep seeing packs with an automotive fuse sitting behind a bms, and I wonder what's the point. Even in a pure redundancy scenario, a proper bms will trigger an OCP way before the fuse would melt, and die if the OCP fails. Even in that extreme case, the main discharge would be instantly cut off, so when exactly would the automotive fuse melt ?
Since a fuse adds some sort of resistance (though I agree: irrelevant in high power system), only reason I could see for such a setup would be to limit the output of a bms too powerful for a given controller. What do you think ?

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serious_sam said:

qwerkus said:

I keep seeing packs with an automotive fuse sitting behind a bms, and I wonder what's the point. Even in a pure redundancy scenario, a proper bms will trigger an OCP way before the fuse would melt, and die if the OCP fails. Even in that extreme case, the main discharge would be instantly cut off, so when exactly would the automotive fuse melt ?
Since a fuse adds some sort of resistance (though I agree: irrelevant in high power system), only reason I could see for such a setup would be to limit the output of a bms too powerful for a given controller. What do you think ?

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MOSFETs often fail as a short circuit. So if the BMS didn't turn them off fast enough in a short circuit condition, they could potentially be overloaded and fail as a short circuit.

I have a BMS with an array of high quality MOSFETs, but I also run a large automotive fuse as redundancy in case shit happens. Have you ever seem the spark that a high power 84VDC battery can make? Boom.

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