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Short-circuit proof mosfet based BMS?

MattiFin

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Quite many BMS's have overload protection but is any of these able/designed to handle short-circuit?
How much delay and is there minimum/maximum tolerable wiring inductance?

Yes, you can and should use fuses but there is still question which one reacts first and if the first device to respond able to withstand the instantaneous current and inductive kickback. Too often the fuses blow only slightly after the semiconductors have already blown. Or the superfast fuses cost more than the semiconductors they are supposedly protecting.
 
I'm designing one which is short circuit proof ;)

Unless you have a super fast ADC and a dedicated core on your MCU you can't do it in software, it'll be too slow. If you want to be able to save the MOSFETs we are talking single digit µs maximum time from current rising over your threshold to current totally interrupted (and that's with using very nice MOSFETs who can handle a peak current of 1 kA each).

Inductance is actually your friend because it will lower the dI/dt. Of course it'll also generate an inductive voltage spike when you interrupt the current so you need to take care of that. If you want some rough idea of the number: I calculated that at least half a meter of 4/0 should be enough inductance in my case.

Given how fast the e-fuse will react it will protect a classic fuse no problem (which is nice given the cost of a MRBF or class T fuse...).

Yes, even super fast classic fuses will not be fast enough in that case, MOSFETs will burn first.

NB: you should still have a classic fuse just in case, as a non-redundant e-fuse isn't safe enough to be used as the only protection device.
 
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Daly for example seem to specify 960 us delay for short-circuit protection. Or about 1 millisecond.
Wiring inductance is slowing down the rate of change during short-circuit. Assuming 48v system with compact layout and 2 meter wiring length gives us roughly 2uH inductance. dI=dU*dt/L = 48v*960us/uH = 24kA at disconnect.
 
Pretty much guaranteed it'll fry its MOSFETs (around 1 kJ of energy to dissipate...). They probably use a software approach as even a slow hardware one would be at least 10 times faster than that.

And 48 V is optimistic, 56 V is more realistic for a fully charged LFP battery ;)
 
I'm designing one which is short circuit proof ;)


Inductance is actually your friend because it will lower the dI/dt. Of course it'll also generate an inductive voltage spike when you interrupt the current so you need to take care of that. If you want some rough idea of the number: I calculated that at least half a meter of 4/0 should be enough inductance in my case.
Inductance is a mixed blessing as it slows down the rate of current rise but increases stored inductive energy.

Your project looks very promising but the thread is miles long.
I wasn't able to find out power section schematic but the board layout looked like you are dissipating the energy in a bunch of TVS diodes?
What sort of wiring length/inductance you figured as the upper limit for yours?

Alternative to TVS diodes would be freewheeling diodes and dumping the energy back to battery/supply.
 
The summary thread: https://diysolarforum.com/threads/summary-of-diy-bms-design-and-reflection.17169/ (go directly to the DPB board section if you want the high current board schematic) ;)

The TVS are here to help the MOFSETs with the inductive voltage spike, most of the energy will be dissipated in the MOFSETs.

I can't remember the exact number (I need to calculate it again to put it in the specs anyways...) but I know anything reasonable (no more than a few meters) is plenty fine.

You can't use freewheeling diodes as the switch is bidirectional.
 
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