to bypass or not to bypass bms?

In defense of fusing your discharge line, and bypassing your bms for discharge. Humans make mistakes, a discharge bms should be be able to stop a human error short circuit and the resulting fire. A fuse may also be able to prevent that fire if setup correctly. You have to ask yourself do you trust complex software (bms), a simple thin wire (a fuse), or yourself (very human, I assume). As a software dev I can tell you unequivocally the human is the weakest link.

It’s also worth noting bigger board manufactures like lacroix frequently use fuses.

To prevent the fuse from vibrating to death you should pad you fuse box thoroughly.


Worth noting is also that if you’re looking to draw 40A you dont pick the BMS that supports 40A.
You pick something that gives you 25% headroom. Beeing superman is cool untill you eat the asphalt


I might be misunderstanding but it looks like the human errors can only really happen in the build process? I guess you could assemble something that just barely works and then accidentally shorts later due to vibrations, but in that case I’d still rather have a chance of the board working long enough to stop and then going up in flames

There are stories I’ve read where human error during maintenance caused a short. I seen builds with leads that could easily be pinched together by flex. Humans just tend to make mistakes, predicting all of them is not practical, so we should assume they may mess up in almost any way. placing the fuse as close as possible the terminal of your pack reduces the chance of a fire by limiting the distance in which a human can mess up the leads. The human can still cause a short with the balance leads, but they may fuse themself out, or you might be able to use pico fuses there, but I have never done that.

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The main reason for this is that DALY bms are over rated 25% on continuous discharge and cut out fast after compared to others that have lower continuous ratings but can accept amp pulses that are much higher.

They are perfectly fine if you bypass but i would personally look elsewhere if you want to use it for all it’s features.

Install a fuse, bypass bms and call it a day

Not on discharge. Never fuse your discharge unless you have non-electrionic braking.

You can easily fuse your discharge, in case of a pinch short or if you don’t trust your stuff. But make sure the fuse and fuse holder rated amperage is much higher than what you will discharge or regen charge if not your board will cut out and that could be fatal.

On the other hand, if you run high amperage, the fuse break wire will be huge and the first thing to blow might be your battery wires or anything else that would melt first. Never say never… :face_with_hand_over_mouth:


Then you probably shouldn’t use it.

Exactly, so a fuse on the discharge line is either dangerous, or unnecessary as the solder would melt before the fuse popped.

I am using daly 10s 60 amp. Two 6374s. 6.6dual. The Daly’s can sustain short bursts of 180 amps. I have never cut off. No glitches. But, I dont really hotrod. 60 amps each.

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Explain that please I’m not following. Fuses blow when you draw too much power which would only happen if you incorrectly configured your ESC to draw too many amps or if a MOSFET blows because chances are it will fail closed. At that point you learned from your mistake and have to buy a new esc or are happy that only your ESC blew and you aren’t kicking a burning battery pack off your build.


If your battery wires melted before your fuse your whole build deserves to go up in flames for using the wrong gauge battery wire lol. The whole idea behind a fuse is that it’s the weakest link in the circuit. It’s not there to save electronics. It’s there to prevent a fire. Putting a diode in parallel with the fuse makes it act like a mosfet anyways so if the fuse does blow the freewheeling motor can still put that electrical energy into the pack, just can’t take it out.


Yes, and when a fuse “blows” it becomes disconnected

Wrong. For most of us, that point is going >40kph down a hill and finding out we don’t have brakes because a fuse disconnected our battery from our VESC. You don’t learn from your mistake in that situation, you die.

Electric BLDC braking happens by your motors and battery exchanging electrons though your ESC. If the battery or motor is disconnected then you can’t have electronic brakes.

The day we start using mechanical brakes this all changes, but for now it’s safer to risk exploding your battery than to risk not having brakes.

Be smart. Have brakes. Don’t fuse your discharge and don’t discharge through the BMS.

The fuse would only blow if:
A. there was a malfunction with your controller, meaning brakes wouldn’t work because the controller is FUBAR.
B. user error with battery amps configuration that allowed more than your set amps to be drawn from the pack.

The diode in parallel with the fuse as I mentioned already would solve both issues anyway. It offers short circuit protection and a path back to battery pack if you really think a fried controller would offer anything useful while still in motion. If the mosfet in your controller blows in the closed position (almost always fails closed) and it has no fuse it’s just going to lock up your motor on that phase and throw you off the front. Then it’s going to melt your motor and then most likely set your pack on fire thanks to the dead short. I’d rather take my chances with free coasting to a stop where maybe my other controller survived and allows braking then having one motor seize up which is guaranteed to throw you off the front.

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A lockup isn’t guaranteed to throw you off the front.

I had a hanger break in half and the motor and wheel detached from the vehicle, riding on the aluminum piece of truck hanger jammed into the street, and didn’t fly off the front. And I was carrying about 15 kg (35 lbs) of groceries.

There are a lot of variables, and hardly anything is guaranteed. It’s all about risk.

Damn that’s nuts. How fast were you going and how quickly did it come to a stop?

Very fast, and the Lyft driver was very confused.

But the point is, you can’t assume anything.

What if a motor was violently ripped from the vehicle, and the fault condition is now gone, and the controller is not FUBAR.

That being said, I wouldn’t personally be completely opposed to using a fuse but I would make sure it was rated for at least double or triple (or more) the maximum power I ever expected to draw, and that it wouldn’t be tripped mechanically by vibrations.

If a motor was violently ripped from the controller it would not blow the fuse because there’s no resistance on the controller. If the motor was ripped out and shorted out the cables the controller would be fried or it would detect a short condition if you are lucky. I’ve used a fuse with a ten amp buffer flawlessly on my last build (60 amp draw, 70 amp fuse). You just look at the datasheet for the time vs current graph. 2 to 3x is probably fine for the most part considering each motor controller will draw 1/2 of the current from your pack so the pack should have no problem outputting that amperage.

Vibration is definitely an important consideration

Yes, that is the problem. To use proper fuse, it would make the safety increase almost nil.