Backpack Battery - What would happen to VESC if battery disconnect while motors were running?

Hey guys - I am considering doing a build and putting the battery in a backpack and running a power cable down to the FOCBOX Unity I intend to use that will be mounted underneath the board. What would happen to the FOCBOX Unity if I came off the board mid-ride while it was powered. Would I do any damage to the Unity? My background is in mechanical engineering with a touch of electronics knowledge so I would really appreciate the help of some people with more knowledge in this area. Namely I am concerned about the TVS diode…

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Welcome to the forum, @electronsareweird. Long battery leads are problematic as they tend to cause problems for the esc. I’m not an electrical engineer but from what I’ve learned in the Rc hobby its best to shorten the distance between batteries and esc as much as possible. Longer leads between the esc and motors tend to be less of a problem but I doubt you want an esc in your backpack, lol.

Heres a video that gives a somewhat short explanation rather than the 66 page thread on RCGroups he mentions. I’m also on RCG and gosh those threads get really long.

Please note the slight error in this video around 1:30. Our brushless motors run on switched DC power, not AC. They only supply current in one direction and any back flow of energy or back-EMF as its called happens when the power to one motor phase is cut off and its magnetic field collapses turning back into voltage that goes into the esc and is either dissipated or cycled out in a process called esc freewheeling. I know, tl;dr right?

Anyways it is doable but you’d need a cap pack and its not advisable because having loose multi stranded wire bouncing around while you ride will only break those tiny copper strands one by one and over time those cables will fail. Could be a fire. Things could asplode. No that wasnt a typo.

For future reference you’d need low ESR electrolytic caps rated for at least 1.5x your expected maximum voltage to cover any voltage spikes that can happen when you suddenly unload a motor/esc and those long power wires act as a capacitor soaking up any extra current that gets dumped into them.

Now whats this TVS doide you speak of?



eMAIL this guy on these videos, he will likely talk your ear off.

Also, I have an EGLIDE board, that I did that. Cant say it is good for the esc, but it hasnt blown up yet.

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I am appalled at that mans lack of socks. But at least the esc looks like its got plenty of caps to handle a few feet of power cable.

That would make me nervous. 50vdc makes one hell of a spark if its short circuited.

Good luck @electronsareweird, whatever you decide.

Thanks @kavic5150 - I am building it with a ThreeSix Downhill Deck (made from waterjet aluminum) so I want to keep that aesthetic, but it is presenting more engineering challenges than I thought…

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That sounds fascinating. Got any pics?