# How does VESC braking work?

Is maximum braking power determined solely by the batterys maximum charge current (Therefore to increase braking power you have to add battery capacity). Or can the VESC dissipate braking energy via heat?

Im a little confused as there is a negative ‘current brake max’ setting and a battery ‘max regen’ option.

Thank you.

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You are limited by the battery pack you use. That means the limitation may lie with the cells, or with the BMS.

Braking current arrives at the pack the same way charging current does. Your pack needs to be able to charge at the current your brakes can provide.

If you send too much current into there you could either overcharge your cells or trip your BMS.

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So if i set ‘battery current max regen’ to -10a. No matter what i set ‘negative motor current max’ to, it will only brake at -10a? In other words 'negative motor current max’set at either -10 or -40 will feel exactly the same, because -10 is max regen.

Oh and I dont use a BMS.

say your max charge current for each cell is 4 amps. If you had a 2P that would be 8 amps maximum charge current. but even with a 3P, you don’t just go down to -12amps and call it a day. Things can get sketchy down there, so i’d say keep it around -8.

then set you negative motor amps to like -40 or thereabouts and you should brake like a bicycle, or at least start approaching that ideal. You’ll need to adjust for your weight, and maybe compromise on the force of the brakes for your application.

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Thanks

I understand all that side of it, its how the VESC works that is baffling and why is has two sets of braking currents (One from motor to VESC and one from VESC to battery) if braking strength is always limited by the battery max regen value.

I guess the only way to test is to leave battery max regen locked at -10a. And try incrementally increasing ‘negative max motor current’ above -10a. If the brake strength increases, it must be dumping excess energy other than into batteries via heat.

Ive tried googling the answer but ive found nothing conclusive. Ill be back…

damn haha I run -20a batt min and -60a motor min

10s6p but limited to 5a charge

my BMS limits me to 6amps allegedly but i use -8 batt regen and a -50 on the motor side.

Ever have a problem braking down hill and causing a reset?

nope. only problems i’ve had are brakes cutting out on full charge. not related to that though

The difference between motor max and batt max has to do with duty cycle. Duty is basically the decimal of top no load speed your motor is currently running at. Its also the ratio of amps in the motor to amps coming from the battery. Eg If the VESC is pulling 10A from the battery, and the motor is spinning at 0.5 duty the motor will be at 20A which is how you can have 2 limits at the same time.

Since RPM = V * kV, if you lower RPM then you need to lower voltage as well since kV is constant. Because of this, the voltage in the motor is not constant, while the voltage coming out of the battery is. The amperage on either side needs to be different for the same amount of power to be moving through the system on either side. Duty just represents that difference.

Duty cycle is lowest at low speed, so the ratio of motor amps to battery amps is high. Say you were at 0.05 duty and you were pulling 10A out of the battery, you’d have 200A in the motor, that’s enough to knock you on your ass, so you’d set a motor max limit to something smaller, usually people set it around 40-100A. The same concepts apply to breaking.

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It also depends what limits it to 5A. A typical 18650 can charge at 4A per cell. 6p means six parallel cells so you can push up to 24A and each cell still gets only 4A. If the 5A limit is from the BMS and u use it for discharge as well, you can’t put battery min to more than -5A. If the limit is your charging port then you can still push up to 24A back (assuming the BMS allows it, if it’s used for discharge as well).

I run -12A per vesc (so -24A per total) on a 10s4p. Rated value is 4*4 so 16A but taking duty cycle into account I will generate 24A only if I apply full brakes from 100% duty cycle. Since I never do a full brake from top speed, I never actually push 24A back into the pack. And if I do, it’s for a good reason (emergency braking) and it will be a short spike so damage is minimal.

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That makes perfect sense if the voltage in the motor is varied, is that only in duty cycle mode? In current control mode is the motor voltage constant and the current controlled directly.

I did a little testing today and it seems even with max regen locked to 10a, increasing negative max motor current does increase braking. It started raining, Ill have to double check tomorrow, maybe im imaging it.

No, you’re not imagining it. Increasing the minimum motor current improves braking especially at low speed. This is true in all control modes.

Both valid reasons for bypassing the bms for charge only!

I discharge through a bms and ride my brakes downhill on a full battery and get no reset. Watching my voltage while doing so, it appears evident that my bms monitors the regen charge and prevents charging when the battery reaches the overcharge detection voltage which is set at 4.28v I have reached that voltage and continued riding the brakes downhill. My battery voltage would not exceed that limit but held there until I reached the bottom and let off the brakes. At that time the bms proceeded to trim the battery back down to 42v while I was coasting. During this test there was no reset and no overheating. This test was with the Bestech HCX-D223V1 I can’t say if this is the same for other bms’s.

Here is an old thread with info on brake settings.

are there resistors in that bms that are capable of burning off as much energy as youre using going downhill? wondering where the energy is going if not the battery.

It’s my understanding that the vesc can “recirculate” the regenerated power through the windings of the motor and the mosfets of the vesc to allow braking without generating any net power to the battery. The heat is spread between the motor and the fets.

if the battery isn’t connected and you brake its a quick doom, how would it be any different in this situation? How exactly would it “recirculate” if you know? sounds like it would have to be shorting the leads to brake