Unless I’m mistaken, assuming a 4 second braking session from 30mph— 8157 joules in 4 seconds works out to 2039 watts give or take.
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but why do you propose wiring 50 cells in series?
Those are small (~19x11x11mm), can handle up to 265 volts, and dissipate up to 15W. wiring 50 of these in series adds the resistance, and if they were in parallel it would divide the resistance by the # of resistors added, correct?
I think that’s what I would want… not sure, because 50 * 8 Ohms, not sure if 400 Ohms is what I need for this sort of thing. Really, I just want a bunch of these to dissipate 15W * however many to equal many watts of energy that can be diverted to a bank of resistors instead of the battery.
why does it need to handle 265 volts? a motor turning at no load speed produces the same BEMF voltage as your battery…
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It doesn’t need to… it’s just one of the first suitable resistors I found for a good price to use as an example. I’m sure there are better options out there, and if I were to figure out diverting the power intelligently, i’d spend a lot more time browsing for better options.
I’m no expert but I’d imagine any time you have a 265V source on your board it can be potentially dangerous. I’m not sure which components would be compatible with such a voltage.
I am pretty sure they are just rated to operate up to that voltage, like capacitor voltage ratings.
sorry… i didn’t click the link at first… i saw 50 18650s in series and assumed you meant lipos. 50 resistors makes more sense. so you need something that can handle 2000-3000w and perhaps up to 60-70v which works out to maybe 50-60 amps, and able to store 8000 joules of heat per braking attempt from 30mph.
@Skatardude10 if you simply want double the braking power for emergency situations I think this is another issue that can be solved with 2wd->4wd… you can get simultaneously double the braking for emergency situations and 1/4th the heating per motor for the same acceleration.
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also there is a physical limit to how much braking force you can actually apply with only 2 wheels… (before your braked wheels slip)
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IMHO yea à rheostatic brake would do the trick… Just need to trigger at proper voltage and be able to dump both lot of watts + any extra voltage spikes rising above the nominal voltage.
Easier said than done in a compact format tho.
Actually, if the VESC firmware didn’t suck implemented dynamic braking, the motors could be used as your “rheostatic brake” by simply connecting all phases to ground and using PWM to modulate the braking current. No additional components needed, but the motors need to be able to dissipate the heat. I’ve found that 6374s can dissipate around 600W for a few seconds without overheating.
This is also inherently antilocking as if the motors stop spinning, then the braking current will decay quickly.
Ok, I am partly wrong with using PWM to modulate the braking current, doing so will result in a small amount of energy being returned to battery. So if your BMS has already cucked you, you’re out of luck…
Another possibility is to charge a supercapacitor array, but that will get expensive really fast.
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