Motor braking.

[hyperasus]

Again......everyone spare us the theories. All anyone has done in this thread so far is confuse the hell out of everyone. I myself agree with what Aragon is saying, but then again.....wtf do I know. Doesn't anyone know something factual on the subject?

[Patrick]

http://www.rc-monster.com/forum/show...?t=6361&page=4
Post #47 Team Teakin prez is talking about regen braking causing voltage spikes, and that it's hard on the esc.

[BrianG]

Personally, this theory discussion is just fine. That's how the subject will eventually become clarified. However, I do agree that the topic could be confusing to someone just trying to find the answer. Maybe the thread title should be amended to something like "Motor Braking - theory discussion"?

[zeropointbug]

Originally Posted by Aragon

I don't think it is regenerative...

If it were, why is the braking force consistent? If it were regenerative, the braking force should be higher if your battery is flat.

Also, what stops the brakes from causing a spike that takes your lipo above 4.2V/cell?

And if it were, why do we see no hint of this on our onboard power meters?

Well, GriffinRU did say that it only spikes your batts when the potential is higher than battery voltage, and enough over the voltage drop on the FET diode. I am guessing it is just shorting the motor, and turns it into heat, but the FET's can control the switching of the 'SHORTING', hence proportional braking.

Also, I have actually tried running the Eagletree in reverse manner, but it does not read anything. I tried using it when charging, it needs to be flipped around I guess.

Originally Posted by zeropointbug

Well, GriffinRU did say that it only spikes your batts when the potential is higher than battery voltage, and enough over the voltage drop on the FET diode. I am guessing it is just shorting the motor, and turns it into heat, but the FET's can control the switching of the 'SHORTING', hence proportional braking.

Yea, the shorting would be easy. You just have heat to dissipate. But if the controller uses the battery as an energy sink to provide braking force then I have to consider the numbers. If it takes, say, 500 Watts to accelerate the car to full speed as quick as possible, surely it's not going to take a large magnitude less power to deccelerate the car back to zero in the same amount of time? Let's assume the controller has to sink only 300 Watts to provide an equal braking speed. Is that 300 Watts going into the battery? If it were a lipo it'd probably puff or be severely damaged.

Is the controller sinking some of the energy to heat and some to the battery? And on what criteria does it perform the split? Are controllers aware enough of battery chemistry to know how much power to limit being generated back into the battery?

[zeropointbug]

Yah, that's the thing, I don't think it would ever WANT to regenerate the batt, unless it's A123 M1 cells, which can take a $HIT load of charge in bursts (they can 5 min charge).

About the acceleration.....try 1500 watts with these XL motors. I have a 7XL, everytime I full throttle blip, it's around 1500 watts, acceleration is super car territory.

For braking, no one knows how much power is being dissipated. But the amount of power you can get from braking is more than you could ever use.

[aqwut]

Originally Posted by Serum

You can use the motorbrake, but the heavier the vehicle, the harder it is to brake.

the motorbrake means more stress on the controller, and the motor will heat up sooner. What it does is 'shorting' the wires of your motor, and the energy is stored back into your cells. (tipical way of braking with electric motors)

Most controllers can handle it though. If you tell us the setup you want to use, we can tell you if it's wise to use a mechanical brake or not.

From my own experiences, Serum is absolutely correct...!!

[Serum]

Thanks Aqwut;

I don't know what the fuzz is at this moment; slowing down a vehicle gives you energy; charging the cells above 4.2V per cell? what did you discovered? a 120 percent efficient setup? The peaks MIGHT be higher (not saying they are) but the lipo won't bother too much from little spikes.

@aragon; 'it's easier to lock the wheels than to spin them' what do you want to do with the heat that is produced by that? stuff it in your esc? no sir, you store it in your batteries again; use the low internal resistance of the batteries for braking.

the braking force should be higher if your battery is flat.

Why? does the internal resistance change when the battery is flat? the brakes are using the batteries internal resistance, not their capacity.

Also, what stops the brakes from causing a spike that takes your lipo above 4.2V/cell?

Physics. A lipo can take more than 4.2V in spikes bytheway. It's too slow, and the internal resistance is too low to be 5V immidiately when 5V is applied for 1/100th seccond.

It's plain simple, it works like that period.

In less words;

Motorbraking means more stress on the setup (motor/controller) > more heat for the motor and the controller (and even the batts). a mechanical brake will heat up too, but that heat is okay since it's in a thin metal plate that absorbs quite some energy before it melts down.

That's why i posted;
'If you tell us the setup you want to use, we can tell you if it's wise to use a mechanical brake or not.'

If a battery can be discharged at 20C for peaks, why can't it be charged at peaks of 20C? Braking delivers less energy than it's taking on acceleration, don't know what ratio it is put in, but i think it would be less than 60 percent difference

Originally Posted by Serum

charging the cells above 4.2V per cell? what did you discovered? a 120 percent efficient setup?

I didn't mean it would charge the cells to above 4.2V/cell, I meant it might charge them at/with more than 4.2V/cell.

Originally Posted by Serum

The peaks MIGHT be higher [than 4.2V/cell] (not saying they are) but the lipo won't bother too much from little spikes.

Well that goes against every precaution I've read about Lipo chemistry. The whole basis of Lipo charging is to:

a. Not exceed the maximum factory rated charge current.
b. Not exceed 4.25V/cell.

Where have you seen that they can survive more than 4.2V/cell?

Originally Posted by Serum

@aragon; 'it's easier to lock the wheels than to spin them' what do you want to do with the heat that is produced by that? stuff it in your esc? no sir, you store it in your batteries again;

Yes, that would be ideal, without a doubt. I'm just not convinced our RC controllers are advanced enough to be doing that...

Originally Posted by Serum

Why?

If a lipo (or any rechargable cell) is connected to voltage source of X volts that is higher than the lipo's voltage, the lipo will draw a current. If the lipo is flat, it will draw a higher current for the same source voltage X.

If the (flat) lipo is drawing more current for the same voltage produce by the spinning motor (ie. same wheel speed), the current flowing through the circuit will be higher, and the braking force will be higher.

Originally Posted by Serum

It's plain simple, it works like that period.

I'll wait for a better explaination and/or real world test results before believing that...

Originally Posted by Serum

If a battery can be discharged at 20C for peaks, why can't it be charged at peaks of 20C?

Would love to know myself. :)

[Serum]

Schulze told me, team tekin told us, what more do you need? I won't invest any more time/effort in trying to convince you.

[GriffinRU]

I do not know what is going on, but:
Batteries internal resistance does change with charge! Otherwise you should be able to drain the battery for long time with now change in voltage drop under load...

Battery will be charged only if voltage generated by motor upon braking is higher the battery voltage (which is usually NOT) so for ESC to charge battery it suppose to have additional circuitry which boosts this voltage to higher levels (assuming we have only two wire going to battery!)

All hands for theory and understanding here!

[zeropointbug]

here here

[Gustav]

Originally Posted by GriffinRU

I do not know what is going on, but:
Batteries internal resistance does change with charge! Otherwise you should be able to drain the battery for long time with now change in voltage drop under load...

Battery will be charged only if voltage generated by motor upon braking is higher the battery voltage (which is usually NOT) so for ESC to charge battery it suppose to have additional circuitry which boosts this voltage to higher levels (assuming we have only two wire going to battery!)

All hands for theory and understanding here!

do controllers have this additional circuitry?
If the voltage generated is lower than the battery voltage and we didn't have this circuit to boost the voltage then where would the energy generated go?

[BrianG]

Well, from what I understand (and from testing), simply shorting the motor leads puts a substantial drag on it. Try it; simply take a motor and spin the shaft. Then, short the motor outputs together and short them again. So, really, it's the motor that is absorbing the energy magnetically. I think the FETs simply apply a "controlled shunt" so there is variable braking available.
Otherwise, I would think the FETs would heat up CONSIDERABLY if they had to take that energy.

Now, if this is true (the shunting theory), I don't understand how there can be a voltage at all to get to the battery...

[zeropointbug]

Yeah, I agree with you Brian, there is a switching to the braking obviously. You can hear it if you apply the brakes and push the truck forward, it sounds like a metallicy sound switching. I've tried the shorting the motor leads too, there is alot of resistance there for sure, as all it is basically a generator with a short circuit.

Maybe we can all understand the regenerative part if we know how particularly the FET's in these controllers work. As FET's have different features. :030: