Do batteries recharge when the car is coasting (off-throttle)? I understand that electric motors are basically generators. But does that current actually flow back into the pack? If so, does anyone know how significant such recharge is?
I took my 2WD buggy without a battery, turned ESC on. Then I took my 2WD truck and turn it on with a battery. Put the cars against each other, in a way that would make the rear wheels touch. Applying throttle to the running car eventually produced enough current and turned on the ESC on the car without a battery - lights on, fan on, beeps, etc.

I would hope not, lipo batteries have to be charged in a very "strict" manner, and i doubt It would be good for them. It would be pretty easy to test, just go pick up one of those cheap night lights, disassemble it, and hook up the esc battery leads to the terminals that come off of the thingy the bulb screws into. Then push it across the floor.

I don't believe it works the way though.

I can not see the ESC letting current run back up the wrong way to charge a battery... even if the motor was trying to deliver it.

In effect when you do brake there is a voltage surge back from the motor to the esc / battery and it's this voltage surge which has been the cause of a lot of esc failures, as the load can be more than the batteries (especially cheap high IR packs) can handle, so the ESC capacitors cop the brunt, boil and then boom.....

I've seen the math worked out somewhere before. But to put approximately 1000mah back in, you'd have to coast about 40 miles.

40 miles... So basically, it does nothing.

For every 10 feet you coast 0.04734848484848 mah gets charged into the battery!

Regenerative braking as it is usually called was available on some of the brushed speed controls. Tekin 411P for example, from about 1990, had it and advertised it prominently. Regen braking made a significant difference with the much lower capacity batteries of the time. It was a deal breaker in 1/12 onroad which was perhaps the most impacted by limited batteries, the fast guys didn’t use full throttle, or at least very sparingly, or they would dump. If I remember correctly a couple laps was immediately gained compared to previous ESCs.

Haven’t seen it mentioned or advertised with brushless, there was discussion that it wasn’t done when brushless first became popular. At least partly because it added a lot of complexity to a unit that was already far more complex than we previously had as I remember it. Although there may be some stray amount as mentioned.

Off subject a touch, full size electric autos do implement regen braking.

ESCs likely have diodes to prevent current from flowing in the opposite direction.

Doubt if there are diodes in the main power path, wouldn't they cause a lot of voltage drop, generate a lot of heat and reduce efficiency, given the relatively high currents going through these?

The diodes are likely "before" the output FETs, meaning that the current can come back through the FETs, but is stopped before flowing through the rest of the ESC and back into the battery.

Again, just a guess. I wouldn't think designers would want current flowing both ways, especially when dealing with a device that has both sensitive sections, and sections made for crazy amounts of amperage output. I think we would need a designer to chime in to get the 100% correct answer.

Another option is the controlling of the FETs themselves. Maybe they are switched off when no throttle is applied, so no reverse current is allowed.

Thank you for the info, guys. Excellent post, Dave H. I had no idea about the history of RC, as I only started the hobby this year.
Would "back" current have to be regulated somehow? Why can't it be allowed to flow back sporadically and spiky?
Also, do 1:1 cars have some form of kinetic energy storage, like a flywheel, for regen braking?

I deleted the eagletree graphs, but I know the losi 1/8 esc allowed reverse current.

Spiking the brakes with a fully charged 4s resulted in a peak voltage above 18v.


I never graphed, but I suspect the tekin RS/pro does also. When I had the RX running directly from the battery the servo would freak out anytime I touched the brakes.




There isn't much danger, the reverse current already came from the battery, you won't overcharge it...

All of the MOSFETs used in our brushless speed controls have intrinsic diodes ("substrate diodes") as part of their construction. These diodes are actually in parallel with the MOSFETs, and are reverse-biased in normal operation. If the motor voltage should spike higher than the battery voltage (as usually happens during partial braking), then current will flow into the battery through these diodes. Every brushless ESC I know of operates this way, so it's not a special feature. In fact, it's usually required to prevent overvoltage damage to the MOSFETs.

Chrysler experimented a fair bit with a flywheel storage system, and there has been other research. Also some have experimented with super capacitors. But as far as I know all current production electric/hybrid cars just recharge the battery. Going back at least to the GM EV-1/Impact from the mid '90s or so.

In the future I suspect we will see suspension energy converted to the batteries. Shock absorbers burn off a noticeable amount of energy.

There is some construction equipment, primarily excavators I think, that use hydraulic accumulators to store energy. Since they already have significant hydraulic and related control systems I've heard it works quite well.

Mazda has a similar system in their new cars....