OK...noob q...where's the power go during hard coasting?

[DanGT86]

Not trying to make a big deal out of it. Just trying to gain a better understanding of how controllers work.

 

[John in CR]

So am I understanding it correctly that the controller can use switching/logic to actively control the voltage during regen but if you are spinning faster than the no load speed of the motor at a given voltage then it will generate regen power above that voltage regardless of the controller regen being enabled or the throttle position?

Also trying to check my understanding of regen in general.

It seems to me that the controller can manipulate the regen voltage any way it needs to make it higher than the pack voltage. Since it works even at low speeds the controller has to be stepping it up right?

Also if the regen voltage max is set higher than the battery voltage max is it a problem all the time or only when the pack is already full. I have my regen max set to 90v via R12 hack. My pack is full at 83v. Should I be concerned or only when regen braking downhill with a completely full pack? At first I was thinking it would only be a problem if I was going faster than the no load speed of my given motor at 83v. Now I'm not sure.

I'm thinking the pack would pull the regen voltage down just like it does when its discharged and I plug in the charger. Charger is at 83v no load but drops to pack voltage when connected. So if that is the case, having the regen voltage set too high will only be a threat to the pack when its full?

You don't want to regen brake going downhill if starting with a full battery at the top of the hill. It would overcharge your battery or if the max regen voltage is set properly in the controller you won't have any regen braking.

Yes, during any charging event there is something similar to the voltage sag of your battery under normal motor load, but it's the opposite. I call it reverse sag for lack of a better term. That's why during regen braking you can see battery voltage go higher and settles back down to a lower voltage when stopped.

Your charger provides a constant current to the battery until it gets up to the cutoff voltage. If it simply turned off then your battery would settle to a lower voltage. Instead the charger holds that cutoff voltage and keeps decreasing the current to some minimal current before turning off. That's the CCCV common charge approach used by most of our chargers (CC for Constant Current and CV for constant voltage).

You're right that during regen braking the controller steps up the voltage enough to charge the battery and they limit that charge current depending on the settings as well as controller mods like a shunt mod.

I don't doubt what Russell and riba2233 say about coasting downhill at speeds above the no-load speed adding something back to the battery, but it must be quite low level reverse current resulting from some type of leakage, not the full potential with no regulation. If an uncontrolled charge was occurring then everyone with a 20mph rig would end up burning systems up coasting down hills at 30mph, which can't be a highly unusual event. My bikes are too fast now, but back when I had a 30mph bike I don't remember noticing any significant drag when coasting well above no load speed, and I also never noticed any voltage increase like I regularly see with regen braking.

 

[Icewrench]

A 10 turn 9C in a 22 inch tire comes with this hogwash regen standard. Happens all the time at 18+ downhill. It is active until I pull the front ebrake or get down to no load speed. The Ca does read it. Do not go all kf on this. Accept it understand it do not fight it.

 

[Russell]

My bikes are too fast now, but back when I had a 30mph bike I don't remember noticing any significant drag when coasting well above no load speed, and I also never noticed any voltage increase like I regularly see with regen braking.

As I've been saying it has to be a slow bike. A "30mph bike" is likely going to have a no-load speed over 38 mph. It's that 38 mph speed you have to exceed to start seeing the regen/braking effect. Hills on my usual routes aren't steep enough or long enough to reach much more than mid to high 30's and with a 42/12 max gear combo my legs spin out in the low 30's so I can't help much. My record coasting speed btw is 50.4 mph down a 13% grade with a geared motor.

Anybody who has used the simulator at ebikes.ca has seen the negative power produced by a DD motor but probably hasn't paid attention to it. Here's something to try; set the parameters for a 36V battery, a 2807 motor, mountain bike, 260 lbs, 26" then set the slope to -10%. Without changing anything else change the motor to a geared motor. The speed prediction will be over 7 mph higher for the freewheeling geared motor.

-R

 

[dogman dan]

I see the same thing as Icewrench. If coasting enough faster than the motors no load, I do see some wattage on the CA.

But it's not big, with the controllers I have, Lyens 12 fet, or E-bikekit stock controllers. So some of that back EMF does leak through some controllers.

But it's nothing at all, compared to engaging the regen on my lyens controller. Regen engaged, I'm lucky to reach 8 mph going down a very steep hill. But just cogging down, you tend to reach a max speed close to your no load speed. Takes a hell of a hill for you to exceed the no load speed, and see 50w on the CA.

I find that with my 5304 and regen engaged, I make a tiny bit of power, enough to go about half a mile after a three mile hill. But I can coast farther than that, if I hit the bottom of that hill at 30 mph, vs 8 mph. So the best compromise is to regen the steepest part on the top of the hill, then the last half mile let speed build up to 25 mph or so.

If you have a hill with a stop at the bottom, regen of course, if you have it.

If you have a hotter than shit motor at the top of the hill, I'd say regen down is cooler than cogging down, but ideally you'd still get more wind than 5 mph on the motor going down. It just depends on weather, if you are too hot, and going 5 mph in a 5 mph tailwind, your motor will roast. Best approach I've found is a couple quarts of water to slowly pour on the motor at the top. Then no worries about heat going down.

 

[Voltron]

Well thanks for all the input. Looks like I need better instrumentation so I can give some quantifiable feedback about it...

 

[John in CR]

At least someone finally actually put a number on it, thanks Dogman. So 50W recovery on a "hell of a hill" is a whopping 0.83 wh per minute spent coasting down the hill. No wonder no one wanted to put a number on it. Since it's not enough current to do any damage I won't worry about it. I need to build some low speed ebikes for people who live in the mountains, and coasting downhill above no load speed it likely to be common. Uncontrollable regen current of significance would need to be addressed in the builds.

 

[Kyle201]

On my H4060, when I reached about 65kph on a downhill my CA reported regen current without actually activating regen with the e-brake. Think it was only about 50-100w, and probably had the throttle open.

 

[dogman dan]

The approximately 50w is all that leaks through the controllers I have. It doesn't always do it, only when above no load speed, which takes one hell of a hill. I have no idea if more is being generated, but it can't be that much, since it doesn't slow you going down a hill very much compared to real regen.

But going down a big hill can definitely continue to warm the motor. That's the heating of the motor, minus the cooling from going down the hill at 25-30 mph, resulting in a net that is still warming the motor.

This won't matter in normal operation, only a problem if you are at 250 F plus at the top of the hill, and frying your halls could happen any second. That's when you need to pour some water over your motor asap to speed the cool down. Or if on the flat, just do a 20 mph cool down cruise for 10 min.

 

[DanGT86]

I let my motor cool by doing hills like this!

https://youtu.be/4UzFbvwuU7U

 

[Voltron]

Oh yeah, well I do that on the uphill to stay cool for coming down! Not.....

But on a serious note, one of those guys from Road Bike Party missed a jump at a show at a bike festival, broke his back and is paralysed now. Wasn't moving real fast but landed in a bad spot....

 

[Voltron]

Thanks for all the input from everybody, feeling good now about the downhills not frying anything, as I go uphill first, so no full pack worries, the motor isn't getting crazy hot on the way up and there's plenty of flatter road along the crest for some cool down, and it doesn't sound like I'm going to shove so much current backwards that fireballs are going to be shooting out of anything.

Thanks again.