Baserunner v4 regeneration voltage limit

[offGridDownUnder]

I have read through the manual, and don't recognize an answer to this question.

If the speed is high enough when regeneration happens, the voltage can exceed the voltage of the battery (48v in my case, but up to 52v nominal for a Baserunner). I've read some suggestion that when this happens the system stops accepting the power and regeneration ceases to slow the motor - the braking effect cuts out.

I already have a 1.54 reduction from my motor to my rear wheel, and I'd like it to be lower. This risks pushing the generated voltage when braking above what the system will accept. Is there a setting to tell the Baserunner "do not put more than this voltage back into the battery"? I would expect that would be a limit on the braking force possible - are there other consequences?

 

[amberwolf]

If the speed is high enough when regeneration happens, the voltage can exceed the voltage of the battery (48v in my case, but up to 52v nominal for a Baserunner). I've read some suggestion that when this happens the system stops accepting the power and regeneration ceases to slow the motor - the braking effect cuts out.

If by "system stops accepting the power" you mean that the battery's BMS disconnects the cells from the controller...yes, that should happen if the BMS is a common port design, (no separate C- and P-), whenever any cell exceeds HVC as designed or programmed into the BMS (or if the BMS has a full-pack HVC, same thing).

When this event occurs, the controller no longer has a load to dump current into, and voltage can then spike beyond what the controller FETs and other parts on the battery bus can handle, damaging or destroying the controller. It could happen very very quickly, faster than the controller can stop the regen process. (the controller is still powered by that process, so it doesn't actually know the battery is disconnected).

Is there a setting to tell the Baserunner "do not put more than this voltage back into the battery"?

It is in the Battery Limits section, under the Max Regen settings, in the Phaserunner Setup Suite program. If you hover over the settings, the tooltip popup explains the details.

 

[offGridDownUnder]

It is in the Battery Limits section, under the Max Regen settings, in the Phaserunner Setup Suite program. If you hover over the settings, the tooltip popup explains the details.

I'll read, but quickly ask - is limiting this adequate to prevent the disaster you outline in the first part of your reply? That is, do you think I may rely on this even if I push the gearing lower, and the overspeed higher?

 

[amberwolf]

theoretically, yes, if the limit is set far enough below the pack HVC so that rapid regen braking voltage increases wont' be so fast that the controller can't stop it in time****. If there is a suggestion or guide in the manual for this limit, I'd follow that--they probably already tested for this sort of thing. If there isn't, I don't know enough about the controller's response times, etc., to suggest anything specific.

I don't use a BMS so there's no chance of the potential failure (well, wires and connections *could* fail, or a fuse could blow, but it's very unlikely under those specific circumstances), so I have mine set to the actual max full charge voltage of the cells x series count (I don't normally charge that high, but the regen is allowed to).


****(this is part of why many controllers cease regen well below the max voltage that the controller itself is rated for...that so many people complain about and start threads to hack the controller to bypass this)

 

[offGridDownUnder]

I also put this question to Grin support, and Justin replied:

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On Tue, Jan 9, 2024 at 3:42 PM B W <bcw1000@gmail.com> wrote:

I have read through the manual, and don't recognize an answer to this question.

If the speed is high enough when regeneration happens, the voltage can exceed the voltage of the battery (48v in my case, but up to 52v nominal for a Baserunner). I've read some suggestion that when this happens the system stops accepting the power and regeneration ceases to slow the motor - the braking effect cuts out.

That is not true. In general if the back-emf of the motor exceeds the battery voltage, there is absolutely nothing the motor controller can do about this fact or stop the current flow. Current will flow through the body diodes of the mosfets and you will experience a regen-like force that clamps the bike speed from traveling much faster, and current will flow into your battery pack. You can't turn off a diode.

However, you can in effect "cheat" this behavior with field weakening which will put positive current into the phases when the back-emf voltage is low, and then that will resist a negative current from flowing the rest of the time. So in that case enabled enough field weakening that the top speed of the wheel unloaded is as high as you ever expect to get cruising downhill. Then you have your normal max regen voltage limit in place and regen is only actuated by an actual regen command.

I already have a 1.54 reduction from my motor to my rear wheel, and I'd like it to be lower. This risks pushing the generated voltage when braking above what the system will accept. Is there a setting to tell the Baserunner "do not put more than this voltage back into the battery"?

The only options are to use active field weakening, or you use a faster wind motor that will not reach this point since the back -emf voltage is less when you have a faster motor.

-Justin

 

[amberwolf]

Well, what actually happens depends on your battery design and the specifics of the situation.

if your battery is below it's full voltage, then what he describes is correct.

No problem exists in this event.


What he describes will also happen if the battery is near full voltage (such that regen current causes the voltage to rise above full), and has no BMS, no HVC, or is separate charge/discharge port so that the BMS cannot disconnect it from the controller, so current can continue to flow.

Only in that event is there a potential problem at the battery (if charging continues long enough, it could overcharge the battery, but there's no problem at the controller.


But if it has a common-port BMS with an HVC that causes it to disconnect from the controller during such a regen event (if it was already near full voltage such that regen current causes the voltage to rise above full) then the current from the controller to it cannot continue flowing (and hence regen braking does not continue).

Only in that last event is there a potential problem at the controller. (the battery shouldn't have a problem because it's disconnected by it's BMS)



If your original question wasn't about any of these events, or your system isn't wired up in any of these ways, then those may not apply to your situation.

 

[offGridDownUnder]

Has anyone measured the back EMF volts/per rpm of the GMAC motor?

That would be very helpful.

 

[amberwolf]

AFAIK the Kv of a motor as a generator should be the same as the Kv of it as a motor.

The GMAC 10T is 8.1 rpm per V; invert that and you get 0.12345679 volts per RPM. (wierd number, huh?!)

The GMAC 8T is 10.2 rpm per V; invert that and you get 0.09803921 volts per RPM

The actual voltage you get out depends on the current flowing thru it at the time, because of the winding resistance.

This page should have the math for everything, but I can't follow it:

Motor constants - Wikipedia

en.wikipedia.org

 

[offGridDownUnder]

My primary concern is to determine how much reduction gearing I can safely use with my fast-wind GMAC 8T or my fast-wind SX2 (with the clutch locked by Grin)? My trike will reach 60km/hr downhill if I don't brake and I want to determine if I need to limit its speed to avoid burning my motor or controller?

What is the maximum motor rpm to not damage the motor or Baserunner or Phaserunner? That is the real question. I am using a nominal 48V battery.

 

[lnanek]

IME the Baserunner just turns itself off and you have to go clear the over voltage fault in the Phaserunner suite software if you go too fast downhill without a clutch. Never managed to fry one that way yet.

 

[alias5000]

Yep, the newer base runners have a 60V limit due to the low Mosfet voltage rating they use. If you have a 52V battery it's very easy to Regen yourself to 60V momentarily (especially when charged full) and see the controller going into voltage protection.
The overspeeding issue that Justin is talking about is another one depending on motor pairing and speed, but a separate issue. Field weakening will indeed get you a few more km/h out of that issue (validated on a standard wind GMAC on 20in wheels and 52V battery)