raising voltage to maximize regen. braking?

[retrocycler]

Hi all!
I've built a bike with regenerative braking using a GMAC10T hub motor. Currently the rig is powered by a 36V 10Ah battery, going through a Grin Tech Phaserunner. For my purposes, it is the braking that is more important than the regenerating.
I've had some difficulties with the regen. braking functionality, as described in this thread: https://endless-sphere.com/forums/viewtopic.php?f=2&t=107765 - Grin Tech has reached out to me, and we're still trying to figure out that problem.
Now I have a theoretical question / hunch: would powering my rig with a higher voltage battery increase both the capacity for and torque of regenerative braking? From my reading this morning, it seems like the hub would be just fine with a higher voltage battery.
My thinking is that under regen. braking, at any given RPM, the wheel would produce less current at a higher voltage. Thus for any given battery, it would take more braking to reach the regen. current limit (typically 1 x capacity). I also wonder if the wheel would produce more (negative) torque when regen. braking at higher voltage? This hunch from my reading, that the motor creates more (positive) torque when powered by higher voltage.
The way my bike is set up now, at 36 V the max. regen. current is -10 Amps, which is a current that the wheel generates very easily, especially at high RPMs (i.e. any speed higher than 15MPH for the 26 in. wheel I have). As a result, presently the braking force is minimal at high speed, and I'd like to improve that.
So, if instead I used a 52V 10Ah battery - likewise tolerating no more than about -10A regen. current - would I get more braking effect/torque due to the higher voltage that the hub needs to put out?
Thanks for any tips and insights!

 

[donn]

It sounds reasonable to me in principle, that a higher voltage battery could tolerate more regen current, but I'm no expert. I'm just wondering if when you get everything working right with your present setup, it will provide plenty of regen braking.

 

[qwerkus]

Hi all!
I've built a bike with regenerative braking using a GMAC10T hub motor. Currently the rig is powered by a 36V 10Ah battery, going through a Grin Tech Phaserunner. For my purposes, it is the braking that is more important than the regenerating.
I've had some difficulties with the regen. braking functionality, as described in this thread: https://endless-sphere.com/forums/viewtopic.php?f=2&t=107765 - Grin Tech has reached out to me, and we're still trying to figure out that problem.
Now I have a theoretical question / hunch: would powering my rig with a higher voltage battery increase both the capacity for and torque of regenerative braking? From my reading this morning, it seems like the hub would be just fine with a higher voltage battery.
My thinking is that under regen. braking, at any given RPM, the wheel would produce less current at a higher voltage. Thus for any given battery, it would take more braking to reach the regen. current limit (typically 1 x capacity). I also wonder if the wheel would produce more (negative) torque when regen. braking at higher voltage? This hunch from my reading, that the motor creates more (positive) torque when powered by higher voltage.
The way my bike is set up now, at 36 V the max. regen. current is -10 Amps, which is a current that the wheel generates very easily, especially at high RPMs (i.e. any speed higher than 15MPH for the 26 in. wheel I have). As a result, presently the braking force is minimal at high speed, and I'd like to improve that.
So, if instead I used a 52V 10Ah battery - likewise tolerating no more than about -10A regen. current - would I get more braking effect/torque due to the higher voltage that the hub needs to put out?
Thanks for any tips and insights!

No. Torque is a function of intensity, not voltage. For a given motor, and at the same power level, higher voltage = higher top speed but lower torque. Not sure about regen, but I guess it's similar.
I'd be curious about your setup though, since in most cases, power gain from regen is neglegeable so there is little incentive to optimize a build for regen.

 

[j bjork]

No. Torque is a function of intensity, not voltage. For a given motor, higher voltage = higher top speed but lower torque.

I am very curious about why higher voltage would result in lower torque :wink:

Well, I suppose higher voltage and the same amperage would give stronger regen. (more Watt) But I mean 10A.. Maybe try 20 or 30A on the same V you have now?

 

[qwerkus]

I am very curious about why higher voltage would result in lower torque :wink:

thanks - Corrected it

 

[AHicks]

I'd call Grin and ask them if they, or anyone they're aware of, has played with that higher voltage concept. Further, they may have another idea or 2 regarding how you might gain more/better braking. Would be a good chance to talk to them about both....

 

[goatman]

heres the rswannabe thread

https://endless-sphere.com/forums/viewtopic.php?f=6&t=103634&start=50#p1526265

 

[donn]

No. Torque is a function of intensity, not voltage. For a given motor, and at the same power level, higher voltage = higher top speed but lower torque. Not sure about regen, but I guess it's similar.

I'd be interested to know, about regen. Is the following reasonable as far as it goes?

He sets the controller's regenerative braking current limit at 10A.

When regen cuts in, the motor voltage depends on speed; let that be VM.
The motor current depends on his variable regen control; let that be AM; power WM leaving the motor is VM x AM.

The controller transforms (?) VM to battery voltage VB. Current is WM / VB ... and no more than 10A.
So (VM x AM) / VB = 10A, and the value he's trying to boost,
AM = 10A x (VB/VM)

QED, braking power limited at 10A at the battery, is greater if the battery voltage is greater.

 

[qwerkus]

I think this is getting OT. His question was if he can improve regen braking by switching to a higher voltage system. I think it's not necessary: the controller should be set to a higher regen current, which will increase braking force. This can easily be done with any cheap KT controller, and should be no problem for a phaserunner. Only a question of setup/config. No need for a new battery. If the regen current is too high, the bms OCP will kick in an protect the battery and all regen power will go into resistive heat. So the question is more about how much heat can his setup dissipate. So only if he reaches the 40-50A max that a phaserunner can take would it make sense to switch to a higher voltage system - which I doubt, and again, at the same power level he will loose torque and gain speed by doing that.

 

[goatman]

you can increase regen amps to 40amps. you can change torque ramp up rate from 200ms to 100ms.

to give you an idea of how regen torque ramp rate works, at 200ms if i hit my regen button at 60km/h id get 1500watts and braking is hard impact, i changed to 400ms and at 60km/h i hit the regen button, it comes on soft and i might get 800watts regen.

id crank up the regen amps and if thats not enough braking and want more change ramp up to 150ms or 100ms. play with it til your happy

 

[donn]

you can increase regen amps to 40amps.

Can he? Just casually looking around for guidance on this, all I see is, a lot of avoiding the question.

My battery's official maximum charge current is 10A.

I'm confident that I could exceed that by a few amps in brief regen braking, and it wouldn't burst into flame, but I have common fixed regen, set fairly low, and in practice I don't go much over 5A. I wouldn't care to throw 40A at it. (52V battery, by the way.)

But when people ask that question here, they kind of get a run-around. You don't need that much regen, energy recovery at high regen is less efficient, bla bla. I mean, interesting discussion, but is there a way to know how hard you can push a battery? If my battery's 10A limit is enforced by BMS, then what happens at the motor - extra current turns into heat?

 

[goatman]

you can increase regen amps to 40amps.

Can he? Just casually looking around for guidance on this, all I see is, a lot of avoiding the question.

My battery's official maximum charge current is 10A.

I'm confident that I could exceed that by a few amps in brief regen braking, and it wouldn't burst into flame, but I have common fixed regen, set fairly low, and in practice I don't go much over 5W. I wouldn't care to throw 40A at it. (52V battery, by the way.)

But when people ask that question here, they kind of get a run-around. You don't need that much regen, energy recovery at high regen is less efficient, bla bla. I mean, interesting discussion, but is there a way to know how hard you can push a battery? If my battery's 10A limit is enforced by BMS, then what happens at the motor - extra current turns into heat?

https://endless-sphere.com/forums/viewtopic.php?f=6&t=103634&start=50#p1526265

jumping the amps to 40 is like putiing the speed limit at 99 in CA. the regen will produce only so many watts and if using throttle for regen, it will roll on at your descretion but simple math by seeing the regen watts will tell you what the max amps being produced are and then decide what to do with the settings.

500 watts regen isnt bad but 800 is better. 36v battery at 20 amps regen is 720 watts. try 20 amps

 

[retrocycler]

Thanks for everyone's replies, some of which make sense to me, even though I don't believe I have an answer yet.

I've already played with all the possible settings on my phaserunner, as described in the other thread that I already mentioned: https://endless-sphere.com/forums/viewtopic.php?f=2&t=107765 In that thread, I include data logs of short rides (data logged from a Cycle Analyst v.3), testing my regen. braking problem. On that question / thread, I'm already in talks with Grin Tech, but the problem has not been solved. This thread about possibly using higher voltage batteries is an attempt at a workaround.

I don't think I can simply set the Phaserunner to permit e.g. 40A of regen. current. Then the battery BMS will disconnect the battery, and Grin tells me this could result in damage to the Phaserunner.

I hope to repeat my question with more clarity - and I wish to direct my question to you, who have experimented with this on your electric motor rig, or who know enough about motors to be able to tell me in theory...

The battery will limit regen. current to 10A. I have a hub motor that can run on forward power with voltages ranging from 37 to 72V. I'm using a Phaserunner, also able to accomodate that range of voltages, and which I'll use to set the max regen. current at 10A. At a given speed, will I get stronger regen. braking torque from a 37V or a 72V battery? Or asked another way, which battery system will hit the 10A regen. current limit sooner, as I increase regen. braking using a throttle?

I understand that in general, at higher speeds I get less torque. This is certainly true for "reverse" torque using regen. braking. I've already observed this with the present setup, and it makes logical sense to me (higher wheel RPM will produce a given level of regen. power with less torque). But will the higher voltage setup get me comparatively more torque at a given speed, or, if not more torque, then comparatively less current at a given speed?

One reply (above) suggests that higher voltage into the motor produces less forward torque. I'm inclined to believe the opposite is true. I imagine that with a higher voltage input, the motor would produce a given torque with less current. (torque x RPM is arguably Power. Power = V x current.) This is the thinking that leads me to the question about increasing voltage in the system, to increase regen. braking torque.

I'm really enjoying the replies, and learning!

Boris

PS
I understand that I can use a 37V battery with lots more capacity (say, 20 or 30 Ah), and in this way get a battery more tolerant of higher regen. current. and thus increase braking. But this solution is more costly and heavier and larger. I don't need more than about 10Ah capacity, so that's why I'm asking about the higher voltage angle. I'd like to minimize cost, weight, and size of battery.

 

[thepronghorn]

For a given battery cell, there is going to be a certain limit on the charge rate which limits the regen power. If you want more regen power, you need more cells. Whether you arrange the cells in series for higher voltage or parallel for higher current doesn't matter from the battery's point of view.

The controller also has current and voltage limits, but with 10A on a 37V battery, the phaserunner has headroom in both departments.

 

[goatman]

what cells are you using in your battery?

 

[serious_sam]

The battery will limit regen. current to 10A. I have a hub motor that can run on forward power with voltages ranging from 37 to 72V. I'm using a Phaserunner, also able to accomodate that range of voltages, and which I'll use to set the max regen. current at 10A. At a given speed, will I get stronger regen. braking torque from a 37V or a 72V battery?

Based on those conditions, the higher voltage battery will allow more regen torque.

 

[retrocycler]

Hi again.

Pronghorn, that is a very interesting statement, and begs questions about definitions. You said:

... there is going to be a certain limit on the charge rate which limits the regen power. If you want more regen power, you need more cells.

Is that true? The charge rate would be the current, wouldn't it? It is the charge current that is limited. But current is not power. Now I think that it is power which gives me the braking torque; more power = more torque (at a given RPM). So then, I think more voltage will give me more power at a given RPM. This is surely true, if I were accelerating the wheel against a load; my question is simply, will it be true in the reverse, when the wheel motor is making power, and the battery is receiving it.

To put your comment about cells to direct use: I'm guessing my 36V battery is 10S4P (40cells). The same capacity 72V battery would simply be a different arrangement, like 20S2P (again 40 cells). It would seem to me, that I could pump more power into 20 cells wired in series, than 10 cells wired in series. The 72V battery could handle more power coming in via regen. braking, and this translates into better braking.

goatman: I have a Ping 36V 10Ah battery with their v.5 BMS - the cell chemistry is LiFePo4
Herewith some specs from the pingbattery.com web page:

Charging Current: <8 Amps (<10 Amps with High Rate BMS)
Rated Discharging Amperage: 10 Amps
Max Continuous Discharging Amperage: 20 Amps (50 Amps with High Rate BMS)
Maximum Discharging Current: 60 Amps
Discharging Cut-off Protection: 25-30 Amps (65-70 Amps with High Rate BMS)


Serious_sam: your statement, though short, meets my expectations. But, before I spring for a higher voltage battery, I'd like some more reassurance.

 

[E-HP]

regen braking strength is going to be proportional to regen energy. if you have a constraint on current, then increasing the voltage will increase regen energy, with a similar increase in braking strength as increasing current would. V x I = P

interesting direction you're taking. i haven't heard of anyone making their decision to increase voltage due to a regen braking need.

 

[goatman]

a question

are regen amps measured as phase amps? ive never put a watt meter in between the battery and controller to see if the regen amps are battery amps or phase amps

also the strong braking you feel, is that the plug braking that kicks in at/below 10mph

 

[E-HP]

a question

are regen amps measured as phase amps? ive never put a watt meter in between the battery and controller to see if the regen amps are battery amps or phase amps

also the strong braking you feel, is that the plug braking that kicks in at/below 10mph

I think the cycle analyst regen amps are battery amps, since the only signal inputs to the CA besides speed (hall input) are from the current shunt between the battery and controller (or shunt inside the controller).

 

[qwerkus]

For a given battery cell, there is going to be a certain limit on the charge rate which limits the regen power. If you want more regen power, you need more cells. Whether you arrange the cells in series for higher voltage or parallel for higher current doesn't matter from the battery's point of view.

The controller also has current and voltage limits, but with 10A on a 37V battery, the phaserunner has headroom in both departments.

Do you have experimental data to support this ? In my experience adding more P cells works better than a longer serie. Reason for that is that internal resistance of cells is greater than properly sized nickel/copper between cells. Hence putting a long S series under high current load will charge ending cells faster than mid cells, and cause balancing issues if repeated / long enough, as most bms only balance 50 to 150ma. This is true for both direction (charge and discharge) and usually means hours or even days of rebalancing.
For an instance I just had to repair a 13s4p battery for a client because of too early UVP. Turns out the 2x mid groups where out of balance because he would push up to 20-25A load and never charge the battery long enough to properly rebalance. Might be an extreme case, but seems to me like a good example.

 

[donn]

Do you have experimental data to support this ? In my experience adding more P cells works better than a longer serie

Sure, but I think you're talking about something else.

The original was, I think, about available regenerative braking force given a 10A charge limit.
If you can put in only 10A at the battery, then a 36V battery can dispose of 360W, a 72V battery can dispose of 720W.
The motor's voltage will in either case be whatever comes from the RPMs, and there's a thought that with greater power going out to the battery, you might(?) get more braking force.

Most of the replies kind of skirt the question. The one you replied to seems to be saying you won't actually have the same charge limit, because the real limit is on charge power, and the same set of cells will accept the same charge power regardless of how they're configured. I don't think that's true, but at any rate it's a different question from "how do I make a reliable battery from some cells."

 

[qwerkus]

Do you have experimental data to support this ? In my experience adding more P cells works better than a longer serie

Sure, but I think you're talking about something else.

The original was, I think, about available regenerative braking force given a 10A charge limit.
If you can put in only 10A at the battery, then a 36V battery can dispose of 360W, a 72V battery can dispose of 720W.
The motor's voltage will in either case be whatever comes from the RPMs, and there's a thought that with greater power going out to the battery, you might(?) get more braking force.

Most of the replies kind of skirt the question. The one you replied to seems to be saying you won't actually have the same charge limit, because the real limit is on charge power, and the same set of cells will accept the same charge power regardless of how they're configured. I don't think that's true, but at any rate it's a different question from "how do I make a reliable battery from some cells."

I did get the OP question but still think going for higher current is the easier way. His 10s4p (i guess) pack is probably too weak anyway, so if he chooses to get a new pack, my point is that more P cells work better under high load than a longer S serie - ergo 10s7p will take a higher regen load than 14s5p. If you distribute 2a charge per cell (which is a lot!), first one will take a max of 42*7*2=588W while the second one will only take 54.6*5*2=546W - that's of course if the first pack is fitted with a 14A-charge-bms and the second one with a 10A. The BMS should not be the limiting factor, but cell type and distribution. If the bms is too weak, but the pack can take it, just swap in a more powerful bms.

EDIT: 14S=58.8V max voltage so 58.8*5*2=588W - total power remains unchanged.

 

[thepronghorn]

Hi again.

Pronghorn, that is a very interesting statement, and begs questions about definitions. You said:

... there is going to be a certain limit on the charge rate which limits the regen power. If you want more regen power, you need more cells.

Is that true? The charge rate would be the current, wouldn't it? It is the charge current that is limited. But current is not power. Now I think that it is power which gives me the braking torque; more power = more torque (at a given RPM). So then, I think more voltage will give me more power at a given RPM. This is surely true, if I were accelerating the wheel against a load; my question is simply, will it be true in the reverse, when the wheel motor is making power, and the battery is receiving it.

To put your comment about cells to direct use: I'm guessing my 36V battery is 10S4P (40cells). The same capacity 72V battery would simply be a different arrangement, like 20S2P (again 40 cells). It would seem to me, that I could pump more power into 20 cells wired in series, than 10 cells wired in series. The 72V battery could handle more power coming in via regen. braking, and this translates into better braking.

goatman: I have a Ping 36V 10Ah battery with their v.5 BMS - the cell chemistry is LiFePo4
Herewith some specs from the pingbattery.com web page:

Charging Current: <8 Amps (<10 Amps with High Rate BMS)
Rated Discharging Amperage: 10 Amps
Max Continuous Discharging Amperage: 20 Amps (50 Amps with High Rate BMS)
Maximum Discharging Current: 60 Amps
Discharging Cut-off Protection: 25-30 Amps (65-70 Amps with High Rate BMS)


Serious_sam: your statement, though short, meets my expectations. But, before I spring for a higher voltage battery, I'd like some more reassurance.

If you reconfigure your battery from 10S4P to 20S2P, wouldn't you have to 1/2 your battery regen current to 5A? The charge rate can be expressed in terms of power or current, but either way I think you are saying it's limited by your cells. If 4P can take 10A charge current, 2P can only take 5A charge current. You'll still have the same strength of regen because you'll still have 360W of regen, but it won't increase.

Personally, besides working with Grin to fix the regen cutout issue when the current limit is reached, I would probably just increase the battery regen current above 10A for your existing battery if I wanted stronger regen. I doubt the BMS will trip at 10A when regening through the discharge leads. The battery can probably take short bursts of charge current higher than 10A with minimal impact on the battery lifespan.

 

[thepronghorn]

Do you have experimental data to support this ? In my experience adding more P cells works better than a longer serie

Sure, but I think you're talking about something else.

The original was, I think, about available regenerative braking force given a 10A charge limit.
If you can put in only 10A at the battery, then a 36V battery can dispose of 360W, a 72V battery can dispose of 720W.
The motor's voltage will in either case be whatever comes from the RPMs, and there's a thought that with greater power going out to the battery, you might(?) get more braking force.

Most of the replies kind of skirt the question. The one you replied to seems to be saying you won't actually have the same charge limit, because the real limit is on charge power, and the same set of cells will accept the same charge power regardless of how they're configured. I don't think that's true, but at any rate it's a different question from "how do I make a reliable battery from some cells."

I did get the OP question but still think going for higher current is the easier way. His 10s4p (i guess) pack is probably too weak anyway, so if he chooses to get a new pack, my point is that more P cells work better under high load than a longer S serie - ergo 10s7p will take a higher regen load than 14s5p. If you distribute 2a charge per cell (which is a lot!), first one will take a max of 42*7*2=588W while the second one will only take 54.6*5*2=546W - that's of course if the first pack is fitted with a 14A-charge-bms and the second one with a 10A. The BMS should not be the limiting factor, but cell type and distribution. If the bms is too weak, but the pack can take it, just swap in a more powerful bms.

I think you have an error in your voltage calculations. 42V for a 10S battery is 4.2V/cell. 54.6V for a 14S battery is 3.9V/cell. If you redo your math with the correct voltages, you'll find that the charge power is exactly the same no matter which way you configure the cells.