raising voltage to maximize regen. braking?

[qwerkus]

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.

Correct - thanks for pointing it out. I had 13s in my mind, which is 54.6V max volt. while 14s is 58.8V , and 58.8*5*2=588W. Total power does not change no matter the amount of cells, but power distribution does. More P cells can take higher current, more S cells higher voltage. So max regen will depend on motor winding - no point in plugging a 72V battery to a motor that only outputs 40-50Vmax regen voltage.

 

[thepronghorn]

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.

Correct - thanks for pointing it out. I had 13s in my mind, which is 54.6V max volt. while 14s is 58.8V. Total power does not change no matter the amount of cells, but power distribution does. More P cells can take higher current, more S cells higher voltage. So max regen will depend on motor winding - no point in plugging a 72V battery to a motor that only outputs 50max regen voltage.

Within reason, motor winding doesn't matter. Just like the controller bucks down the battery voltage to the motor phase voltage when driving the motor, it boosts the motor phase voltage up to the battery voltage during regen. So even if you're travelling at 1/2 speed and motor voltage is only 1/2 the battery voltage, the controller will boost the motor voltage up to the battery voltage to keep the regen current flowing.

This is why regen tapers off at lower speeds, once the motor voltage drops too low, it's difficult to boost it all the way back up to the battery voltage.

 

[qwerkus]

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.

Correct - thanks for pointing it out. I had 13s in my mind, which is 54.6V max volt. while 14s is 58.8V. Total power does not change no matter the amount of cells, but power distribution does. More P cells can take higher current, more S cells higher voltage. So max regen will depend on motor winding - no point in plugging a 72V battery to a motor that only outputs 50max regen voltage.

Within reason, motor winding doesn't matter. Just like the controller bucks down the battery voltage to the motor phase voltage when driving the motor, it boosts the motor phase voltage up to the battery voltage during regen. So even if you're travelling at 1/2 speed and motor voltage is only 1/2 the battery voltage, the controller will boost the motor voltage up to the battery voltage to keep the regen current flowing.

This is why regen tapers off at lower speeds, once the motor voltage drops too low, it's difficult to boost it all the way back up to the battery voltage.

OP question was about optimization, not possibility. Sure the controller acts as a DC regulator, but at some efficiency cost. If you want max output, first find out your average speed, than check regen output at that speed and match the battery. But even then, we are talking about very little effective gain - probably not something worth the trouble. The real deal would be a controller optimized for regen and even than, there are soo many posts in this forum with real-life regen values showing that optimizing a bike for regen is never worth it. It should always be optimized for lowest consumption in the first place and thats why a geared hub with no regen will net you a higher mileage in many scenarios than a dd hub with regen.

 

[donn]

Correct - thanks for pointing it out. I had 13s in my mind, which is 54.6V max volt

Just nitpicking here, but note that he has a LiFePO4 battery. I don't know details, but A123 26650 cells are rated for 10A charge, so I've been assuming a 1P configuration like mine.

Of course it doesn't matter what type of battery cell you use to illustrate the principle of the matter, but just wanted to point out that while some people here seem to think "14S" for example tells you the voltage of a battery pack, of course it doesn't really. You need to also know the cell voltage.

 

[goatman]

if it is the A123 at 4p for 10ah, it can handle 40amps. the bms is the issue.

i dont know why the bms could fry the PR, the PR is kicking off where OP has it set to kick off. thats why the regen braking stops

amps are amps

add S if you want more watts

upgrade the bms if you want more amps

as far as safety, add amps, in an emergency you dont want the PR kicking off the regen because of a 10amp regen limit

 

[Steph]

Within reason, motor winding doesn't matter. Just like the controller bucks down the battery voltage to the motor phase voltage when driving the motor, it boosts the motor phase voltage up to the battery voltage during regen.

OP question was about optimization, not possibility. Sure the controller acts as a DC regulator, but at some efficiency cost. If you want max output, first find out your average speed, than check regen output at that speed and match the battery.

What efficiency cost?
To me, it’s not like the DC regulation comes on top of the regen and adding it results in extra cost. The motor coil and the half-bridge make a buck-boost (btw, I think buck can also be involved in regen, especially with street legal bikes) circuit that is used for capturing the energy and adjusting it at the desired output voltage, all in one go. What will change when you change the battery or the motor winding is essentially the frequency at which the high fet is switched.

So what I can see as potential efficiency impact are the switching losses (a few tens of nC per switch, I don’t think that will amount to noticeable effect) and impedance losses (rc filter with caps and battery internal resistance) on higher harmonics of the output signal but I think that’s again quite negligible in front of the dc component.

Is there anything I missed in the picture?

 

[Addy]

What will change when you change the battery or the motor winding is essentially the frequency at which the high fet is switched.

So what I can see as potential efficiency impact are the switching losses (a few tens of nC per switch, I don’t think that will amount to noticeable effect) and impedance losses (rc filter with caps and battery internal resistance) on higher harmonics of the output signal but I think that’s again quite negligible in front of the dc component.

Is there anything I missed in the picture?

There is also the change in duty cycle when any DC/DC converter operates at different ratios of Vin / Vout. The greater the difference between Vin and Vout, the more narrow the duty cycle will be for the high/low FETs, depending on the conversion type. With a narrow duty cycle, a FET would spend more of it's time in the inefficient switching region and less time fully-on conducting efficiently.

Here's an example of a random boost converter's efficiency:



You can see that it's less efficient when Vin and Vout are further apart.

 

[Steph]

Thanks Addy,

Indeed, I didn't account for the losses during the switching phase of the fet. I was rather assuming that with a strong enough drive, the transition is sharp enough to be neglected, but with a tight duty-cycle the switch on and switch off phase may come too close and start to overlap (I think that's when the input cannot provide enough power anyway) and they cannot be neglected.

Then, I want to discuss the comparison with a pure DC-DC regulator.
In the case of the regulator, the regulation will be driven by the demand; that's why the the graph you shared has the output current in X-axis. That partly explains why, for a given output current, when the voltage difference is higher the efficiency is lower: the input current has to be that much higher, and that increases ohmic losses (in the coil; in the fet, all the more indeed when duty-cycle become tight).
In the case of the regen circuit, I think the regulation will be rather driven by the production: considering any input power, the circuit will try and produce a voltage that matches the charge voltage of the battery, with output current being the variable factor. So, considering ohmic losses, efficiency would be the highest towards lowest input power (speed . braking_force).
Now, I suppose the braking_force is determined by the switching pattern (I still have to look into this part to understand it better) and this is were it may become interesting: assuming we are hitting a limit with the duty cycle, maybe higher (or lower?) battery voltage could enable better regen (for braking, I think power output is in the form of heat so battery voltage doesn't really matter).

What do you think?

 

[Addy]

Steph, you raise a good point about the input current, I agree.

As for your question, I think a lower battery voltage would give you higher efficiency in recovering the momentum of the bike, for the reasons we discussed. When you are braking, the motor voltage (BEMF) is being reduced proportionally with your speed until it reaches 0 and you have come to a stop. With a higher battery voltage and the same motor, braking from the same speed, the motor voltage will be further away from the battery voltage, pushing the duty cycle closer to the limit.

 

[retrocycler]

Wow, so many interesting replies! Thank you all. Sorry I can't follow the discussion so closely / every day.
Now I'm somewhat confused, however!

Goatman wrote,

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

My question - What is "plug braking?"

querkus wrote,

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.

My reply: if this is true, then my plan to go to higher voltage is doomed. Is this also why thepronghorn wrote:

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.

My question - why is that? If individually the cells can take a certain maximum charge current, why would that be decreased at each cell, depending on the arrangement?

thepronghorn wrote, in reference to the motor controller transforming/boosting the motor phase voltage,

This is why regen tapers off at lower speeds, once the motor voltage drops too low, it's difficult to boost it all the way back up to the battery voltage.

My question - my experience has been the opposite. With my 36V battery, I'm getting the most braking torque at low speeds. I get quite useful braking say from 10 to 5MPH, but not much useful braking at say 20 to 15MPH. (at any given indicated regen. current, the braking torque is much lower at higher speed - but that makes sense to me).

donn pointed out,

but note that he has a LiFePO4 battery. I don't know details, but A123 26650 cells are rated for 10A charge, so I've been assuming a 1P configuration like mine.

My reply - I did recently learn this about the battery I have. It does not have the 18650 cells, but instead a lower voltage pouch type of cell. I also read up on the LiFePo chemistry, and learned that it is more robust and safer than other flavors of Lithium cells.

Goatman, following on donn's post, said,

if it is the A123 at 4p for 10ah, it can handle 40amps. the bms is the issue.

My reply: this is very interesting! Perhaps for my braking needs I should stick with LiFePo chemistry! I also learned that in my setup, the regen. power does not flow through the BMS. The BMS only controls charge current coming through the separate charging wires. So I don't think the BMS is causing the "cutout" of braking (see the other thread). If it's true that I could put 40A of regen. power back into this kind of battery (for brief periods), then my problem is solved, and I don't need to buy a new battery.

I tried to read/understand the discussion about "efficiency costs," most of which went over my head. Note that I'm not so much interested in the recapture of energy. I'm interested in maximizing the braking torque using a motor with regen. capability. It could just make the motor hotter, with zero energy going into the battery... that would be fine with me (except for I don't want to damage the motor)!

 

[goatman]

theres a link in this thread about plug braking to another thread about plug braking.
https://endless-sphere.com/forums/viewtopic.php?f=1&t=107940

if i was in your shoes id just bump the regen to 20 amps and try it but thats me.

 

[donn]

Goatman, following on donn's post, said,

if it is the A123 at 4p for 10ah, it can handle 40amps. the bms is the issue.

My reply: this is very interesting! Perhaps for my braking needs I should stick with LiFePo chemistry!

Not really ... I use LiFePO4 myself, but it's essentially the same as LiCo here. More pouches in parallel than you have, would give you a higher max charge than you have -- but if your battery configuration is essentially like mine, you have 10A max recharge in a "1P" configuration. I.e., you can ignore that.

I also learned that in my setup, the regen. power does not flow through the BMS. The BMS only controls charge current coming through the separate charging wires. So I don't think the BMS is causing the "cutout" of braking (see the other thread). If it's true that I could put 40A of regen. power back into this kind of battery (for brief periods), then my problem is solved, and I don't need to buy a new battery.

I guess you could try it, and if the battery doesn't last very long, for the replacement battery you could dial that back. The nice thing is that LiFeP04 is a little less likely to burst into flames when abused, though I believe it can happen.

 

[ichiban]

.


Just a thought :

I am no expert and still learning. Just want to share some idea, so please bear with me :

Enthusiasts here on ES are interested in regen braking regarding its motor's back-EMF vehicle stopping capability as an alternative but can be an effective primary brake. While energy re-capped from regen is minor 3%, 5%, or 10% to almost not attractive. So, charging back is just a small bonus. Bear in mind that, in order to utilize regen braking, regen charge back will always come together as a package. Or seeing in a different perspective, when we brake, motor will generate back EMF that we have to send to somewhere. And at that moment, regen braking force can be utilized.

One great thing about regen braking is that it can be a very effective main brake without depending on friction between disc rotors, pads, tires and road surface. It brakes by using internal force similar to car's engine brake. I recall reading from somewhere that regen brake can reduce mechanical brake usage as much as 80-90% if set correctly. :thumb:

Regenerative braking is all about “energy conservation”. When a vehicle travelling at a speed and has to stop, the momentum (= mass x speed, or energy) need to go somewhere, either to a mechanical brake (dissipated as heat) or electrically dumped to the battery as re-covered energy. Either way, this "resevoir" must be adequately large enough to absorb the brief but pwerful transferred energy.

Re : Pack's capacity & Cells type

The cells config (P & S) we choose for the batt pack, will that affect regen effectiveness ? Any other concerns / issues I should be awared ?

For examples, if I choose 3Ah cells, max chg at 1C= 3A, max dischg at 6A will it be less effective (in term of absorbing regen braking) to stop the vehicle than 3Ah cells, max chg at 2C 6A, max dischg at 20A ? To what degree ?

FMPOV, it is the max charging rate of the cells that matters (for regen braking power that will charge back to the batteries). Number of parallel strings is also important, since 5P can receive more current safely than 4P as a whole. This more Ps is like a bigger resevoir that can absorb more returning energy. More Ss is also important, since it will have high enough voltage (for the controller to convert volt generated from the motor to this batt pack's volt). Less Volt of pack means more Amp to charge back to pack, from Power = Volt x Amp and might cause it to exceed our overall Amp charge back since we have limited number of Ps. When regen power is a certain amount dictated by overall momentum.

However, regen charge back is just for seconds at a time when we normally brake to slow down. We never brake for minutes and will never do so. Will that hurt the batt pack much if we keep exceeding max Amp charge rate by not much and briefly but oftenly ? Considering we brake countless of time per ride. But the regen energy recovering rate of only 3%-10%, it is not much at all and average out with braking throughtout the trip, seconds at a time. It is not like charging the battery continuously for 30 minutes or so that over-current charging can cause damage and hurt longevity.

So, I think it will be unlikely that the regen charge back will damage our pack, if sized properly. And yes, I believe that higher pack's voltage will make your regen brake works more effectively. And setting higher regen amp will make you brake more powerfully. But you wil have to choose the right BMS as well.


Re : cells' chemistry

IMHO, I still believe in our old lovely Li-Ion for its robust, energy dense, super mass-produced, reliability, availability, cost (per kwh) and long established history. Of course, from the reputable brands and suppliers. Though there are some concerns like safety issues for Li-Ion, but those are well taken care of by proper protections (BMS, fuse, connections, wire gauge, etc.), good design and fabrication. As long as we do it properly and use it within limit, li-ion can be very dependable. Countless of Li-Ion packs have been in usages for decades and there have been very minor issues (per lithium batteries capita) in the scale of ppm or ppb. In my case, my 1st pack 14S5P of LG18650MJ1 has been charged about 250 times and still strongly going (takes longer to charge with same wh). I believe it should last over 30-40kkm with some degradation (may be 60-70% life left in it) and that's a lot for bike ! :thumb: :thumb:

After its retirement, I will use it for some re-purposed projects, might be for DIY power-wall, portable power pack or etc. That's how nice Li-Ion is.

Though LiFePO4 might be safer in some regards, but it is considerably less energy dense and cost more (wh/l, wh/kg and wh/$), less accessories to choose from, etc. So your batt pack will be bigger, heavier, and more expensive. From my own survey, LiFePO4 can be 2-3X less energy dense compared to Li-Ion. That means, for same wh pack, it can be 2-3 times bigger, heavier, and about double the cost.

So, it is a matter of choice since we know all the pros & cons.


.

 

[donn]

Re : cells' chemistry
... As long as we do it properly and use it within limit, li-ion can be very dependable. Countless of Li-Ion packs have been in usages for decades and there have been very minor issues (per lithium batteries capita) in the scale of ppm or ppb.
...
Though LiFePO4 might be safer in some regards, but it is considerably less energy dense and cost more (wh/l, wh/kg and wh/$), less accessories to choose from, etc. So your batt pack will be bigger, heavier, and more expensive. From my own survey, LiFePO4 can be 2-3X less energy dense compared to Li-Ion. That means, for same wh pack, it can be 2-3 times bigger, heavier, and about double the cost.

So, it is a matter of choice since we know all the pros & cons.

I would normally let this pass, but given your obvious interest in making sure we all know the pros & cons, I hope you won't mind.

"Lithium ion" is actually generic term that includes all Lithium battery chemistries, including LiFePO4. They're all safe enough to be widely used, when used "properly", but some people here have lost their houses anyway, so it's appropriate to regard them with some caution and not let our affection for them rule entirely.

The common lithium chemistry in ebike batteries is Lithium cobalt. LiCo does have a higher energy density than LiFePO4, but for example if I compare a Ping 48V LiFePO4 15Ah against a Luna 48V LiCo 13.5Ah, the LiFePO4 is only 1.5 times larger (and supposedly has more capacity, but who knows.) It isn't like a I need a trailer to carry my LiFePO4 battery around. It also has a relatively high maximum discharge current, which I believe is characteristic of LiFePO4 but haven't researched that.

LiFePO4 won't catch on fire when it's punctured, which is nice, but it also has a longer useful life, and it's made of materials that are more easily found in nature. That might be the principal attraction for me - cobalt comes largely from central African mines, many of which have been acquired by China and are operated under the most deplorable conditions, in human and environmental terms. So for me, that belongs among the pros & cons.

 

[ichiban]

I would normally let this pass, but given your obvious interest in making sure we all know the pros & cons, I hope you won't mind.

"Lithium ion" is actually generic term that includes all Lithium battery chemistries, including LiFePO4. They're all safe enough to be widely used, when used "properly", but some people here have lost their houses anyway, so it's appropriate to regard them with some caution and not let our affection for them rule entirely.

The common lithium chemistry in ebike batteries is Lithium cobalt. LiCo does have a higher energy density than LiFePO4, but for example if I compare a Ping 48V LiFePO4 15Ah against a Luna 48V LiCo 13.5Ah, the LiFePO4 is only 1.5 times larger (and supposedly has more capacity, but who knows.) It isn't like a I need a trailer to carry my LiFePO4 battery around. It also has a relatively high maximum discharge current, which I believe is characteristic of LiFePO4 but haven't researched that.

LiFePO4 won't catch on fire when it's punctured, which is nice, but it also has a longer useful life, and it's made of materials that are more easily found in nature. That might be the principal attraction for me - cobalt comes largely from central African mines, many of which have been acquired by China and are operated under the most deplorable conditions, in human and environmental terms. So for me, that belongs among the pros & cons.

Thanks donn. I actually don't mind your comment at all. It is great that each of us can contribute our experience for the DIY community

Feel like you are a knowledgable person and I actually need some help for that important build (as a gift for my son's BD - must be finished within this November). Please go & take a look at :

https://endless-sphere.com/forums/viewtopic.php?f=6&t=108610

Since I really have limited knowledge re hub motor and the system. Time is rushing too. Hope someone can help me with that. Grin Tech is very sluggish in responding to my e-mails, I will have to spend month(s) before I can get enough info to make purchase from them and it will be too late. Anyone know their back-door channel where I can get quick help ?

Hope the owner of this thread do not mind me hi-jacking some help here.

Appreciate any help there, please.

 

[thepronghorn]

....

querkus wrote,

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.

My reply: if this is true, then my plan to go to higher voltage is doomed. Is this also why thepronghorn wrote:

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.

My question - why is that? If individually the cells can take a certain maximum charge current, why would that be decreased at each cell, depending on the arrangement?

thepronghorn wrote, in reference to the motor controller transforming/boosting the motor phase voltage,

This is why regen tapers off at lower speeds, once the motor voltage drops too low, it's difficult to boost it all the way back up to the battery voltage.

My question - my experience has been the opposite. With my 36V battery, I'm getting the most braking torque at low speeds. I get quite useful braking say from 10 to 5MPH, but not much useful braking at say 20 to 15MPH. (at any given indicated regen. current, the braking torque is much lower at higher speed - but that makes sense to me).

I'm not saying the per cell charge current decreases, just saying the total pack charge current decreases. 4P at 10A is 2.5A/cell, so a 2P pack would only be able to do 5A total.

For the low speed question, I meant at very low speeds under a couple mph. As you understand, your charge power limit results in much stronger braking at lower speeds 10-5MPH than higher speeds 20-15MPH.

 

[donn]

I'm not saying the per cell charge current decreases, just saying the total pack charge current decreases. 4P at 10A is 2.5A/cell, so a 2P pack would only be able to do 5A total.

Sure - but 5A at 72V, vs 10A at 36V, isn't it? = 360W. The actual voltage from the hub is something else, transformed by the controller to the battery voltage. If the controller is going to limit the battery side amperage to 5A@72V or 10A@36V, that should be at least roughly the same motor amps in either case, if the transform is done so that watts in = watts out.

 

[thepronghorn]

I'm not saying the per cell charge current decreases, just saying the total pack charge current decreases. 4P at 10A is 2.5A/cell, so a 2P pack would only be able to do 5A total.

Sure - but 5A at 72V, vs 10A at 36V, isn't it? = 360W. The actual voltage from the hub is something else, transformed by the controller to the battery voltage. If the controller is going to limit the battery side amperage to 5A@72V or 10A@36V, that should be at least roughly the same motor amps in either case, if the transform is done so that watts in = watts out.

Yes

 

[retrocycler]

Ladies and Gentlemen, thanks again for all your replies!
I think I'll play a little more with my existing battery (36V LiFePo), see what difference 20A regen. current makes. If it makes a good difference, then I'll have something to go on for any future battery purchase - be it soon after (depending on how long the LiFePo lasts), or later on.
I understand now what was being said about current handling considerations in 1P vs. 2P vs. higher numbers. It's very obvious to me now... Assuming the same voltage, of course higher parallelism can take higher charge current.
Interesting to learn about the sourcing of Cobalt for LiCo batteries. I'm adding that to my considerations.

 

[retrocycler]

Thinking some more about what pronghorn wrote (among others), and wondering:

thepronghorn wrote: ↑20 Oct 2020, 22:46

I'm not saying the per cell charge current decreases, just saying the total pack charge current decreases. 4P at 10A is 2.5A/cell, so a 2P pack would only be able to do 5A total.

to which donn followed up:

Sure - but 5A at 72V, vs 10A at 36V, isn't it? = 360W. The actual voltage from the hub is something else, transformed by the controller to the battery voltage. If the controller is going to limit the battery side amperage to 5A@72V or 10A@36V, that should be at least roughly the same motor amps in either case, if the transform is done so that watts in = watts out.

My question: assuming the same capacity in the two battery packs (36V vs. 72V), why would the 72V pack take only 5A maximum charge current? Max. charge current for most packs appears to be related to capacity, i.e. =1C. (i.e. a 10Ah pack can take 10A max. charge current).

Wheel motor produced power is what makes braking torque. I want to maximize braking torque. With the current limited to a common value (e.g. 10A), it would seem that higher voltage would allow for more power.

Observed: comparing two battery packs, assuming the same voltage, higher parallelism (higher P number) means more capacity. More capacity means higher regen. current tolerance, which means more regen. power at that voltage, therefore better braking. But this higher capacity means more cells, and that means more cost.

Proposed: comparing two battery packs with the same capacity, a higher voltage pack (i.e. lower parallelism) can tolerate a higher regen. power, because at higher voltage, the same maximum current would mean more power.

I'm still hanging on to this idea of higher voltage pack giving better regen. braking performance, all other things being equal.
Somebody break my logic!

Maybe I'm wrong about the same capacity packs allowing for the same max. charge current, even with the higher voltage packs?

 

[E-HP]

Observed: comparing two battery packs, assuming the same voltage, higher parallelism (higher P number) means more capacity. More capacity means higher regen. current tolerance, which means more regen. power at that voltage, therefore better braking. But this higher capacity means more cells, and that means more cost.

Proposed: comparing two battery packs with the same capacity, a higher voltage pack (i.e. lower parallelism) can tolerate a higher regen. power, because at higher voltage, the same maximum current would mean more power.

I'm still hanging on to this idea of higher voltage pack giving better regen. braking performance, all other things being equal.
Somebody break my logic!

Maybe I'm wrong about the same capacity packs allowing for the same max. charge current, even with the higher voltage packs?

Personally, I don't think it matters much how much regen current goes into the battery, since it's not sustained long enough to heat up the pack. But, if you want data, I can regen down a 10%-15% hill and see what speed the bike slows to at 52V and 72V, with the same regen current setting. Per your theory 72v should provide more braking since it's more regen power if the max regen current is the same. I recall my bike slows to around 15-17 mph on a 15% grade, with my settings, but I haven't done an A/B comparison.

 

[John in CR]

The easy way to increase regen if the program setting is already maxed out is to do a shunt mod to trick the controller into raising the current limit of regen. Then lower the current limits for forward motion to tune it back down to the acceleration you already have.

Since braking force is limited by a fixed current amount in common controllers, I doubt increasing the pack voltage will increase braking force at all. Whether for forward motion or braking torque is limited by current with voltage not playing a role.

 

[thepronghorn]

Thinking some more about what pronghorn wrote (among others), and wondering:

thepronghorn wrote: ↑20 Oct 2020, 22:46

I'm not saying the per cell charge current decreases, just saying the total pack charge current decreases. 4P at 10A is 2.5A/cell, so a 2P pack would only be able to do 5A total.

to which donn followed up:

Sure - but 5A at 72V, vs 10A at 36V, isn't it? = 360W. The actual voltage from the hub is something else, transformed by the controller to the battery voltage. If the controller is going to limit the battery side amperage to 5A@72V or 10A@36V, that should be at least roughly the same motor amps in either case, if the transform is done so that watts in = watts out.

My question: assuming the same capacity in the two battery packs (36V vs. 72V), why would the 72V pack take only 5A maximum charge current? Max. charge current for most packs appears to be related to capacity, i.e. =1C. (i.e. a 10Ah pack can take 10A max. charge current).

Wheel motor produced power is what makes braking torque. I want to maximize braking torque. With the current limited to a common value (e.g. 10A), it would seem that higher voltage would allow for more power.

Observed: comparing two battery packs, assuming the same voltage, higher parallelism (higher P number) means more capacity. More capacity means higher regen. current tolerance, which means more regen. power at that voltage, therefore better braking. But this higher capacity means more cells, and that means more cost.

Proposed: comparing two battery packs with the same capacity, a higher voltage pack (i.e. lower parallelism) can tolerate a higher regen. power, because at higher voltage, the same maximum current would mean more power.

I'm still hanging on to this idea of higher voltage pack giving better regen. braking performance, all other things being equal.
Somebody break my logic!

Maybe I'm wrong about the same capacity packs allowing for the same max. charge current, even with the higher voltage packs?

When you say "capacity", what units are you using? If you mean amp-hour (Ah) capacity, then yes a 72V, 10Ah, 720Wh pack will accept more regen power than a 36V, 10Ah, 360Wh pack, but it will also be twice as heavy and twice as expensive. 1C charge rate on a 720Wh pack is 720W of regen vs. 360W for 1C on the 360Wh pack.

If you mean watt-hour (Wh) capacity, then no a 72V, 5Ah, 360Wh pack will not accept any more regen power than a 36V, 10Ah, 360Wh pack since 1C is the same 360W for both packs.

 

[John in CR]

I'm still hanging on to this idea of higher voltage pack giving better regen. braking performance, all other things being equal.
Somebody break my logic!

You need to let that go. Regen force is based on current not voltage or power. Controllers with regen have a separate current limit for regen, and just like current determines torque for acceleration it also determines the torque for braking.

 

[serious_sam]

I'm still hanging on to this idea of higher voltage pack giving better regen. braking performance, all other things being equal.
Somebody break my logic!

You need to let that go. Regen force is based on current not voltage or power. Controllers with regen have a separate current limit for regen, and just like current determines torque for acceleration it also determines the torque for braking.

Regen force/torque is proportional to phase current, yes.

But, the controller converts that Vphase x Iphase to Vbatt x Ibatt. So the higher the battery voltage, the lower the battery current to maintain that relationship.

And if the battery current limit is the limiting factor in the system (as it is on my bike - my 12F can push more regen than I'm willing to put into my pack, to maintain longevity), then raising Vbatt will increase capacity to accept more Aphase. Within the voltage limits of the controller of course.

On the flipside, if you used those extra cells in parallel with the pack (instead of series), you would also increase regen capacity of the pack by the same amount, and thats probably a better solution in many cases (as it would be on my bike, since I'm already at 20S).

At the end of the day, it's the addition of cells to the pack that increase it's capacity to accept or deliver more power, and if the pack is the limiting factor, then that addition will help increase regen power.