Low Power, High Regen?

I'm considering to build an LTO (lithium titanate oxide) ebike using 40ah cells...

Since the energy density kind of sucks, i want to keep the power low (around 20mph). But seeing as these cells can take a massive 10C (400a), i want to take advantage of regen braking. I don't mind using an oversized motor just for this purpose if i have to, since i want to have extremely strong regen braking that can be used almost exclusively. Weight isnt a big deal.

But my question is, what kind of controller would i need for this? I guess i need a high power controller? But could i still limit power while keeping the high regen?

Very odd to shop for a motor based on cell chemistry.

Do you already have the cells, getting them for free or something?

The Ah capacity of the cells are not relevant, since you can parallel as many as you need to get to a certain pack capacity.

Regen is not a significant source of overall energy, realistically might add 10-15% in most scenarios.

Certainly not enough to be a factor in how powerful (heavy) a motor you select.

Also, the fact that density is low, means you need **more** power to carry the pack around, not less.

Weight is **always** a big deal for a moving vehicle, that **is** the use case we're discussing here right?

Speed has little to do with power, the power required to get around at 20mph top speed can vary enormously depends on the total vehicle weight, slopes involved and acceleration desired.

When discussing power, use watts instead, or amps even better once you've decided on a system voltage.

Limiting your amps is done by lightening the load, avoiding hills, driving slowly and only accelerating gently.

In the end you need enough power to do the job, with enough extra available for when it's needed, even if you only use it once in a while.

Having a controller **capable** of feeding high amps to the motor when needed

does not make the overall system less efficient, as in consume more Ah per mile.

No i don't have the cells, i'm just interested in using this chemistry due to it's characteristics. The AH of the cells are relevant, because these are massive 40ah cells that cannot be paralled due to lack of space. And i mention their capacity because of regen braking...

I was under the impression that regen is insignificant on most ebikes because the batteries can't handle enough current (also the reason the braking sucks). If a battery can take 400a then can't i regen brake extremely hard and feed all that power to the battery?

I'm really asking about the controller. I'm pointing out that one one hand i'm using low power, but on the other hand want to generate high power through regen braking. My point about the motor was just that i may need to oversize it a bit (in comparison to the low output) to avoid overheating during hard regen braking. But maybe not since they are only temporary spikes.

Username1 said:

I was under the impression that regen is insignificant on most ebikes because the batteries can't handle enough current

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Nope, just not that much energy to recover, except in a steep long mountain context on the downhill half of your trip.

I hope you don't think activating brakes unnecessarily will be a net gain? 8-D

> I'm really asking about the controller.

I realize that. My point is you misunderstand so much about the big picture, you don't realize that detail is a very minor one.

Any decent controller with the variable regen feature will do, but as I said the main overall benefit not the energy recaptured but savings on not replacing brake pads so often.

I consider regen to be **very** important, because a heavy cargo / tandem going down long steep hills gets very dangerous without it.

But dissipating the heat through heat sinks or resistors is just as valuable as putting it back into the battery.

> I'm pointing out that one one hand i'm using low power

Why do you still think that? For the umpteenth time, carrying around a lot of extra weight means you need to use **more** power.

In reality that power needed to propel the vehicle is greater and more important than anything to do with regen.

> but on the other hand want to generate high power through regen braking

1. why?

2. you won't

Username1 said:

No i don't have the cells, i'm just interested in using this chemistry due to it's characteristics

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If that really is your motivation, better to scratch that itch with a non propulsion use case, maybe set up some solar panels in the back yard?

john61ct said:

I hope you don't think activating brakes unnecessarily will be a net gain? 8-D

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Of course not.

john61ct said:

Why do you still think that? For the umpteenth time, carrying around a lot of extra weight means you need to use **more** power.

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I never said more weight doesn't use more power, of course it does. I'm saying that i'm limiting my speed and power to a modest level regardless of the weight. It will weight what it weighs.

john61ct said:

In reality that power needed to propel the vehicle is greater and more important than anything to do with regen.

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So if i'm using only 30a max for riding, but generate 300a when braking, the 300a figure is meaningless? I dont think so.

john61ct said:

> but on the other hand want to generate high power through regen braking

1. why?

2. you won't

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1. Because like i said, i want a very powerful rear regen brake (capable of locking the wheel if possible).

2. I will. Hundreds of amps is not high power? Should i try running that through a 30a controller then?

john61ct said:

If that really is your motivation, better to scratch that itch with a non propulsion use case, maybe set up some solar panels in the back yard?

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As i said in my other thread, i'm not doing it just for novelty. I mainly want the extremely long cell life and subzero abilities. Fast charging also.

Current can be very high but not involve much energy gained or lost.

I've measured amps used to crank a big engine at 800+ amps.

However, the 20Ah battery used was not depleted more than half a percent.

You seem to think i'm expecting massive energy gains. Not the case. The main reason is like you said, to negate wear on the brakes. But it's also the case that a heavy, wide tire, 10c charging capable ebike is perfectly suited to take advantage of regen energy.

I could just get a 30 amp controller (or whatever small size), but then my braking force (and also energy recovery) is limited correct? So i'm asking can i use a very powerful controller for the braking capability, while also limiting the power for acceleration (meaning not available at the throttle)? I probably should have been a little clearer in my first post.

Yes, thought I already said, use a CAv3 if the controller can't be programmed that way by itself.

Username1 said:

The main reason is like you said, to negate wear on the brakes. But it's also the case that a heavy, wide tire, 10c charging capable ebike is perfectly suited to take advantage of regen energy.

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Just keep in mind, if you want to use regen, then you need to charge your cells lower than full charge so they can accept the regen charge. The thing that most affects the reduction in SOC needed is your cell/pack internal resistance.

If you have regular brakes too, then you can just run your pack down as you ride, and your regen braking ability will increase as the pack drains. But if regen is your only source of braking, then you need to take the initial SOC into account, to enable the level of braking you want. This can be tricky, because regular braking might not need much current, but emergency braking will. Do you keep enough voltage headroom for emergency braking ? How does that affect your range ?

As a random example, see the attached image/file. Fairly self explanatory. Green cells are input variables. Red cells are results. This is a simplified view, but gives a ballpark estimate of what you're looking at.

View attachment regen.xlsx
Also note that the controller has to boost motor voltage above pack voltage, to feed current back into the pack. As the motor slows to a stop, it becomes increasingly difficult to extract that braking power and convert to electricity. So regen effectivness is reduced at very low speeds. Some controllers can't brake you to a full stop. Some controllers have the ability to feed power back into the motor in reverse to enable full stop braking, but not all controllers can do that. One example that can is Nucular. That is the only one I have experience with personally.

serious_sam said:

Also note that the controller has to boost motor voltage above pack voltage, to feed current back into the pack. As the motor slows to a stop, it becomes increasingly difficult to extract that braking power and convert to electricity. So regen effectivness is reduced at very low speeds. Some controllers can't brake you to a full stop. Some controllers have the ability to feed power back into the motor in reverse to enable full stop braking, but not all controllers can do that. One example that can is Nucular. That is the only one I have experience with personally.

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Does this mean a lower voltage battery can regen brake down to a lower speed, since it requires less voltage to feed the battery?

Amberwolf is currently working on a controller, that he wants the features that you are talking about in a good price range. It's a work in progress.

Username1 said:

Does this mean a lower voltage battery can regen brake down to a lower speed, since it requires less voltage to feed the battery?

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Not necessarily. In my understanding, pulling the power out of the motor at lower speeds becomes more difficult for the controller to handle. At a certain point, the braking effort tapers off to zero before the bike has fully stopped. Like I said, some controllers can brake to a full stop (like the Nucular). If you're looking to replace a physical brake with regen, then you should get a controller with some programmability that can do a full stop.

Out of production now but the adaptto controllers had the ability you are after. You can set all the parameters for power, Regen, charge voltage termination (say 80%soc) and then Max allowable voltage (for Regen).
Super helpful for me as I live on a hill and Regen after about 5mts of riding for a bout 1km.
Regen is awesome, and esp useful with heavy loads on long decents.
I would imagine you'll need to get a controller than can handle high power levels and program the power limits to limit battery current but leave the regen current set to high amps. I doubt you'll find a low power controller that has powerful regen as the mosfets need to be able to handle both.

Do not "replace" mechanical braking with regen kit,

the regen helps save wear and tear, great for long-run drag braking without brakes fading from heat, and

as a side benefit recovers a little power.

For **safety** your mechanical brakes should reliably **every time** let you panic-stop the rig

in worst-case conditions, i.e. max'd out weight load, going fast down a steep hill

without **any** help from the regen braking.

The voltage / SoC thing would only be an issue if your home is at the top of the mountain where you need good drag braking.

In which case charge to 5% below your normal "Full", now no problem at all.

If you don't live on top of a mountain, then never will this issue be any problem, at all.

I won't be replacing the mechanical brakes. Just not using them most of the time. I was already planning to not charge the battery to 100% so no problem there.

That Nucular controller looks awesome. It has everything i need and the top version supports 300a. It's not even crazy expensive. Hopefully they will be back in stock soon.

john61ct said:

The voltage / SoC thing would only be an issue if your home is at the top of the mountain where you need good drag braking.
In which case charge to 5% below your normal "Full", now no problem at all.
If you don't live on top of a mountain, then never will this issue be any problem, at all.

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Once again, talking bullshit.

Do you even regen ?

Username1 said:

That Nucular controller looks awesome. It has everything i need and the top version supports 300a. It's not even crazy expensive. Hopefully they will be back in stock soon.

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Best thing ever. I'm 100% happy with mine. Everyone that has one is jizzing all over the place (metaphorically. And possibly literally, it wouldn't surprise me).

Since you own that controller, maybe you can answer a question for me... It says the voltage range is from 20-90. I'm currently thinking about doing a 24v battery with a low turn motor for 20mph top speed. I wasn't planning to drain the battery below 20v anyway, so you you think this controller would work well, or might it run rough somehow because of being right on the lower limit?

Username1 said:

Since you own that controller, maybe you can answer a question for me.

It says the voltage range is from 20-90. I'm currently thinking about doing a 24v battery with a low turn motor for 20mph top speed. I wasn't planning to drain the batteries below 20v anyway, so you you think this controller would work well, or might it run rough somehow because of being right on the lower limit?

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I can't help with that one sorry. I'm running 20S.

One thing I can say, Nucular offer superb support. Basically real-time if you join the Telegram group. See the bottom of the OP from Vasiliy at Nucular for the link:
https://endless-sphere.com/forums/viewtopic.php?f=31&t=75494

Download the Telegram app for your phone, and post your question. Too easy.