Electric Motorcycle Armchair Engineers Discussion Thread. Truths, Half-truths, and Myths.

Discussion in 'Electric Motorcycles' started by T.S.Zarathustra, Mar 22, 2019.

  1. SteveAZ

    SteveAZ Long timer

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    Catching regen energy from hard braking is difficult - batteries just can't take the energy quickly enough

    Maxwell makes "supercapacitors" up to the task but that adds size, cost and weight that in smaller vehicles may be hard to justify

    https://www.maxwell.com/products/ultracapacitors/
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  2. liberpolly

    liberpolly Nu, shoyn, nudnick!

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  3. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    Yeah, probably not that important for a motorcycle where most of the energy is spent on overcoming air resistance.
  4. WagonWillie

    WagonWillie Been here awhile

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    I think it could be done; mine has the Charge Tank, so I theoretically I could use more of the regen, but that's not how it's set up. I do ride in a custom mode most of the time and I've set the regen while "coasting" to zero; I actually get more range that way if I can plan my riding to not have to back off quickly. I still get "full" regen from the brakes.
  5. ctromley

    ctromley Been here awhile

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    Gonna have to call you on that one. Generally speaking, batteries can be recharged as fast as they can be discharged. If you have a 1000 hp EV, you can theoretically regen brake at a 1000 hp rate. (At least from the batteries' perspective.) Reality gets in the way here for a variety of reasons. Chassis balance, driver/rider expectations and/or skill, electronics capabilities, losses, etc.

    Hobbyists have tried for stronger regen than what you usually get, but without a fully integrated approach it can get a bit squirrelly. (For example, a FWD drive car using heavy, real-time user-variable regen braking on front wheels only - that kind of thing.)

    If you wanted to design an EV with absolute killer regen braking, the match of any highly-developed, monster-disc-equipped, perfectly dialed-in racing braking system, with built-in electronic ABS that ensures absolute maximum use of all available traction, you could do it. The batteries wouldn't mind a bit.

    One reason you don't get much back from regen in everyday EVs is in the conversions. When you accelerate, the power you put out to do it doesn't all reach the wheels. That's a loss. The momentum you built up that will be tapped to recharge the batteries will go through another conversion loss before it gets back to the batteries.

    Also, it's not just acceleration that uses battery power. Just maintaining speed takes power to overcome aero losses, friction losses, climbing hills, system overhead, yada yada yada. Maintaining speed doesn't take much power compared to accelerating, but you spend a lot more time maintaining speed. That's where much (most) of your driving power goes, and you're not getting any of that 'maintaining speed' power back. It all gets sucked up in losses, so it's gone. So generally speaking, 10 - 15% recovery is what most people should expect. OEMs are terrified of adding more, even though they could, because that would make the vehicle too 'different' and would lead to lawsuits. Even if they did, you could only get back the power you put out to accelerate, minus losses.

    BTW, if you did build that EV with killer regen braking, it would still need a complete hydraulic braking system. FMVSS standards require redundancy in braking, and an electrical failure could cause a braking failure requiring hydraulic backup. Those rules were written before EVs became mainstream, and there might be room for redefining what constitutes 'redundancy'. But at least for now, don't expect any EV to rely on regen without having hydraulic brakes too.
  6. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    All true, but those losses don't go up with the weight increase, to close the logical loop. So reducing horsepower to reduce weight of the motor and the battery is not a good investment.
  7. ctromley

    ctromley Been here awhile

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    Weight has a direct effect on climbing hills. That's a big one, at least while you're climbing the hill. (And you only get part of that back when you go down, because of all those other losses.) Weight also has an effect on rolling friction losses, but those are pretty small. (Land speed record competitors know that from experience, and prefer more weight within reason for the stability at speed it provides.)

    Lower mass in performance vehicles is always a good idea (LSR attempts being an exception). This is especially true for EVs. But just knocking a few pounds off the motor or a few hundred off the pack is not the point and is going to do virtually nothing or you. The real advantage is when you design for low mass from the start and everything gets smaller and lighter, because everything works with everything else. Do it right and you've got an entirely different vehicle. If you're thinking about this in piecemeal terms, you're doing it wrong.
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  8. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    Electric bicycle?
  9. SteveAZ

    SteveAZ Long timer

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    Let's run numbers

    Say we have a 300kg total weight going 100kph and we're going to decelerate to 50kph in 5sec (which to me isn't silly hard braking)

    Initial energy = 300kg * 100kph^2 / 2 = ~116kJ(oules)

    End energy = 300kg * 50kph^2 / 2 = ~30kJ

    Energy lost = ~116kJ - ~30kJ = ~86kJ (joule = Ws)

    Power during deceleration = ~86J / 5sec = ~17.2kw


    We need to charge at ~17kw (~236A at 72V) presuming completely steady deceleration (not going to happen) and this is a reasonable decel - any harder it goes up proportionally... at some point exceeding the ability to charge... what is the zero charge rate?

    The potential energy to recover is ~86kJ = ~23wh which isn't really all that much too and in an ideal world the same amount was used to do just the acceleration

    Feel free to check my numbers, I'm just doing this on-the-fly so plenty opportunity I goofed...


    Frankly I think regen is likely most effective going down steep hills... at least that's where the math makes sense...
  10. SteveAZ

    SteveAZ Long timer

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    Just looked up zero sr/f charging rate - 3kw standard; 6kw rapid... far less than the >17kw in my example...
  11. ctromley

    ctromley Been here awhile

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    Not sure where you're going with this. EV charging rates are limited by charger capacity, not batteries. Regen does not use the vehicle charger, it uses the controller. Think of it as a two-way valve.

    Tesla's latest V3 Superchargers charge at 250 kW. That's nowhere near what the battery pack can take, it just shows the challenges of really high-rate quick charging. You can find similar numbers at the drag strip where some competitors are running home-made dump charging setups, using a big pack in the truck to charge the race vehicle through a high-amp DC controller (2000 A controllers are available off the shelf). The ENEL dump charger for the Moto-E series is a more polished version of that, presumably made to charge multiple bikes at a time. (And yes, they did have that embarrassing garage fire, but I think they learned their lesson about how careful you need to be when managing high-amp charging and the dump pack it frequently requires. It's better not to learn those lessons the hard way like they did.)

    If you're going to make a charger for an EV, you make it as low-capacity as customer expectations allow to keep your costs down. If you want high power regen, you already have the expense of the power stage in your controller paid for. You just need to design the system so it takes power in both directions. In fact, I believe there are some EVs with controllers (I forget who does this) that include the normal battery charging function in one unit. Saves money and space. I believe it's not used widely because it's patented.
  12. SteveAZ

    SteveAZ Long timer

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    I'm very skeptical that the regen charging somehow bypasses other charging circuitry... charging is charging and it needs the same circuitry for the battery management and I doubt the amount of regen power that can be managed is all that much more than the charging electronics can handle if it all. I don't have insight into the EV world on this point so could be completely wrong but I do have a firm grasp of the technologies in general.

    My point is that regen power can and will exceed the ability of the electronics and battery to recapture that energy... My very simple example was to point out how much power that is. I'd suggest that based on those numbers (unless my math is wrong) that there are going to be times where that power is as much as 100kw or more on a moto and that will exceed that limit and the considerable excess will be dumped as heat...
  13. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    The limitations on electronics are immaterial and can be overcome very efficiently.

    The limitations on batteries depend on the charge level - an empty battery can charge at much *faster* rate than it can discharge.

    If we consumers demand efficient regen, it will be supplied very shortly.
  14. ctromley

    ctromley Been here awhile

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    Look at the wiring diagram for any EV. The charger power connections go to the mains on one end and the pack on the other. Nothing else. The controller connects to the motor on one end and the pack on the other. The only path from the motor(s) to the pack is through the controller.
  15. SteveAZ

    SteveAZ Long timer

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    If you can point me to the wiring I'm eager to be educated... but running the numbers the way I did it appears very clear that braking power can exceed the maximum charging currents of the battery (again, please recheck them but it's very straightforward math)... I am also skeptical that they are able to be charged effectively and safely as quickly as they can be discharged.

    I was working with maxwell on very different uses for the "ultracapacitors" and they are the ones that educated me on the market for them in EV's specifically for taking the braking energy that exceeds the ability of the EV's' batteries, particularly larger EV's, to absorb that energy...
  16. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    Total braking power, for sure, but the braking power supplied by motor-generator - no. Also, most of braking power on motorcycles comes from front brakes, and they are not connected to the circuit.

    Of course they would say that, this is their business model. There's a different designs for fast-discharge vs. high-capacity batteries, but it's symmetrical for both charge and discharge rates - provided it is below 70% full.

    Here's some very superficial reading:
    https://batteryuniversity.com/learn/article/ultra_fast_chargers
    https://batteryuniversity.com/learn/article/discharge_characteristics_li
  17. SteveAZ

    SteveAZ Long timer

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    I like the battery university articles.... call it confrontational bias... I see confirmation :wink:

    "The battery must be designed to accept an ultra-fast charge and must be in good condition. Li-ion can be designed for a fast charge of 10-minutes or so but the specific energy of such a cell will be low."


    Specific energy is too important so most EV's aren't going to have these types of cells

    More that suggest cells able to take high charge currents are inappropriate for EV use:

    "The maximum charge current a Li-ion can accept is governed by cell design, and not the cathode material, as is commonly assumed. The goal is to avoid lithium-plating on the anode and to keep the temperature under control. A thin anode with high porosity and small graphite particles enables ultra-fast charging because of the large surface area. Power Cells can be charged and discharged at high currents, but the energy density is low......

    Apply the ultra-fast charge only when necessary. A well-designed ultra-fast charger should have charge-time selection to give the user the option to choose the least stressful charge for the time allotted. Figure 2 compares the cycle life of a typical lithium-ion battery when charged and discharged at 1C, 2C and 3C rates. The longevity can further be prolonged by charging and discharging below 1C; 0.8C is the recommended rate."
  18. MJSfoto1956

    MJSfoto1956 Been here awhile

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    I've read articles that discuss using large capacitors to store the regen, then add the power back to the pack when appropriate.
  19. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    You are still missing the point, dude - the point is that whatever is the design, they take charge as fast as they are discharged.
  20. smdub

    smdub Adventurer

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    You know not of what you speak.
    250kW charging a P100D is only 2.5C. Big whoop. We do that all the time. We discharge LiPos at 20C continuous (and 100C burst.) That's the equivalent to 2700hp in a P100D. You simply can not charge lithium that fast. The cell resistance will cause the voltage to jump so high you lithium plate/gas. Faster charging shortens the CC part of the charge cycle but extends the CV part.
    You can only regen as hard as the peak battery voltage will allow. A dead battery can regen much more than a charged one. This leads to variable braking capability which is a big red flag. You'd have to blend hydraulic w/ regen to give a consistent amount to the user to be safe. Consistent is more important than max available. Ford wouldn't allow us to give the user more power when the control/motor was cooler. That lead to variable acceleration rates. User would complain they were able to pull out into traffic safely one day but the next it 'bogged' in the same situation and they'd get hit by an oncoming vehicle. You often have to cap well below what's possible.
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