EV tech explained: What are the limiting factors of an EV’s regenerative braking force?

Tesla Model S (charged EVs)

In many EVs, regenerative braking is limited by the manufacturer to a modest level, and usually offers little or no ability for the driver to adjust it (aside from the few cases in which regen is made proportional to the initial movement of the brake pedal). Is tepid regen braking the result of some sort of technical limitation with the drive system components most commonly used in EVs: a Li-ion battery, a three-phase voltage source inverter and either a permanent-magnet synchronous or asynchronous induction motor? Maybe, maybe not…

Let’s start with the motor. Both types of AC motor commonly used in EVs – the asynchronous induction and the permanent-magnet synchronous – work equally well as a generator or a motor, so this shouldn’t be a limiting factor when it comes to regen braking force. The inverter can also work equally well in either operating mode. Of course, regen braking becomes less effective as motor RPM approaches zero, just as the rate of acceleration declines once motor RPM exceeds a certain value, but aside from these limits – and the other laws of physics, road conditions, etc. – the motor and inverter won’t intrinsically limit regen braking.

Electric Motor

The battery is the part of the EV drivetrain most likely to be the limiting factor for regen braking. The Li-ion cell types most commonly used in EVs are capable of at least a 2C continuous discharge rate, but only a 0.8C to 1C charge rate. This suggests that the rate of charging has to be limited to no more than 40-50% of discharging, at least on a continuous basis. Of course, Li-ion batteries can tolerate much higher discharge rates for short periods of time; this is implied by the ratio of the motor/inverter power rating to the battery capacity (e.g. a Nissan Leaf with a 24 kWh battery and an 80 kW motor/inverter rating subjects its battery to a maximum discharge rate of 3.33C). However, an excessive discharge rate mainly results in a higher cell temperature, whereas charging at too high a rate can cause the lithium ions to plate out as metal onto the cathode, which results in a permanent loss of capacity, or can even cause catastrophic failure of the cell.

Finally, there might be more subjective considerations for limiting regen braking, such as driving feel and intuitiveness of operation. For example, aggressively engaging regen as soon as the accelerator pedal is lifted can make for a jarring driving experience and will generally be less efficient than simply coasting (though regen braking is always more efficient than mechanical braking, of course). There doesn’t seem to be as compelling an argument for not providing some means of adjusting the strength of regen manually – via a knob, say – so perhaps this will become a standard feature on all EVs in the future.

 

SEE ALSO: A closer look at regenerative braking

 

  • Freepat 75014

    Interesting. I had the chance to drive a Tesla Model X P90D last summer and was quite surprised that regen was linked to lifting the accelerator pedal, rather than pushing the Brake pedal, or at least it was not clear if the latter would also trigger regen or not at all. Initial Regen setting was quite aggressive, so as soon as I lifted my foot from accelerator pedal the car was kind of “really braking” instead of just coasting, as when I do same in my old BMW 530DA. I changed the setting to lower option and retrieved the same coasting feeling as on my BMW but then I had no clue if I was still getting any real regen benefit 1/ When lifting my foot from the accelerator pedal 2/ When pressing the brake pedal. Interested to know more on how that works on Tesla today as I’m waiting for next Model X 100D (or more than 100) refresh with 2170 battery cells by year end, before I order my Model X. These new cells are said to be 30% more efficient than previous 18165 cells (So I dream a 130D model at 100D price), plus to last 2X longer, hence they may allow more Cs in charge and in discharge, typically to charge up to 3X faster on next gen Supercharger v3 tweeted by Elon end of 2016, or for Regen too.

  • Ozzie Perch

    Mostly a compromise as the driver is not usually an engineer. With high regen rates you may approach 100& battery and then have to discard the regen energy as heat and most vehicles have no ability to do that!

    • nordlyst

      I think you’re right. Of course this is technically easy to solve by automatically limiting available regeneration according to SoC, but it seems more risky to have inconsistent braking the stronger that braking is. Regen in my LEAF at 100% SoC is absent, but since it’s never strong anyway this isn’t a big deal.

      Don’t get me wrong, I’d absolutely love to have regen like a Bolt. But to me, having a mode where the car coasts completely freely with no regen at all is at least as desirable as a good “one pedal” mode, since I drive mostly in very light traffic and not much in stop-and-go traffic.

  • Ron Richings

    All of which can be moderated by a suitably sized capacitor to ‘smooth out’ the recharging energy transfer.

    • nordlyst

      Yes, but is it worth it? How much more would the car cost as a result (not simply the part cost, but total cost, including production time, testing/QA, profits etc.)? And how much space would it take? How much weight would it add?

      I think the benefit is quite small, especially with 60 kWh packs and larger, so to my mind such a supercapacitor would have to be very small, light, cheap, safe and easy to integrate into assembly and testing to be worth consideration.

  • Ed

    Tesla makes it all work pretty seamlessly….but I suspect there is a Supercap in the future to maximize recovery.
    BTW: I picked up about 12 miles of rated range driving down from Mt. Palomar. That feels pretty good!

  • nordlyst

    I actually put my LEAF in neutral a lot exactly because the car can’t coast freely in any other way. A lot of people seem to forget, or perhaps never thought about, the fact that ALL braking implies energy waste. Regenerative braking wastes less energy, but will obviously never be 100% – and probably is at best 50% in real use in EVs today.

    The best way to stop, in energy terms, is actually to gain altitude. Hardly practical for drivers individually, but it’s an idea that is now sometimes used when designing subway tracks. By placing the track a bit deeper than the stations you not only waste less energy on braking (regeneratively, of course!) but you also get up to speed faster exiting the station. The added passive safety even means the safe distance between trains is a little bit shorter. This means that both latency (time to travel from A to B) and throughput (capacity, number of people that can be moved from A to B per hour) increases.

    In flat places like Denmark or Holland I can imagine it would be possible to apply this principle to roads as well (possible, but perhaps not practical or economical). Imagine a road in a city that’s basically a low-amplitude sine wave, with intersections at each peak of the wave. You could almost roll from one peak to the next and expend little more energy than going at constant speed with no intersections! Pretty cool.

  • jerryroane

    The regen motor can be separate from the car’s motor and tied directly to the power grid. Then it can accept all the power in real time not requiring expensive capacitors or super-capacitors. Patented system uses launch motor and catch linear generator to reclaim the energy of going fast. No mechanical braking is required in this system. The generator needs to be about 230 horsepower not something most cars have as a drive motor used backwards. More than that sending the regen energy into a storage system will lose the inefficiency of that transformation. The typical loss mechanical to electrical is 4% for the generator. Going straight to the grid is 100% recovered by definition of the meter. The mass and speed of the vehicle then becomes the “rotor”. No need for an extra large battery pack to absorb the 9.3 second pulse of high energy for a slow down from high speed to road speed.

  • Ramon A. Cardona

    Illusion or fact? I have not recall reading numbers as to rehén on the Leaf. Range increase while descending are due to little to zero energy being used not due to battery gaining a charge. Much like to solar panel on many Leafs, regen in my view, is minimal and an illusion compared to driving efficiently. At least that is my observation. Any data collected as to recen is welcomed.