Speak EV - Electric Car Forums banner
21 - 34 of 34 Posts

·
Registered
Joined
·
5,251 Posts
I don’t think that It has much to do with motor/drive efficiency. I suspect that it’s the battery performance which is highly susceptible to excessive discharge rates when accelerating hard.

The discharge rate, even for something like a Model 3P, is pretty trivial. 20 years ago I was building battery packs that would happily support a 20C rate of discharge with no problems or cell overheating. A Model 3P, at absolute max power, is not even hitting close to half that.
 

·
Registered
Joined
·
586 Posts
I don’t think that It has much to do with motor/drive efficiency. I suspect that it’s the battery performance which is highly susceptible to excessive discharge rates when accelerating hard.
Yeah I wondered about that, it's definitely the case with other battery technologies like lead-acid that the total amount of amp-hours you can get out of the battery depends on the rate of discharge, discharging it more slowly gets more energy out of the battery before it is fully discharged, discharging it quickly gets less energy out in total. I haven't studied lithium ion technology enough to know how much of any impact that has on this type of battery though.
 

·
Registered
Joined
·
5,251 Posts
For the sort of small discharge rates from an EV battery it isn't really a significant issue. Tha capacity isn't affected directly, all that happens is that the I²R losses from the pack internal resistance generate a bit more heat losses, so the overall efficiency is a bit lower if a lot of current is pulled. There's nothing like the voltage drop seen on a lead acid battery at modest discharge rates, as even really small and cheap lithium cells will comfortably deliver way more current than a lead acid car battery (hence the reason that there are pocket sized lithium booster packs that will crank an engine happily).
 

·
Registered
Joined
·
532 Posts
There's nothing like the voltage drop seen on a lead acid battery at modest discharge rates, as even really small and cheap lithium cells will comfortably deliver way more current than a lead acid car battery (hence the reason that there are pocket sized lithium booster packs that will crank an engine happily).
I didn't know that.
Thank you (y)
 

·
Registered
Kia E-Niro 2021 4+
Joined
·
317 Posts
…..Tesla as one of the most acceleratey…
Acceleratey. My new favourite word.

But interesting as the OP’s question is, the only thing that matters is how much fun accelerating in an EV is….. Goodbye BMW that’s been on my rear bumper all through a 30mph zone. Here is a 60mph zone and you now look very tiny in my rear view mirror.
 

·
Registered
Joined
·
871 Posts
We know that Lithium batteries can support high discharge rates but we also know from research that lifetime is adversely affected in the same way that high charging rates of 2C and above have a detrimental affect on the life. Motor and drive efficiencies are extremely high so I am simply suggesting that reduced range my be more to do with battery performance than drive components, however, I haven’t seen any curves of discharge rate v capacity.

One thing I do know, if I put my foot down hard in Eco modes I get flashing warnings on the dash!
 

·
Registered
Joined
·
5,251 Posts
The effective internal resistance of lithium cells is very low, much lower than the resistance of all the stuff the battery is supplying. Taking a Tesla Model 3 LR pack as an example (only because the data is available) each cell has an internal resistance of about 12mΩ, and there are 46 cells in parallel in each cell group, so that gives an effective cell internal resistance of 0.2609mΩ. There are 96 cell groups in series, making the total pack internal resistance ~25mΩ.

I ran TeslaMate with my Model 3LR and the highest power I ever managed to record was about 235 kW, and then only for maybe 2 seconds (at the power level the car hits the legal speed limit rather quickly). The typical battery pack voltage is around 355 V, so that equates to a battery current of ~662 A. The battery I²R losses will have been about 11 kW for those couple of seconds. The motors/inverters combination is around 85% efficient at that power level (they are much better at lower power levels, up to about 94% efficient), so the losses in the motors/inverters would be around 35 kW.

Using more sensible numbers, the average current from the battery pack is around 50 A, and at that current the battery pack I²R losses would only be around 63 W, whilst the motors/inverters would be around 94% efficient, so their losses would be 1.065 kW, massively higher than the battery pack internal resistance losses.
 

·
Registered
Joined
·
5,289 Posts
In addition to other replies, internal losses in the battery are higher under heavy current discharge. So a slow steady motor output will be best for economy.
Lithium Ion cell columbic efficiency is very close to 100% even at relatively high charge/discharge rates, so these cell losses are almost entirely regular I^2*R losses (internal cell resistance) as already discussed. (If you consider "battery" to include all the interconnects between cells those have I^2*R losses as well)

Not the case with some other battery types - for example columbic efficiency of Lead Acid cells goes WAY down at higher discharge rates, even 1C, so the increase in losses is a lot more than the measured internal resistance would suggest so a golf cart with Lead Acid batteries would see a much heavier range penalty for being hammered than one using Lithium Ion cells with the same Ah capacity as Lead Acid just isn't efficient at high discharge rates.
 

·
Registered
Joined
·
5,289 Posts
Most electric motors are highly efficient, I wouldn't expect significant differences between permanent magnet motors from Tesla/Jaguar/VW/Hyundai etc etc. Mass of vehicle & aerodynamics cause far greater variations in m/kWh. You want a really efficient EV? Get a Hyundai 28 kWh. Compact, light, really slippery aerodynamics, fast on Rapids, what's not to like!
At lower RPM this is true, but at high motor RPM near top speed bog standard Permanent Magnet Synchronous motors as seen in cars like the Leaf are not very efficient, with significant fall in efficiency at the top end, hence poor efficiency at high speed, (beyond just aerodynamics) and also a relatively limited top speed of about 100mph to avoid gearing it too high and killing low speed performance.

This is why EV's are starting to move to hybrid designs like the Switched Reluctance motor used in the Model 3 - which depending on the phasing of the current relative to the rotor position can operate anywhere between permanent magnet mode and reluctance mode. (In practice it is always operating somewhere between the two but the operating point is varied with motor speed to stay at optimal efficiency at all speeds)

A motor like the Model 3's has a much wider RPM/torque operating range and maintains a high efficiency over a much wider RPM range. It can give all the benefits of a PMS motor at low speeds without any of the drawbacks at high speeds.
 

·
Registered
Kia e-Niro 4 MY20, Zoe Z.E.50 GT Line
Joined
·
1,688 Posts
I wonder if we'll see any capacitative diractance/magneto reluctance hybrid motors
 

·
I'm not crazy, the attack has begun.
Joined
·
30,973 Posts
ICE cars trained us all to avoid a heavy right foot if we want economy. Accelerate hard and revs are high.

Most people seem convinced the same applies to EVs.

But why? Obviously speed matters. And regen isn't 100%. But is the acceleration itself "expensive" in efficiency terms? If you intend travelling at 60 mph, is there a cost of getting to that speed in 5 seconds vs 15?

What would be happening inside the motor to cause inefficiency?
It's 'different' to ICE, the full low-down here if you want the master-class;-

 

·
Registered
Joined
·
970 Posts
It's 'different' to ICE, the full low-down here if you want the master-class;-

Donald,
I practice what you preach and believe it does pay in efficiency.

In my Golf i accelerate at power setting 1 if i can, which i know from experience gives a 5 mls/kwh, I can hold this setting up to c50mph and then the air drag starts to kick in.
I have no idea how the power setting i see ties up with the torque i cannot see and so wonder how this would appear on the torque/speed map.
 
21 - 34 of 34 Posts
Top