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Re: REGEN Question

Thu Jun 15, 2017 3:09 am

GerryAZ wrote:It really depends how you measure or define regeneration efficiency. The electric motor and inverter/converter system are quite efficient (probably over 90%) in both directions (the only difference between power generated by the motor and power consumed by the motor is direction of current flow). Since mechanical friction (bearings, tires, etc.) and aerodynamic drag are present at a given speed, the power required by the motor is the amount required to overcome those losses plus the amount of power required to move the car up the hill. Conversely, the power generated by the motor going down the same hill is the amount of power generated by the car going down hill minus the sum of mechanical and aerodynamic losses. The mechanical and aerodynamic losses make it appear that regeneration is not efficient.

10 kW for mechanical and aerodynamic losses at given speed
20 kW for energy to move car up hill and recover going down hill at given speed

Assume 100% efficiency in motor and inverter/converter system:
Net power from battery going up hill is 10 kW losses + 20 kW hill = 30 kW
Net power to battery going down hill is 20 kW hill - 10 kW losses = 10 kW

Assume 90% efficiency in motor and inverter/converter system:
Net power from battery going up hill is (10 kW losses + 20 kW hill) / 0.9 = 33.3 kW
Net power to battery going down hill is (20 kW hill - 10 kW losses) * 0.9 = 9.0 kW

This is why many people claim that only 1/3 of power is recovered by regeneration when regeneration is probably about 90% efficient.

I think your calculations are a bit on the optimistic side. But you do have a good point.

Still, let's do a bit more in calculations.
According to Tony Williams' range chart a Leaf traveling at 60mph uses 15.38kW. Yet according to the aerodynamic and rolling resistance calculations on Ecomodder's aerodynamic and rolling resistance calculator with a CRR of 0.10, a total weight (including driver) of 3,700lbs, a CR of 0.28, and a frontal area of 24.8 square feet the total power needed to overcome rolling resistance and aerodynamic drag is just a bit over 12kW. So we get a total efficiency of around 78% from battery to wheel. Mind you that this is including drive train losses. If we regen then we have such an efficiency both charging and discharging, or 61% total regen efficiency. But keep in mind that that's more or less a best case scenario figure, and that during regen you normally run much higher charge rate than normal which can potentially reduce battery efficiency by 10% or more. It can also reduce inverter efficiency. Add to that that the regen energy recovered is normally used for a higher accelerating discharge rate which also reduces battery and inverter efficiencies once again. By the time you've ran all the numbers 40% regen efficiency doesn't sound that far off.
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2013 Toyota Avalon Hybrid CURRENT

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Re: REGEN Question

Tue Jul 04, 2017 8:29 am

Finally got to test my regen question - I tested on a highway over a total 7 mile stretch in a 2013 S under ECO mode with the following results, using GIDS as an energy metric:

1. Steady 60 mph: 13 gids
2. Accelerate to 65 from 60, regen from 65 to 60, repeat: 21 GIDS
3. Accelerate to 65 from 60, coast from 65 to 60, repeat: 19 GIDS
4. Steady 65 mph: 20 GIDS

We have to accept that this is only one data set, so I can't represent that this is a definitive test. I did think I would get more out of coasting, but I think if you pick where to do it it really pays off - at least that's what I've found.

Also interesting to me that the energy difference between steady 60 and 65 is a 54% difference, and there was virtually no difference between the steady 65 and the coasting/regen cases. So like the fable, sure and steady wins in the long run.
2017 Leaf S with QC, JUN mfg date

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Re: REGEN Question

Tue Jul 04, 2017 12:14 pm

Interesting and not surprising. It is not possible to gain energy by speeding up and braking or coasting. The conservation of energy principle says you can't get more energy out of a closed system than you put into, and there will always be friction and other inefficiency losses.
2013 Leaf SV - lease ended, 2016 Leaf S30 - purchased

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