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IssacZachary
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Joined: Fri Jan 13, 2017 9:57 am
Delivery Date: 15 Nov 2016
Leaf Number: 420789
Location: Gunnison, CO, USA

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.

Example:
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.
2013 SL 45,000 miles.
12 bars until 44,300 miles on June 2, 2017. :D
11 bars current. :)
Other vehicles: 1972 VW Super Beetle 1600cc dual port engine. 1985 VW Golf non-turbo 1.6L diesel.

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