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Calculating theoretical efficiency.

5544 Views 4 Replies 3 Participants Last post by  madderscience
I'm working on my first EV conversion and really want to run through a lot of theoretical configurations for my battery pack to get the performance I want out of it, but one thing I'm having trouble with is getting an number for "efficiency" or rather, an estimate of watts/mile. This seems to be the key to making any calculations relevant to my vehicle (1986 Mazda RX-7) especially when it comes to the battery pack. What kind of numbers do I need and is there an equation to plug them in to, even if it is just a rough estimate it would really help.
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for a small, aerodynamic car, the two biggest sources of drag on a car are rolling resistance and aerodynamic drag. Rolling resisitance is more or less a constant value that is proportional to the weight of the vehicle. Commonly it is around 1 to 2% of the weight of the vehicle. The aerodynamic drag is a product of the Cd A (drag coefficient multiplied by frontal surface area) of the vehicle and the square of your speed. So at lower speeds, rolling resistance is the biggest factor. At higher speeds, aerodynamic drag is the largest factor. The "crossover" speed is usually around 40mph. There are also losses in the motor, drivetrain, and electronics.

Your mileage may vary, but for all my calculations I used a combined efficiency of 78% for battery-to-wheels energy conversion efficiency.

Look here for the CdA of mazda vehicles:

this page lists an '86 RX7 with a CdA of 0.55m^2 which is a good number.

Rolling resistance is affected by your tires, brakes, alignment and a host of other factors, but if you know the weight of your vehicle and the CdA, and you have about a mile of flat, deserted roadway available you can measure it using a coast down test and some math.
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