Electric Vehicle Frequently Asked Questions

CALCULATIONS

How much power will I need? (Calculating power used to move a car)

The force required to move a car may be determined mathematically if you know:

• The weight of the vehicle (the total converted weight can be closely estimated by weighing what you take out and what you put in)
  
• The rolling resistance (This can be measured by pulling the car at a constant speed on a flat surface using a spring scale, but it depends on weight, so make sure it is close to the finished weight when you do the measurement)
  
• The drag coefficient and the frontal area of the vehicle (this may be obtained from the dealer, or may be estimated based on similar vehicles)
  

1. The rolling resistance is pretty much a constant force
   
2. The force due to moving on an incline is Weight multiplied by the sine of the angle of the incline from horizontal
   
3. The aerodynamic force is (drag coefficient) * (frontal area) * (speed)^2 * (air density) / 2
   

NOTES: These calculations are general and intended for estimates only. Appropriate unit conversion factors must be applied. These equations are for steady state and do not take acceleration into account.

The resultant force must be changed into torgue at the tire using T = Force * (Tire radius), and then the gear ratios in the transmission and differential must be taken into account. This will tell you the torque that your motor must generate for your vehicle to go that speed. Use of the appropriate motor curves will help to find the ideal gear and how much electrical power must enter the motor. The motor curves and your controller characteristics will tell you how much headroom you have for acceleration at that speed.

For a more detailed estimate, including range, amperage consumption, and voltage sag for different batteries, plug your weight, drag, and rolling resistance numbers into

How can I calculate the weight and balance of my conversion project?

Before removing any of the ICE components, take the car to a grain elevator, weigh station, or truck stop and weigh the car. All of these locations have scales, and if you explain what you are trying to do, they will likely let you use them for free. Even a certified weight is not very expensive. Weigh the car with just the front wheels on the scales, with just the rear wheels on the scales, and with the entire car on the scales. This will tell you how the weight is distributed between the front and rear axles. To maintain handling and performance, this distribution should be maintained after the conversion.

The vertical weight distribution should also be maintained. The height of the center of gravity of the unconverted car is difficult to measure, but can be estimated as a height just at the top of the engine block. If the converted car maintains a CG height lower than this, the rollover tendency will generally be reduced. The way to do this is to place as many of the added components below this height as much as possible. By far the heaviest portion of an EV is the batteries, so if they can be placed at or below this height (and they usually are, by being sunk into the floor), the overall CG height is usually maintained or reduced.

A more tedious (but more accurate) way to establish that your converted CG is lower is to use a spreadsheet to calculate the normalized rolling moment of the removed items and added items. It is not necessary to know the "before" CG height when using this spreadsheet, but by carefully recording the CG height of each item removed and added with respect to a fixed reference point on the chassis, the change in CG height can be computed. This is commonly done when aircraft are modified.

What kind of range can I expect from my conversion?

Uve's EV calculator is available to give a lot of information, including range. It is at www.geocities.com/CapeCanaveral/Lab/8679/evcalc.html.

For quick back of the envelope calculations, however, a simple empirical equation which seems to give good results is available. This has been developed by the EV discussion list from a combination of formulas, and has been dubbed the "Neon-Dube" equation by David Dymaxion, its creator, in honor of Paul "Neon" Gooch, and Bill Dube', the primary contributors. It is:

Miles of range = (original weight/converted weight) * (mpg as an ICE car/500) * pounds of batteries