Published October 1, 2012:
EV Range Calculator
Here is a simple spreadsheet that calculates the range you can expect to get in your own EV. It is based on coefficients of drag (road and air friction) as well as battery voltage and capacity. It assumes there are no head/tail/cross winds, there are no elevation changes and that you are driving at a constant speed the whole time (no traffic lights).
It has the default parameters for my Geo EV. Simply modify the cells in orange with the parameters from your vehicle. I put in multiple columns so you can see at a glance what speed does to the horsepower requirements and to the battery range.
- Vehicle Weight (add your own body weight too since you will also be in the car).
- Coefficients of wheel/road drag and air friction for your vehicle. A list of vehicle Cd-airs can be found here.
- Vehicle's frontal area (scroll down toward the bottom of the link).
- Battery voltage and capacity (assumes Li-Fe batteries otherwise Peukert coefficient will be much lower, eg 0.57 instead of 0.98).
- This spreadsheet also assumes Li-Fe battery weight has offset the ICE component weight. If you are using Lead-acid batteries, add their weight to the total weight of the vehicle.
It is amazing to me how speed is so important to battery range in an EV, (or fuel economy in any vehicle for that matter). If you double your speed, the force to overcome air drag alone increases 4 times and the energy requirement increase 8 times. To demonstrate this, change the Cd-road (row 6) to 0 for a couple of columns (this allows all calculations to only account for air friction). In one column set the vehicle speed to some value (50 mph for example) and in another column set the vehicle speed to double the first column. The horsepower will be 8-times higher when you double your speed. Wow!
Once you get a working EV and can confirm its actual battery range with real conditions and your driving habits, you can start tweaking the drag coefficients.
If a typical car has a Cd-road of 0.015, it can be lowered to 0.010 quite easily by removing unnecessary weight, getting new wheel bearings, using better gear oil and installing low rolling resistance tires.
Aeromods (which make the most impact) can lower your Cd-air significantly. I wanted to reduce the Geo's Cd-air from 0.34 to under 0.25. Any improvements in reducing this coefficient will geometrically reduce the energy required to overcome air friction. While aeromods won't do anything for you at very slow speeds, this can greatly increase your EV range at freeway speeds (which would otherwise be very poor).
The same goes for driving slower. Are you really that excited about going to work today that you need to speed? Who cares about the rat-pack race fighting over the passing lane just to be the first one going to work? Just cruise on in using the slow lane and drive happy.
If you are able to change your behavior so you can safely stay in the slow-lane and drive 55mph instead of 75mph, your fuel efficiency goes up by 43% or more. The gains for me are well worth the extra 6 minutes of drive time in each direction of my commute each day. When you wake up each morning, just hit snooze 1 less time and you will gain an extra 6-minutes to be put toward your 55mph daily commute.
For EV's with Lead-acid batteries, (like my EV truck), there is also the Peukert coefficient. At very low current draw this coefficient is close to 1 but at high currents (greater than 100A) this can be as low as 0.57 or lower, in effect making your battery act like a much smaller battery. This means pulling more amps while driving fast will reduce your range even further. Lithium batteries also have a Peukert coefficient but it is nearly 1 (0.98 typically) even at high current draws. It is almost an ideal battery.
Driving slower, reducing the Cd-road (LRR tires) and Cd-air (aeromods) can move than double your battery range or fuel economy.