One of the first questions we get is, “how far will it go on a charge?” The answer to that question is it depends on what battery pack we put into the car. It all comes down to how much energy you carry and how much weight you are pushing. There are some other factors like wind resistance and rolling resistance as well. There are some rules of thumb that people in the industry have found that usually hold pretty true most of the time for any average car*. You can kind of guess how much energy you are going to need by taking the cars weight and dividing it by 10. This will give you the number of watt hours that you are going to need to drive 1 mile.
Lets take a look at this rule of thumb and apply it to a car. Say the car you want us to convert for you weighs about 2800 lbs. Another rule of thumb is that once we take out all the internal combustion stuff (ICE) and put in the electric stuff and batteries, its going to be about 200lbs more weight. (I know this isn’t accurate, its just a rule of thumb.) So we are looking at a vehicle with a curb weight of about 3,000 lbs. We divide that by 10 so we get 300. So its going to take us about 300 watt hours to go 1 mile. That means to go 100 miles on a single charge we are going need 100 x 300 =30,000 or 30 kwh of energy. That means a 30kwh battery pack.
Now if our system is going to be a 300 volt nominal system, that means that we are going to need 100 ah batteries cells to go 100 miles. Since each lithium cell is about 3.2 volts, we need about 94 cells to make a 300 volt pack. Therefore our pack to make this 3000lb car go 100 miles is going to be made up of 94 – 100ah lithium cells.
The question you need to ask yourself when looking at an electric car is “How far do I need to drive?” Don’t answer this one with emotion. Do a test. Zero out your trip odometer each morning and then write down how far you went. Do this for 2 weeks. Now take a look at what your daily driving habits look like. Most people find that they drive less than 40 miles in any average day. The exceptions being of course when they go on a trip somewhere. There are some, like myself who have found themselves commuting to work driving 30-40 miles each way. To figure out how far you really need, take the numbers you found in the above test and add 15% for the occasional detour and then add another 15% for degradation of the battery pack over time. This means our average drive really only needs to go about 52 miles to handle about 95% of his driving. So if we create a car that will do 100 miles on a charge, he is buying way more than he really needs. Since the biggest cost in the whole electric car equation is the batteries, his easiest way to cut costs is by agreeing to a smaller battery.
Next week we will look at the second question everyone asks –“How long does it take to charge?”
*Jack Rickard from EVTV.me has made this rule of thumb famous on his weekly show. Please don’t give me credit for it. It is a pretty good rule of thumb and I thank Jack for it.