I have driven an MG Midget for 35 years. About 10 years ago I thought it would make an excellent car for electric conversion. The older models only weigh 1600 lbs, and there is no power brakes, steering, or air-conditioning to deal with. I finally decide to do the conversion when I realized that Colorado was offering $6,000 in rebates to do it. This ’77 midget was started by people in California. I bought it with just the motor in it and added Lithium batteries, a controller, BMS, and other items. I was able to keep the weight down to 150 lbs over original. The car drives just like a gas midget, and I have over 100 miles in range. I have a site that shows much of my work. www.electricmgmidget.com.
Now that I’ve finished my conversion and have driven it for over a year I have some opinions on how a electric car conversion should be done. Since this site is designed to help people considering buying or building an electric car I thought this blog would be appropriate. I’m a mechanical engineer by training, and I have a good understanding of electronics as well. I did all of my own work on my cars for the last 35 years, so tackling an electric conversion was not out of the question for me. I had driven and maintained a MG Midget for 35 years, so I knew everything about that car. This made converting it much easier than a car I was not familiar with. I did 1 month of intensive research on the web before starting the project. Even with that research there were many things I didn’t understand till I was finished. I have a few things that I think are necessary when considering a conversion.
1) It will cost a fair amount of money if done right. Mine cost $15,000 including the car.
2) Lithium batteries are the only way to go if you want a truly drivable car.
3) Understanding electric motors, torque, voltage characteristics is very important when designing the car.
4) Lightweight cars will make a much better conversion that heavy cars or trucks.
5) A correctly installed BMS can make the car’s daily operation as simple as plug it in at night.
Obviously this blog could get very long, but I’ll try elaborate on the above points.
A large portion of the cost of converting an electric car comes from the battery costs. I feel that if you try to skimp on that, then you’re better off not bothering with the project. Many people either try lead acid cells, or think they will start with lead acid cells and later convert to Lithium cells. From what I’ve read you range with lead acid batteries will be very disappointing. They don’t like to be drawn down very far, so the range you think you’ll get isn’t really there. They don’t last very long, and shorter if you really do use them aggressively. Converting from lead acid to lithium cells won’t be trivial, and if done right will require all new mounting hardware, a new BMS, and new charger, and most likely a new controller. So seriously consider using Lithium cells for your conversion.
Many people think that because an electric motor has great low end torque they can do away with the transmission and or clutch and drive in 1 gear. While this is true to some degree, it will seriously compromise your driving experience. When designing a system you must know what torque the original gas engine provided and design from there. Picking a larger electric motor with tons of torque isn’t the correct way to go in designing an electric car. You need to know if the drive train can handle the extra torque, can the batteries you want to use deliver the amps you need, and how much voltage will be required so you don’t limit your drivable RPM range to 2,000 RPM. I can’t go into detail on how this all works here. You can research this in different places on the web, and there are EV calculators that can help you pick the correct combination of motor, batteries and gearing. I can say that maintaining your transmission and clutch will open up your choices tremendously and I cringe when I see people talking about eliminating them in their build.
Keeping your car light will greatly add to your range. For around town driving and dealing with hills, weight is very important. On my 5 mile commute to work, I have enough stop and starts to have acceleration losses use up 35% of my total energy. It isn’t just good aerodynamics that make for good range, it’s also weight. By using a recording GPS and some math I was able to calculate the various forms of energy use in my commute. From that I showed that if I could reduce my car’s weight by 100lbs, I could increase the range by 3%. This kind of calculation depends on the commute, so be careful if you try to simply use this as a rule of thumb. My commute has 6 stops in 5 miles, and a maximum speed of 50 Mph.
I have a Elithion BMS system in my car. It took considerable time to install and test it, but the time was worth it. I haven’t had to even look at the cells voltages in the last 9 months. I know if something is wrong with my batteries, the BMS will tell me. I drive my car until I’ve gone about 50 miles, and then plug it in overnight. In the morning the light is green on my charger and I unplug it and drive away. The BMS controlled the charge, and now shows my fuel gauge as full. When driving, the BMS keeps track of the energy used and with a small circuit shows me through my original fuel gauge. When a cell gets too low on voltage, the BMS tells the controller to limit my current so I can limp home. I know the use of BMS systems is often debated, but for me it means complete carefree driving and charging. I would recommend them, but many electric conversions do very well without one. It is your choice.