Dont Go There

EV Conversion Mistakes to Avoid

Mike Brown & Shari Prange

©2003 Mike Brown & Shari Prange

You're all fired up about electric cars, and you're ready to make the big leap. You're going to convert one yourself. Already, your mind is bubbling with ideas for it.

Before you make any serious commitments—like spending large amounts of money or taking your car apart—let's look at some of the common mistakes electric vehicle (EV) beginners make, so you can avoid them and build your car like a pro.

Donor Vehicles

Choosing the correct donor vehicle is one of the most important parts of the conversion process. Picking the wrong car could result in an EV that will not meet your requirements for range, passenger and cargo capacity, or suitability for your terrain and climate conditions.

No Heavy Metal. The weight of the donor vehicle has a direct effect on the range of the finished conversion. The more an EV weighs, the more power or torque is required to accelerate it and keep it moving. More motor torque means higher amperage drawn from the battery pack, and reduced range. Using a big vehicle with room for more batteries doesn't work because more batteries equal more weight, which requires higher amperage, etc.

Continuous, high-amp draws can also damage components like the motor and speed controller. For these reasons, we don't advise considering any vehicle with a curb weight of more than 3,000 pounds (1,360 kg) for conversion. Light pickups, such as the Ford Ranger or Chevy S10, can be good candidates for conversion. But stay away from larger 3/4 ton or 1 ton rated pickups, and SUVs and minivans.

Size Matters. So if a heavy, large vehicle is out of the question, is the lightest, smallest car the best choice for a conversion? In a word, maybe. The small size of an Austin Mini, Honda 600, or MG Midget leaves little room for batteries. An on-road conversion EV needs at least a 96 volt battery pack because voltage equals speed. A 96 volt system with enough capacity to give decent range requires sixteen, large, heavy, 6 volt golf cart batteries. The total weight of this battery pack is nearly equal to the total weight of a Mini, and has about the same footprint.

Eight, 12 volt batteries would give us our 96 volt system with much less weight, and would take up much less space. However, because pounds of lead equal miles of range, the 12 volt EV's range would be much less than a 6 volt powered conversion. You must decide if there is enough room in the vehicle for the number and type of batteries needed for the range you want. The lack of space for batteries also eliminates many fiberglass kit cars, even though they are physically bigger than the little cars we have been discussing.

Choosing a donor vehicle is kind of a Goldilocks thing— not too large and not too small, but just right.

Parts Supply

The expense and work of doing the conversion makes owning the EV a long-term commitment. So the vehicle you pick should be a popular make and model. This ensures an available supply of affordable replacement parts like brake and chassis pieces. Some vehicles weren't built and sold in large enough numbers to make it profitable for the aftermarket industry to manufacture and stock parts for them. The dealership is often the only source for these parts.

If there is no longer any dealer presence for your donor car, you may be forced to pay inflated prices to a specialty parts house to keep your conversion on the road. So if Auntie Em has a Fiat that she will give you for your conversion project, try to remember the last time you saw a Fiat dealership.

Shifting Gears

A conversion EV with a series-wound DC motor (the most commonly used type) needs a transmission. Direct drive is only suitable for small, lightweight, built-from-scratch vehicles. Even those cars have to compromise when selecting their drive gear. One gear ratio will give good

Compact cars and light pickups are good vehicle choices. Don't use tiny cars or large trucks or vans

Compact cars and light pickups are good vehicle choices. Don't use tiny cars or large trucks or vans

acceleration but a low top speed. If you gear for high speed, it takes a while to get there, and is less efficient at lower speeds. The best way to get speed, acceleration, and efficiency is with the torque and speed multiplication of a transmission.

So our conversion needs a transmission, but which kind—automatic or manual? Unfortunately, most automatic transmissions don't work well for several reasons. The first problem is the lower efficiency inherent in an automatic transmission, which has a bad effect on the range of the EV. The second problem is with the shift points. These will not be set to match the needs of an EV, and can be particularly bad on hills. Finally, an automatic is designed to work with an engine that idles at stops. Electric motors don't—they simply stop. This leads to a delayed throttle response when the stoplight turns green again. For these reasons, automatics are rarely used. They can be made to work, but this isn't really a project for an EV novice.

Clutch or No Clutch?

One of the oldest controversies in the conversion EV world is whether or not a clutch is necessary. A gas or diesel engine needs a clutch to prevent stalling when the vehicle comes to a stop. Disengaging the clutch also makes shifting gears easier.

Some people feel that since the electric motor freewheels when the electricity is shut off, a clutch is not necessary. Another perceived benefit is the weight saved by eliminating the flywheel and clutch assembly.

Like the majority of other EV builders, we believe that a clutch should be used. While it is not necessary when coming to a stop, a clutch makes starting off a much smoother process. Smooth gearshifts in a clutchless EV are possible after much practice, and upshifts are easier than downshifts. The amount of practice and the occasional missed shift might keep some people from driving the conversion on a daily basis, or at all.

The main reason for retaining the clutch is safety. A slow jerky start or a missed shift in traffic could lead to a rear end collision. Missing a downshift while attempting to accelerate out of harm's way could also be dangerous. Being able to disconnect the motor from the rest of the drivetrain in case of a motor or controller failure is a necessity. Most people who are familiar with a manual transmission would want a clutch. Eliminating it doesn't save that much weight. Leave it in.


People get themselves into trouble several ways when choosing their conversion components. Usually, they are either trying to save money or increase performance. If you're a novice with EVs, you're safest to stick toward the well-traveled middle of the road in your conversion. The more you stray and improvise and experiment, the more knowledge you need to do it safely and successfully. The components you choose should be:

• Appropriately rated for use in a car

• Compatible with each other

• Not homebuilt

• Not obsolete or out of production

• Not experimental or preproduction prototypes

Let's look at each of these criteria, and see why they are important.

Appropriate Ratings. This is usually a problem when people are bargain hunting. They find a "great deal" on a surplus aircraft starter/generator or a forklift motor. The problem is that these units were designed for an entirely different kind of duty cycle. For example, the aircraft starter/generator is designed to be a starter or a generator, not a traction motor. Its voltage rating is too low for a car because it's keyed to the battery voltage of an aircraft.

The forklift motor is designed to move a vehicle, but not very fast. It's designed to use low voltage for low speeds and high current for lots of torque. Unless you want to pull stumps, this motor is not for you.

The same principles apply if you try to use a household AC circuit breaker instead of a DC breaker, or ordinary automotive starting batteries instead of traction batteries. Make sure your car has the right stuff.

Homebrew electronics with inappropriately rated components can be very costly in the end.

Compatibility. This is another issue that comes up due to bargain hunting, especially when people are searching out used parts from multiple sources. You may find a good deal on a motor here, a controller there, and a charger somewhere else. They might all be perfectly valid components to use in a conversion—but not the same conversion.

Different motor types require different controller types. Also, many components only operate within specified voltage ranges. A 72 volt charger is no good with a 96 to 120 volt controller. Shopping used and piecemeal can be done, but you need to be fully informed to make the right choices.

Homebuilt. Some people also think that they can save some money (and have a little fun) building a few of their own components. Usually, this means the speed controller, the charger, or the motor-to-transmission adaptor. Or they may come across a "great deal" on someone else's homebuilt components.

Homebuilt controllers and chargers are often pretty crude compared to manufactured pieces. In the case of a charger, this can damage your batteries and end up costing you more money. More important, they usually lack many fail-safe features and safety redundancies. With the level of voltage and current involved, this is no place to skimp on safety.

The motor-to-transmission adaptor is also more subtle and complex than people realize. This piece is carrying a large amount of torque, at high rpm, from the motor to the transmission. Even small errors in measurements, design, or fabrication can lead to stresses that will cause failures. Any adaptor failure will be a major hassle, and possibly expensive if it damages the motor. This is a precision piece of design and machine work that should be built by someone with experience. Your local machinist buddy may have the tools, and the skills to operate them—but not the knowledge of electric cars to do the design.

Obsolete or Out-of-Production

Components. Obsolete components probably will not provide the same level of performance as current models. If your needs are modest, this may not matter much to you, but you also need to beware of the compatibility issues mentioned earlier.

Some out-of-production units, such as Prestolite motors, are still perfectly valid for many vehicles. The main problem here is the lack of factory support. If you have a problem with a component that is out of production, you may not be able to find the pieces or services needed to fix it. If you then have to redesign your car for a different component, this can be an expensive and frustrating experience. While some of these components can be quite good, you need to make an informed decision about whether you feel comfortable with that level of risk.

Experimental or Preproduction Prototypes. The opposite of obsolete and out of production units is experimental or preproduction prototypes. These components may offer improved performance. The problem is that they are not fully developed yet. You will be essentially volunteering your services as a guinea pig for testing and debugging these units.

If there are problems, you may have some vehicle downtime while the inventor tries to identify the cause and repair it. There won't be any quick factory exchange for a working unit. This is another situation where you have to make an informed decision about risk levels. As a novice, you might want to stick to safer territory. Even an experienced EVer shouldn't combine too many risks in one vehicle.

Use batteries like these that are appropriate for powering a vehicle. Don't use starting, marine, or RV batteries.

Use batteries like these that are appropriate for powering a vehicle. Don't use starting, marine, or RV batteries.


Batteries deserve some attention all by themselves. First, let me repeat what I said above about getting the right components for the job. This is especially true with batteries. Many novices get suckered by "great deals" (there's that phrase again!) involving very cheap or free batteries. They think, "If the batteries don't last very long, who cares? They were cheap."

But cheap batteries can have other problems, some of which are expensive. For one thing, they may not have the energy density to give you the range you need. If your vehicle isn't usable, what good is it to you? And if you want to switch to a more appropriate battery later, you may have to completely redesign and rebuild your rack and box system. Remember—bargain batteries aren't.

The other common battery issues are more a matter of oversight than economics. The first is that people don't always stop to think about battery maintenance. Battery tops need to be easily accessible. If you can't easily reach the batteries to check and refill the water, you won't do it, and the batteries will die very young. This is often an issue with kit cars.

The second issue is battery containment. The battery pack represents a lot of weight. In the event of a collision, it will try to keep moving while the chassis is crashing to an abrupt stop. If this battery monster is sitting behind you, a collision could be very deadly.

Take the time to do some serious study on battery containment. You need to secure them against movement in all directions—front-to-rear, side-to-side, and up-and-down in case of rollover. Even a 30 mph (48 kph) collision involves significant G-forces. (For more info on proper battery containment, see HP78, HP79, and HP80.)

Finally, don't run out and buy your batteries first. Buy them last. If they sit in your garage for six months while you build the car, they will be in the way, and will self-discharge and sulfate if they are not being regularly charged.

Hybrids—Don't Try This at Home

Gas or diesel range extenders for EVs have always held a lot of interest. The common scheme involves carrying a small generator in the back of the vehicle. The problem with this idea is that a commercial genset with enough capacity to do the job would end up being nearly as big as the engine you removed. The biggest drawbacks of the "genset in the back" plan are the noise, vibration, and air pollution.

Another hybrid scheme involves leaving the existing drivetrain of a front-wheel drive car in place for long-range trips and adding an electric drive to the rear wheels for around town driving. The major drawback of this plan is fitting two complete drive systems—with a fuel tank for one and a battery pack for the other—in a small car, as well as a clutch system for engaging and disengaging them. We saw it done once and it wasn't pretty.

The Sun, the Wind, & the Rolling Wheel

Solar-electric panels and electric vehicles seem to be a natural match. If an EV with the roof covered by solar-

electric panels is sitting in the sun at work for eight hours, the batteries should get back most of the energy it used getting there, right?

Wrong. The output of a solar-electric system the size of a car's roof is so low that all it does is trickle charge the batteries. I was involved with a Voltsrabbit with a PV array on the roof, and after a full day of sitting in the sun, its range was only increased by 5 miles (8 km). A large, stationary PV array at home can be used to charge a car, but the panels you can fit on the car itself just aren't enough to be effective.

Another idea that we hear from time to time is putting a wind generator on the roof of the vehicle. The theory goes that the air passing by the car, while you're driving, turns the wind generator, which in turn charges the batteries. The reality is that the air drag produced by the wind generator eats up more energy than it produces.

Hooking an alternator to the nondrive wheels is another idea that would work if it didn't take mechanical energy to produce electrical energy. The only way this would work at all is if there was a clutch between the alternator and the wheels that was engaged only when the power to the motor was turned off. This would be a kind of regenerative braking that would produce more braking action than electricity. Finding a source for the alternator, clutch, and control system might be difficult. It has been done, but the results are not impressive.

Off to a Good Start

You now know how to avoid electric vehicle conversion mistakes that have cost some novices considerable time, money, and disappointment. You're already ahead of the game. With a little homework and realistic expectations, you can build an electric conversion that will be a solid, reliable workhorse for you, as well as carefree and fun to drive.


Mike Brown & Shari Prange, Electro Automotive, PO Box 1113-HP, Felton, CA 95018 • 831-429-1989 • Fax: 831-429-1907 • [email protected][email protected]

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