EV Conversion Basics

Written by: Rob McClement, MEVA President

Converting a gas vehicle to a battery electric vehicle has been until recently the only viable option for early adopters. Many factory EVs are either unavailable in local markets or may have a relatively high cost of ownership. Converting a gas car or truck to all electric drive is being done by thousands of people around the world. Many converters begin with no special mechanical or electrical skills. There are tremendous online resources for Do-It-Yourself converters. Online forums such as diyelectriccar.com have thousands of topics and threads discussing every aspect of conversion that you can imagine. Forum members are generally very helpful, willing to share their extensive knowledge and expertise with newcomers.

The process of completing a conversion can seem daunting at first, but it can be done. Do your research, be persistent and take one step at a time!

What is a conversion?

An EV conversion is essentially recycling a factory built gas car or truck, turning it into an efficient and clean electric vehicle. The vehicle that is being converted is usually referred to as a “donor”. The donor has already been engineered for safety and is road legal.

Why do a conversion?

Many of the reasons for building a conversion are the same as for driving a factory built EV:

  • reduce your carbon footprint (cleaner car, global warming, etc)
  • use domestic energy (hydro electricity is made in Manitoba and keeps our energy dollars in our economy)
  • health benefits (Would you drive a gas car if the exhaust came out of the steering wheel?)
  • economic benefits (energy cost for an EV is about 1/8 that of a gas car)
  • efficiency (An EV is about 75% efficient compared to 17% gas efficiency)

How to do a conversion?

Research: Begin by learning all that you can! Join the Manitoba Electric Vehicle Association. Several of our members are driving conversions that they have built and are using as daily drivers.

Googleelectric vehicle conversions”, you can expect to over 800,000 results! You will find links to conversion suppliers, many of which offer complete conversion kits for various models, conversion forums, blogs and EV associations. You will not be alone in your effort.

Buy a book or two on conversion. An excellent place to start is “Build your own Electric Vehicle” by Seth Leitman and Bob Brant, although there are now several other excellent books appearing on the market.

Choosing a Donor

The most important piece of advice is this: “Choose a vehicle that you will want to drive.” You will invest a lot of time, energy and money in your conversion, so choose a vehicle that you will like and enjoy.

Consider what you need your conversion to do. How many passengers? What range will you need? What about cargo capacity? What budget do you have for your conversion? The answers to these questions will affect whether you choose a compact sedan, a two seat sports car or a small truck.

Manual transmission vehicles are easier to convert than automatic transmission vehicles, though they may also be converted.

TIP: Avoid a donor with rust, even if it is free!

Preparing for Conversion

Remove the stuff you don’t need; engine, radiator, fuel tank, fuel lines, fuel pump and filter, exhaust system and related wiring. Use appropriate safety equipment and procedures. Your hands will get really dirty doing this work. Your hands will be surprisingly clean working with the EV components!

Selecting EV Components

This is perhaps the hardest part of doing a conversion. There are several key decisions to make that will affect your choice of components.

AC or a DC system

AC (Alternating Current) systems are considered to be superior and are more efficient. All of the factory EVs use AC systems. AC systems will use higher voltage, which is more efficient and can use lighter gauge wiring. AC will also incorporate regenerative braking more easily. The downside to AC is that it is significantly more expensive, tends to have less torque and components are less available to EV converters.

DC (Direct Current) are far more popular and available to DIY EV converters. They are about 30% less expensive than AC, but do not easily incorporate regenerative braking. DC is likely 10-15% less efficient than AC systems.

Type of Battery Chemistry; Lead or Lithium

Until recently, deep cycle lead acid batteries where the only choice for conversions. These batteries are inexpensive, but have a low energy density and relatively short cycle life. Conversions using lead batteries tended to be very heavy, would accelerate fairly modestly and would have very limited range. Small cars would have a range of 30-60 kilometers while trucks with 50% battery load might travel 80-120 kilometers. Battery packs may need replacing every 3-5 years. Neglect or abuse would often shorten pack life even further.

Lithium iron phosphate (LiFePO4) cell technology is now dominating the conversion scene. This chemistry has roughly four times the energy density of lead acid batteries! These lithium batteries are sealed, requiring no service. They may also be discharged to a far greater extent than can lead batteries. Cycle life is 4-8 times longer. Even though the initial purchase price is much higher than lead acid, lithium batteries are considered a much better value. A conversion that uses lithium cells will be far lighter, quicker, have far better range and will be more fun to drive.

Size of Battery Pack

The size of the pack , voltage and amp hours (or kilowatt hours) depends on what you want your vehicle to do. An electric race car will have a very different pack than will a commuter vehicle. The race car needs a pack that will deliver a lot of power for a short period while a commuter vehicle uses lower power levels for a longer period.

Most of us will answer the battery pack question by considering the range that we need. If you only need a range of 30 kilometers, you could likely use a lead pack. If you need a range in the neighborhood of 100 kilometers, you’ll likely need a lithium pack with about 20 Kwh of available energy. This would mean a pack size of 25 Kwh, since you will only want to discharge the pack by 80% (called 80% DOD, degree of discharge).

A typical LiFePO4 pack with a 25 Kwh capacity would consist of 45 individual cells rated at 3.2 volts, each with an amp hour rating of 180. The cells would each be connected in series (positive to negative) to result in a nominal pack voltage of 144 volts.

There are a variety of ways to achieve a desired pack capacity. AC systems would likely use more cells with a lower amp hour rating and would result in a higher voltage. AC systems are designed to operate at double or triple the voltage of DC systems.

Choice of Motor

Most DC conversions use brushed, series wound DC motors. These are compact, robust, simple, powerful and reasonably affordable. There are several popular brands of DC motors, several of which are built specifically for use in EVs. The Warp9 motor, from Netgain, is likely the most popular choice for conversions. It weighs about 70 kilograms and is about the size of a watermelon. The life expectancy of this motor is about 1,000,000 kilometers! There is only one moving part. The only service requirement is to change the brushes every 100,000 kilometers or so (easy and cheap to do).

Electric motors are given very conservative horsepower ratings. The Warp9 is rated at a whopping 32 hp. This is a very misleading number. The motor is capable at putting out up to 700% of this power for brief periods of time, such as while accelerating. Torque is the real story of electric motors. Unlike gas engines, electric motors make all of their torque available right from zero rpm. It makes for an amazing driving experience! Even more amazing is that it does this while being over 80% efficient. Most gas engines are less than 20% efficient.

AC motors have several advantages over DC motors. They are brushless, so will never require any maintenance. They are even more efficient, often over 90%. They can spin twice as fast as brushed DC motors. This makes them suitable for single speed transmissions. They are more expensive and must be carefully matched to a suitable controller (also more expensive).

Choice of Controller

The purpose of the controller is to supply power to the motor. There are many excellent DC motor controllers available to the DIY market today. The choice of controller depends on the type of performance you want from your vehicle. Controllers are rated by an amperage output. Usually a continuous rating and a short-term intermittent rating. Moderate, but acceptable performance controllers will have an intermittent rating of 300-500 amps and will be suitable for a small car. 500-1000 amp controllers will produce performance better than the original gas engine. The racing crowd will use 2000 amp controllers with very impressive results (silly, but impressive).

Almost all controllers work in conjunction with a small device called a “potbox”. This is short for potentiometer in a box. The pot box is connected to the accelerator pedal by a cable and to the motor controller by wires. The pot box tells the controller what to do. The harder you press on the accelerator, the more power is sent to the motor.

It is important to remember that the motor controller will only be able to transmit as much power as the battery pack is capable of delivering. A small power pack will not supply power to make the most of a large motor controller.

Choice of Charger

Most conversions will carry an on-board charger. Having the charger installed in the vehicle will let you opportunity charge whenever you are away from home. If you are able to charge while at work, you can effectively double your daily range.

Chargers are available from many companies with many different specifications, i.e. voltage input and output and amperage ratings. The larger the charger, the shorter the charge time will be. 240 volt chargers have the disadvantage of being able to plug into readily available 120 volt receptacles. Some chargers will can operate with either 120 or 240 volt plugins.

Smart chargers are programmable to work best with your battery chemistry and will not overcharge your pack.

Battery management systems (BMS) will monitor each cell individually to protect against over charging or discharging.

Transmission- To clutch or not to clutch?

The majority of conversions will use manual transmissions. Automatic transmissions require an idling engine to maintain oil pressure to the transmissions. Electric motors do not need to idle, so are not a perfect fit with automatics. They can be made to work though.

Electric motors are easily mounted to the manual transmission in a car. It requires that a motor adapter plate and spacer plates be fabricated or purchased from a supplier.

A choice to be made is to keep the clutch or not. It is easier to couple the motor to the transmission without the clutch. The rotational mass of an electric motor is so small that the clutch is usually not required. The synchronizing gears can do the job without disconnecting the motor and transmission to shift gears.

Shifting if rarely needed for EVs, due to the torque of the motor. You will not easily stall an electric motor, unlike a gas engine. All the torque is available from zero. You can easily start off in second or third gear. Electric motor, unlike gas engines, run more efficiently at higher rpm. Fourth gear is suitable for highway speed.

Putting it All Together

The main components have been listed, there are other details, such as power steering and power brakes. There are several solutions to these which are fairly simple and inexpensive. There are also wiring details such as safety fuses, a main breaker, a few relays. Again this may seem daunting at first but will become less intimidating as you work your way through it.

There will also be some work with metal and plastic. Components will need to be securely and safely mounted. Battery boxes will take some effort to configure. The good news is that it has all been done before and there will be someone out there who will be willing to assist.

Summary

Building a battery electric conversion is a very worthwhile and rewarding experience. The cost of the project can vary widely, depending on your expectations. A typical quality conversion will likely cost between $10,000-$25,000 for components. The vehicle should last a very long time and will save energy costs each time you drive it. It will be quiet, fun, efficient and will have no tailpipe emissions! Does it get any better than that?