The best part about building your own EV.
The best part about building your own EV is YOU get to choose all the parts to build the EV that best suits your needs.
Here’s some things you should be thinking about at this stage of the game:
- The platform you want to convert
- Your range requirements (and driving style)
- Is this a performance build or a commuter?
- What is your budget? The most meager of EV conversions still cost around $8-10k.
What’s your chosen platform?
The platform you choose will determine what you can and can’t do. Are you looking for the easiest conversion? Willing to trade simplicity for something more unique? If you need a lot of range, you need a lot of battery—does your chosen platform have enough room? OEM conversions have the luxury of building a battery compartment ground up, you may be limited.
Some considerations for what components you plan to buy are highly platform dependent. Today there are solutions for every type of drivetrain layout, new or used OEM.
Popular OEM components seem to be from the Nissan Leaf for a FF setup or a whole Tesla subframe for an IRS setup. These are good options, got a CRX or other Honda that’s already tricked out, Leaf.
A 240 or 3-series drift missile? Maybe the Tesla. Bear in mind that most Tesla motors will have to come with proprietary VCUs (Vehicle Control Unit) developed by parts houses, an additional cost to think about.
Maybe a muscle car or some other FR setup with a live axle? There are hundreds of different motors and adapter options which can be considered.
- A used Leaf motor and gearbox setup can be had for around $1,000-$1,500.
- Most brushed DC setups found in mid-weight FF/FR vehicles run $3,000 for motor + controller.
- BLDC setups start at around $6,000 for 100kw motor + controller.
- Tesla rear-drive units ready-to-run are in the $11,000 range.
Here are a few tips on keeping a first conversion simple.
First using off the shelf components will make any process much smoother. Transmission adapter plates can be bought to save having one designed and machined (many DC brushed motors share a common “B-face” bolt configuration). Choosing a vehicle that has an off the shelf adapter is a good way to go for a first conversion. (Miatas, Toyota trucks, S10, VW Rabbit)
Using OEM batteries comes with its own perks. Safety fusing, bussing and disconnecting means are usually already built in. Depending on the route you go they could also include a casing which could cut down on fabrication needed for safely containing the batteries. (Leaf batteries are plentiful as are Chevy Volt)
Pickups are a popular choice for first conversions because they are built for the weight unlike a little unibody car and they have a space that’s easily adapted to carry batteries, either in a bed-workbox configuration or underneath the bed between the frame rails.
We will discuss motors and transmissions further in but a good rule of thumb is keeping a transmission is a cheaper way to retain factory acceleration and top speed than having an “all motor(s)” build. Manuals are simpler than automatics and can even be driven without a clutch if you choose simply by welding components like the center of the flywheel and clutch hub together.
The main components of an EV conversion are the batteries, motor, motor controller, controller cooling system, motor cooling system, battery cell management, battery monitor, VCU, charger, charge port, contactors, emergency disconnects and fuses. I’ll cover the big three in this article.
Brushed vs. Brushless DC Motors
There are three types of electric motor commonly used in conversions. The first and the original is the series wound brushed DC motor (think De Walt battery drill). This is by far the cheapest setup today and as such is very popular and is still seen as the gateway drug into custom Evs.
Brushed motors do have their drawbacks though, they rely on motor brushes pushed against a rotating commutator, these create messy carbon dust and require replacement (around 70,000 miles) for high current applications, as they are a crucial component that can be subject to extreme heat.
The torque band of a brushed motor is actually quite narrow with its peak dropping around 3000 rpm. From zero to there you will have full torque, for this reason most conversions using this type of motor retain the original transmission of the vehicle and mate to it with an adapter plate.
The other drawback of these motors is most are open frame air-cooled designs, even though these are cheap and can be quite high power motors, air cooling has its limits. Max rpm of most motors of this design is around 6-7000rpm.
The next type of motor is a brushless DC motor (BLDC). With the advent of modern semiconductors, the elimination of a rotating commutator was made possible, no brushes or copper surfaces to wear out.
The big advantage here is that a BLDC motor shares with an AC motor is regenerative capabilities. Re-gen is possible with brushed designs but it’s much more of a headache than any EV conversion should consider, especially with off the shelf BLDC and AC setups available.
Most AC and BLDC units are often sealed and liquid cooled, this is a massive advantage that cuts out service and temperature concerns.
The RPM limits on these motors are also higher with most able to do between 7500 and 16000 rpm at the top end. Torque curves are also flatter and hold more out to higher rpm.
Transmission—should it stay or should it go?
A big part of choosing a platform is whether to retain the transmission. Automatic transmissions can be used but by far the simplest option if you need to retain the transmission is a manual. Automatic transmissions that don’t retain a torque converter (reduce driveline mass) and motor idle will require a pump and RPM cutout sensor for when the motor does spool up to turn off the pump.
Both the clutches in an auto and a conventional manual with a flywheel will need to withstand the rated peak torque of the motor you choose, this is not an area to skimp on and motor torque can change depending on controller and battery choice.
You don’t have to retain the transmission but, depending on the RPM limit of your motor and the available motor amps (torque), it is advisable to consider it. In most single motor, brushed DC conversions this is done but direct drive is possible with a short final drive ratio (2.7-3.5). Usually it’s either a very high torque motor or an inline twin motor setup. The short ratio ensures you can reach a reasonable top speed with a motor that loses torque around 3000rpm.
A Tesla reduction gear is 9:1 because its motor can spin at 16,000 rpm so it can have a tall ratio for starting torque and still achieve respectable top speeds. Most factory EVs use a similar single gear approach.
A motor controller or inverter is basically the device that modulates voltage and current across a load.
For AC and BLDC setups, most of the time the motor and controller/inverter come paired or will list only a few motors they will be compatible with. For brushed DC there are a wide variety of controllers ranging from 50kW to 1MW. Most are in the 100kW to 300kW range and will work with almost any brushed DC motor within the voltage output constraints of the controller.
The big thing to realize about controllers is their configurability. Most have a voltage input window with a minimum. Lower power controllers may be 72-144V and high power controllers can go from 270-550V. Voltage outputs are very motor specific. Brushed DC motors usually top out around 170V but BLDC and AC setups can use full pack voltage as they typically draw less current than DC setups.
Controllers will have provisions for vehicle inputs and control including things like contactor drivers (a contactor is a just a large relay capable of passing and breaking high currents), RPM governing, auto-idle and cruise control, etc.. Throttle inputs can be varied by including simply a potentiometer or a hall-effect sensor. High end and OEM controllers also use CANBus capable VCUs to manage I/O.
Batteries = Power
The biggest shock (aah..) most people get from learning about electric vehicles is that the motor is not always the thing that determines how much power or even torque a vehicle has. This is more true of DC brushed setups, as usually the motors are capable of huge power peaks—if set up correctly—that usually exceed the battery budget of the conversion.
It should be intuitive that the battery is what determines total power, how much power can you dump at any one time. The main thing governing this is capacity—how many amp-hours is the battery?
Batteries have a “C rate”. C for capacity so its charge rate is expressed as C/? and its discharge rate Cx? Or C?
One of the most popular DIY EV battery chemistries at the moment is Lithium Iron Phosphate (LiFePo4) and they commonly have a charge rate of C/3 and a peak discharge rate between C5 and C8. This constraint means a 50ah battery can dump a maximum of 400 amps peak and a 100ah battery can dump 800.
OEM EV batteries are usually Lithium Manganese Cobalt due to their higher energy density, learn more about battery chemistries here.
Kilowatt-hours. The EV version of whp?
When talking about battery packs it is also common to describe the energy as kilowatt-hours, Kwh. This is handy for estimating total power, kW which is easily convertible to hp (100 kW or 100,000 watts / 746 = 134hp) and also in estimating range given an EVs efficiency in watt hours per mile or miles per kWh.
Larger capacity batteries with C6 to C10 discharge rates are the rule for higher performance. A 60kWh Tesla battery can dump around 400kW of power for 10-20 seconds, but of course, this makes for a ~4500lb car. There are other options we will look at.
Ready to shop for different EV performance parts?
There’s a LOT of information in this one, but think of it like explaining the powertrain of a conventional vehicle to someone from scratch. EVs might seem different, but at the end of the day, they’re still cars and trucks like any other—they just use different parts to make the wheels spin.
Here’s a few of the more reputable vendors in the space, offering all the bells and whistles mentioned above, so you can start exploring different performance parts.
EVSource – A supplier of predominantly NetGain and HPVS motors with a good selection of other components.
EV West – A wide selection of components including full Tesla swap packages.
HSR Motors – A company dedicated to selling Tesla drivetrain packages and motors.
eRepairables – Salvage site with listings for EVs.
EV Trading Post – used EV components and half finished projects.
Don’t forget the little things, either.
Don’t forget all the other little things you don’t want to forget, like bulk wire, wire terminations, specialty tools, etc. These are also available from the above suppliers.
Coming in the next episode…
In addition to the major components, in the next installment, I will cover everything else which needed —with a price breakdown including high voltage wiring and 12V control systems with a simple diagram explaining what goes where.
Hopefully this has answered some basic questions about EV components and some of their characteristics, capabilities and limitations, please ask any questions you have below to generate useful discussion!