Gas prices hit the $4 mark for the first time a few years back. That’s also when Ken Watkins had an epiphany while languishing in traffic: Day after day, month after month, he sat behind the wheel stewing while his Corvette burned gallons of the precious stuff. Finally …
Turning a Red Miata Green
Seven batteries fill the underhood compartment. The Miata’s original throttle cable operates the potentiometer (left), telling the motor controller (behind the aluminum plate) how fast to turn the electric motor (center).
Ken is a retired electric engineer who built lasers for the military. He’s always loved working on cars and has restored two Corvettes. These days he lives in Lake Mary, Florida, and teaches in the automotive department at a nearby college. His students’ mechanical projects have included building a Chevy Cavalier EV.
After he sold the S-10, Ken searched for a small, nimble car. He soon found a sound, undamaged 1991 Miata at a good price. Of course he didn’t care about the engine and most of its supporting equipment. These heavy, greasy parts were soon piled on his garage floor, prime candidates for eBay listings.
Ken knew from his S-10 experience that transforming the average car into an EV is much like building a race car: Performance is a compromise between power and weight. When building an EV, weight is the enemy, but heavy lead-acid batteries are your friends. Ken spent a lot of time punching his calculator, evaluating components and testing various battery layouts within the Miata’s small available space.
When the adding and subtracting was done, Ken had removed about 500 pounds and added nearly 1500. In finished trim, the Miata weighs about 3000 pounds and has a range of about 40 miles on a full charge.
Since Ken has a degree in electrical engineering, he is well versed in how tiny electrons work together to motivate a car. However, he maintains that it doesn’t take geek-level knowledge to build an EV.
“Anyone who can turn a wrench can build a car like this,” he says. And to make the task easier, several online suppliers offer complete kits and support. In all, this project took Ken about six months—not counting the bodywork and painting, which he farmed out.
Amping It Up
The lowered trunk floor accommodates 12 stacked batteries. The box on the bottom left houses a contactor and fuses. The one on the bottom right is the DC/DC converter, which transforms storage battery voltage into a 12-volt supply for the EV’s lights, radio and wipers. The test points make it easy to check the voltage on the lower set of batteries.
The four basic components of any EV conversion are a motor, motor-to-transmission adapter, motor controller and battery pack. Choices abound, but final selection depends on the vehicle’s size and available battery space, the desired range and, of course, the budget.
Ken chose a 9-inch-diameter DC motor that weighs about 90 pounds, turns up to 4500 rpm, and generates about 20 horsepower with 100 lb.-ft. of torque. While that doesn’t sound like much oomph, electric motors generate full torque at startup, so that power is adequate for a car this size.
Mounting such a small motor in an engine bay is not difficult, but it requires accurate measuring and fitting. Ken designed and built his own motor mounts, taking advantage of the Miata’s original front engine mounts.
Ken also crafted the motor-to-transmission adapter, which aligns and secures the motor to the transmission’s bell housing. Inside the adapter, a coupler connects the motor’s shaft to the transmission input shaft.
This Miata came with an automatic transmission, but automatics are seldom used in conversions. First, they require an external pump for circulating transmission fluid because electric motors don’t “idle” at stop. Second, they’re generally heavier and less efficient than manual transmissions. Ken’s solution: Install a five-speed manual without a clutch and leave the rest of the drivetrain standard.
An EV’s motor controller (Ken used one with a capacity of 650 amps) might be compared to a gas engine’s carburetor or fuel-injection system, since it regulates the flow of juice to the motor. A variable resistor attached to the go pedal translates the throttle input to an electric signal for the controller, which provides smooth control of motor rpm and vehicle speed.
For this Miata, 19 six-volt, 200-amp, aqueous gas mat (AGM) lead-acid batteries provide the power. Wired together, their output adds up to nearly 120 volts. With proper care—which includes avoiding total depletion and applying regular recharges—batteries like this should last about five years. Ken stacked these cells for the best possible weight balance within the available underhood and trunk space.
Sorting It Out
Like any newly built project, Ken’s required some sorting, mostly because of the added weight. After he installed the batteries, the Miata’s rear suspension rested on the bump stops.
So he did some research, weighed the car’s nose and tail, and called Ground Control Suspension Systems with the numbers. These suspension specialists built a set of coil-over shocks designed to handle the extra load. Once installed, this stouter suspension leveled ride height and dramatically improved handling.
To handle the increased poundage, the Miata’s brakes also needed attention. Ken learned that 1994-and-up Miatas were equipped with larger brakes, so he located a set and installed them. Braking was much improved.
After some road time, Ken discovered that the motor controller was generating too much heat, so he built a water-cooled heat sink that’s connected to a heat exchanger mounted in the car’s front grille opening. Now the controller runs cool even in the Florida heat.
A set of spoked aluminum wheels from Team Dynamics Motorsports gave the roadster a bit more flash and saved a few pounds over the original steel wheels.
For the final touch, Ken installed “Electric Vehicle” decals. “Some of my friends thought I ought to go with the stealth look,” he said. “But I wanted to spread the word.”
The car does attract a lot of attention, and curious motorists ask questions at every stop. Ken is always happy to oblige, and he has even written a how-to book on his S-10 and Miata experiences. You can find more info at evsbyken.com. “Many folks are eager to learn about EVs,” he says, “especially when the price of gas goes up.”
Living With a Miata EV
Plug-in stations are popping up in business and public parking lots across the country, allowing EVs to stray farther from their home chargers.
A charger mounted to Ken’s garage wall handles battery duties. He plugs it in after he parks each night, and when the charge is complete, it shuts itself off.
Ken says the electricity needed for a 40-mile trip costs between $1 and $1.50. His normal commute is about 25 miles, so a full charge is not needed each day. Charging stations are now showing up at hotels and other locations in many communities, but hookup plugs can differ depending on charger type.
This Miata is responsible for zero tailpipe emissions. In fact, it no longer has a tailpipe. But since no household energy from the grid is emission-free, it’s safe to assume that this car’s emission contribution is about the same as any appliance that consumes an equal amount of electricity.
The Miata has served as Ken’s daily commuter for the past three years, and he reports that taking care of it is as simple as, well, flipping a light switch. “It’s been very reliable,” he says. “I’ve had no trouble with it, and it’s fun to drive.”
Since he’s always ready to take on another project, he’s itching to build a third EV. He’s hoping his next conversion will be able to take advantage of lithium-ion batteries, which weigh some 70 percent less than lead-acid units but currently cost about 10 times more.
“Interest in EVs is growing,” he says. “People are getting enthusiastic about electric cars, and there is plenty of support out there. You can buy complete kits with very detailed and illustrated instructions for specific vehicles.” There are also plenty of online forums and EV clubs.
Ken estimates that an average gearhead can convert a small car like a Miata to an EV for about $6000 to $8000, not including the cost of the car and any paint or bodywork. As with any home garage project, labor is always considered free.
Lightning, Hold the Grease
The DC ammeter (left) measures current power use. Stomping the throttle pegs this meter past 400 amps, but a light foot conserves charge and maintains motor draw at around 100 amps. The voltmeter (center) indicates remaining battery capacity. When this falls to around 80 volts, indicating that about 80 percent of the charge has been used, you’d better be near a charging station. The right meter monitors voltage for the normal, 12-volt circuit—think lights, radio, wipers, etc.
For a commuter, this car works quite well. Because of recent increases in gas prices, it can allow its occupants to feel a little smug as they silently glide past all those gas stations they’ll never need to visit.
One caution, though: No matter what the source, you can still run out of juice, so you’d better keep a watchful eye on that voltage meter.
Photography by John Webber
