An Electric Miata?

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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 stew-ing while his Corvette burned gallons of the precious stuff. Finally he reached his limit.

But since Ken is a mild-mannered guy, he didn’t make like the deranged TV anchor in the movie “Network” and hang out the window screaming, “I’m mad as hell, and I’m not gonna take this anymore!”

Instead, he kept his mouth shut and built an electric vehicle.

First, he bought a theft-recovery Chevy S-10 and converted the little pickup to electric power. He enjoyed the project and learned a lot.

He drove the truck every day, relishing the attention it drew and pocketing the gas savings. Two years later, someone took a shine to the vehicle and made Ken an offer he couldn’t refuse.

Ken parted with the S-10 but was determined to build another EV, this one sportier and more fun to drive. Like a Corvette, maybe, only smaller and quieter.

Turning a Red Miata Green

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 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 “Many folks are eager to learn about EVs,” he says, “especially when the price of gas goes up.”

Living With a Miata EV

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

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.

Behind the Wheel

That whirring you hear when you turn the key is not an electric fuel pump; it’s a vacuum pump for the brake booster. When the booster tank is evacuated, silence prevails.

Select a gear—no clutch needed, since the electric motor idles at zero rpm—and release the emergency brake. Then, press the throttle; just don’t call it a gas pedal. Ken Watkins’s electric-powered Miata glides off with a slight motor whine.

Since the electric motor packs such a broad torque curve, you can start in almost any gear. Ken generally starts in third or fourth, but either one provides enough punch to keep up with normal traffic. For quicker launches, you may start in a lower gear, although first doesn’t get much use.

When you want to shift, lift your right foot and ease—don’t yank—the shifter into the next gear. The electric motor lacks the rotating mass of a reciprocating engine, so the transmission’s synchronizers can match motor speed to drivetrain speed and handle deliberate shifts without a problem. Ken reports that at 65 mph in fifth, the motor is turning 3000 rpm. He has run it up to 70 mph and says it could go even faster.

With all that lead stashed up front, slow-speed steering is heavy, but it lightens up once you’re rolling. You feel the extra weight when putting this Miata through quick transitions, but overall it seems well balanced.

Acceleration won’t snap your neck, but the car moves out smartly enough. The beefed-up suspension handles corners well, and the brakes manage even panic stops without drama.

The tachometer and speedometer work as normal, but the instruments to watch are the three mounted above the stereo head unit. The most important one tells you the remaining charge. It’s your “gas” gauge.

Ken says this EV has changed his driving style. Mindful of its limited range, he strives to conserve battery charge with smooth takeoffs and moderate throttle inputs. “It takes more charge to accelerate,” he says. “You can maintain speed pretty economically.” Once up to speed, he plans ahead to maintain momentum whenever traffic allows.

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View comments on the GRM forums
te72 Reader
7/11/18 9:59 p.m.

I wouldn't mind trying the lithium ion version, if his math of 1/10 the weight is ballpark accurate. Figure 1700 lb Miata with instant torque? Now that... that could be fun. However, the "10x the cost" part would be rough. Figure a typical battery is $100, and there's 19 of the things in this car? So, by that (likely outdated, thanks technological progress and economy of scale!) math, we're talking around $19k just in batteries? Sheesh...


All in all though, it makes me wonder where the cost per mile math ends up on something like this. Driving experience on paper sounds horrible, but only because weight is the enemy of the Miata chassis. The less of it you have, the better it is, and vice versa. Figure the cost of energy, gas or electron, plus you wouldn't have to ever change the oil, filters, etc, BUT... you have to change the batteries around the 5 year mark.


I dunno why, but the math aspect of this is more exciting to me than the driving aspect.

exST165 Reader
7/13/18 2:56 p.m.

What if the goal was to build an autocross weapon?  Just enough battery for a 60 second run and power to rival a supercharged CSP?  There would be some gain from not having to shift if you could launch in second or third.  Could the batteries be small and light enough to not hurt handling?  For a 50 or 60 second run how much time is at WOT?  

Given the differences in torque curves what kind of electric motor would be needed?

7/13/18 5:01 p.m.
exST165 said:

What if the goal was to build an autocross weapon?  Just enough battery for a 60 second run and power to rival a supercharged CSP?  There would be some gain from not having to shift if you could launch in second or third.  Could the batteries be small and light enough to not hurt handling?  For a 50 or 60 second run how much time is at WOT?  

Given the differences in torque curves what kind of electric motor would be needed?

Team Frozen Nuts Racing

It should be noted this article was made in 2013, and the advancements in battery technology since then have honestly been legion in number- Lithium-Sulfur and Solid-state seem to be the next types of energy storage we will see, though Lithium-ion will be hanging around for far longer. And thanks to the Leaf, Volt, and other EVs battery prices are falling quickly.

As for a motor? Depends honestly on price and maintenence. D/C wound motors are cheaper and have some interesting abilities over A/C due to brushing and affects of how they generate the field, but for preformance an A/C motor is what you want since they 'convert' electrical power to rotation faster. The other problem (in cost) is your controller- all batteries can do A/C power no problem, but the controller that takes consistent battery power and coverts to A/C will be expensive.

Since you don't need a transmission with motors (in some cases) an easy upgrade to this car would be two smaller D/C motors linked (somthing like a WARP9, since you can buy them with a diff-yoke pre-welded to the output shaft) for high torque, a limited slip diff for safety, and a controller than can handle sudden discharge but doesn't require any regen braking stuff. From there, you'd need to source a battery system, a vacuum pump for your brakes, a charger, and your gauges- lots of electric guys use crank position sensors on the main motor shafts for Speedos and there are plenty of voltimeters for EVs now that show charge in easy-to-read precentages.

SVreX MegaDork
7/13/18 5:21 p.m.

I really want to do this!

SVreX MegaDork
7/13/18 5:24 p.m.

With Leaf prices dropping through the floor, wouldn’t a wrecked Leaf be an excellent candidate for a drivetrain donor for an electric Miata?

Regarding the Autocross weapon... I considered this for the Challenge.  The car only needs to run for 60 seconds at a time. Spare fully charged batteries could be on site, Ready to swap in. Since they are not on the car, the spare batteries should be budget neutral. 

Justjim75 Reader
7/13/18 5:25 p.m.

In reply to te72 :

Not trying to be a butthead but it may help or hurt someone's decision to know, 19 batteries times $100 each is $1900, not $19k

kb58 SuperDork
7/13/18 5:42 p.m.

^ beat me to it...

7/13/18 5:42 p.m.
SVreX said:

With Leaf prices dropping through the floor, wouldn’t a wrecked Leaf be an excellent candidate for a drivetrain donor for an electric Miata?

Regarding the Autocross weapon... I considered this for the Challenge.  The car only needs to run for 60 seconds at a time. Spare fully charged batteries could be on site, Ready to swap in. Since they are not on the car, the spare batteries should be budget neutral. 

Orange is the new Flat Black

Yes. Leaf batteries are about 4.3 amp/hours each. The stock motor for a leaf is somewhere in the 150 horsepower range with about ~200 'torque', but with aftermarket controllers and battery maintenence you could get more. Your main problem would be keeping them properly cooled.

kb58 SuperDork
7/13/18 5:51 p.m.

True, but being run hard for only 60-seconds, then being switched out, it seems like the thermal mass alone would keep it from getting too warm.

SVreX MegaDork
7/13/18 5:54 p.m.

In reply to kb58 :

As a Challenge weapon, I’d run lead acids. Because they’re cheap. 

The Leaf battery would be better for a full time commuter. 

kb58 SuperDork
7/13/18 6:47 p.m.

I never decided whether I liked the semi-reality show "Fastest Car", but there was a Datsun 1200 in there that had been converted to electric. I won't give away the ending but it certainly had the power/torque to win... whether it did or not, you'll have to watch :P

(If you want to cheat and save some time, you can just jump to the last episode where they have their overly-dramatic run-off.)

SVreX MegaDork
7/13/18 9:23 p.m.

In reply to kb58 :

That would be The White Zombie. It was built nearly 20 years ago, but now runs over $100,000 batteries which are Navy prototypes. It’s allegedly the worlds fastest electric car.

It may be wearing the skin of a 1200, but it is a very serious drag race car. 

exST165 Reader
7/13/18 9:31 p.m.

Just found a Warp11 on eBay for $3,900 so not Challenge material but 400 ft:lbs at zero rpm and goes to 4500 rpm weighs 226 lbs!  didn’t see a diameter but get a diff yoke adapter and mount it in the transmission tunnel straight to the diff.  Math would be needed to double check that it won’t run out of rpm at autocross top speed and probably some massaging of the tunnel.  Mount the batteries in the passenger seat area and the polar moment of inertia would be minimal!  But would that 226 lb motor and batteries be lighter than the Miata engine and transmission?  And to be fair supercharger.

Dream is to do something like this for Challenge money but until electric motors start appearing in junk yards in some volume I don’t see it happening.

te72 Reader
7/14/18 12:40 a.m.
Justjim75 said:

In reply to te72 :

Not trying to be a butthead but it may help or hurt someone's decision to know, 19 batteries times $100 each is $1900, not $19k

Perhaps my explanation wasn't clear, or maybe it didn't read well. I was saying the lithium version was of interest to me. The article states that the builder of this Miata wanted to do a lithium powered version, due to them being about 1/10 the weight for the same power, but at a cost of 10x as much.


Typical lead acid battery, around $100. 10x that much for a lithium would be $1000 per battery. This Miata has 19 batteries. Hopefully that makes more sense. =)

DrBoost MegaDork
7/14/18 1:49 a.m.

$8,000 buys a LOT of gas. But I agree, it's cool and something I've always wanted to do 

BirgerBuilder New Reader
7/14/18 9:01 a.m.

Chevy Volt Batteries have a similar range to this, 40 miles before you start dipping low enough to damage the cells. On ebay they currently go for... $2500. 

Get building guys. 

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