EV Design Considerations. So you want to build an EV?

I’ve been asked to comment on what makes a good donor vehicle for an EV conversion.  There are a couple of books on the subject of EV conversions, but I mean that literally.  Perusing Amazon, you can find loads of books on other alternative energy projects; solar hot water heating, wind turbine power,  solar photovoltaic, etc…  But converting a car to electric power, that’s a tricky one.

The most well known book on conversions is probably Bob Brandt’s, “Build Your Own Electric Vehicle”.  Another is Michael P Brown’s, “Convert It!”.  Both were originally written in the mid-90s, so while the theory found within is as relevant today as it was in 20th century, some of the technology is different.  If I had to recommend only one, it would be “Build Your Own Electric Vehicle” because it’s quite a good reference tome with lots of charts and graphs, equations and examples.

At the heart of it, EV conversions still remain the pervue of hobbyists and hackers,  environmentalists and gearheads.  And thankfully the internet exists, because its probably the best place to gain insight into building your own EV.

Let me start with a fundamental rule of EVs.  EVs cannot replace a conventional car 100% of the time for 100% of the people.  But an EV could play the role of a second vehicle perfectly, even if it’s the vehicle you do 90% of you driving in.  The simple reason is range.  The batteries just can’t practically store enough potential energy for  significant range.  You won’t take your EV on a long driving vacation, but depending on your commuting habits it could be a winning solution.  GM thinks that 75% of Americans commute an average of 33 miles a day.  It’s why they’ve designed their upcoming Chevy Volt to run 40 miles on a single battery charge.  How far do you drive a day?

GM's Chevy Volt plug-in hybrid.  Will do 40 miles on electric power alone.

GM's Chevy Volt plug-in hybrid. Will do 40 miles on electric power alone.

This kind of subjective thinking becomes the driving force behind choosing a donor vehicle for EV conversion.  What do you want out of it?  Do you need ultimate range because of a long commute, or will 20-40 miles be enough?  Do you mind spending lots of money on state-of-the-art lightweight, high capacity battery technology, or is budget more of a consideration?  Do you need to dust everyone in the vicinity away from a stoplight, or is more leisurely performance acceptable?  Do you need the utility of a large vehicle, or will a compact suffice?  Do you need the latest automotive gadgets at your disposal, or will the technology from a decade do the job?  Homebuilt EVs become a very personal decision and design, but findamentally they all come back to one imporant factor.

Designing an EV centers around efficiency.  Since the battery technology is limited in this regard, everything about designing an EV takes efficiency into consideration.  Sometimes, ultimate efficiency isn’t the desired goal or even necessary,  but efficiency is what ties everything together; it dictates the design, component selection, packaging, range, performance, and cost.  At the end of the day, choice of donor vehicle often comes down to availability and desireability.

Tesla Roadster EV can do 250 miles on charge and 0-60 in under four seconds. Costs $109,000!

Tesla Roadster EV can do 250 miles on charge and 0-60 in under four seconds. Costs $109,000!

There are a few basic tenets of EV design.  Weight is bad.  Friction is bad.  Both of these things will sap the energy out of batteries in short order.  And, I’ve chosen to convert a heavy, 4wd, aerodynamic brick of a vehicle!  Well, that’s not entirely true… except the aerodynamics part.  Let me explain my reasoning.

Let’s look at weight first.  The less weight, or mass, you have to move the less energy is required to move it. Keeping a heavy mass moving at a constant speed doesn’t really use much more energy than a light mass, but accelerating the heavy mass to speed will consume more power.  The trick is finding a vehicle chassis (or “sled”) that manages to be lightweight yet strong enough to carry the EV power system and have the utility you desire.  Geo Metro’s make great conversion candidates because they weigh very little, but they may not be practical from other standpoints, such as some of the ones that didn’t make them very attractive as conventional vehicles.  I really dig that this particular Metro has a scoop on the hood.  I don’t know why (I don’t know I dig it or why it has a scoop on the hood.)

It's a Geo Metro!  With a hood scoop!  Why?!?  Why not!  Awsome...  Sadly, this particular car isn't an EV, but if it was it would have made the hood scoop that much better.

It's a Geo Metro! With a hood scoop! Why?!? Why not! Awsome... Sadly, this particular car isn't an EV, but if it was it would have made the hood scoop that much better.

My Land Rover is actually fairly light for the type of vehicle it is.  Most modern cars have bloated in recent years due to the addition of safety equipment and amenities; things like soundproofing and complex electronic systems.  Today’s Toyota Camry weighs close to 3400 lbs.  My Land Rover, a 4wd truck that stands 6-1/2 feet tall with a boxed-steel ladder chassis, weighs a little over 3100 lbs.  This is mainly due to its short length and weight saving aluminum body panels… and zero sound insulation or amenities.

Back in 1999, the Toyota Camry weighed about 3100 lbs, as well.  In 1986, the last of the first generation models, it weighed a scant 2300 lbs!  Now of course, it has grown dimensionally over the years, it’s gotten more powerful,  and it has, no doubt, become safer and more comfortable.  But the Toyota Camry remains a perfect example of where cars have come over the past 20 years.  The current hybrid version of the Camry weighs in a 3700 pounds.  What a porker!

Weighs a ton... or two.  But I still managed 45 mpg in one.

Weighs a ton... or two. But I still managed 45 mpg in one.

I mention it because it’s something to think about when choosing a donor car.  Do you want all the amenities and luxuries of a modern car?  Or can you live with something a bit older?  Certainly, going with the older vehicle is a more efficient option.  Using older vehicles as donor platforms for an EV conversion not only becomes a less expensive option, but the lack of electronic doo-dads in the older models makes a conversion much easier.

VW Beetle is a popular EV conversion, with kits supplied by several companies

VW Beetle is a popular EV conversion, with kits supplied by several companies

That covers weight.  What about friction?  There are a couple of things which contribute to friction.

One is the drivetrain layout.  A four wheel drive system will be less efficient than a front-wheel drive layout due to the increased number of moving parts.  The same thing is true, but to a lesser degree, with rear wheel drive vehicles… unless they’re rear or mid engine designs, like VW Bugs or Porsches (or Lotus Elises or Toyota MR2s… or Lamborghinis.) Every time power has to make a turn, such as with a direction changing differential, the system loses power to friction.   Front wheel drive transmissions do have differentials, but most designs are transversly mounted, which leads to no directional change in the differential… more efficient.

My Land Rover is indeed a 4wd vehicle capable of wasting useful power, but it’s not full-time 4wd.  Unlike most modern 4wd systems, I can choose to run the vehicle in 2wd with power going to the rear.  I can also unlock the front hubs which disconnects the front driveshafts and differential from creating parasitic drag, so it essentially becomes a rear-wheel drive car.  Not the most efficient, but not terrible either.

Ford Ranger EV, popular with converters due to its high load capacity, its relatively light weight, and good utility

Ford Ranger EV, popular with converters due to its high load capacity, its relatively light weight, and good utility

Second on the friction list is aerodynamic drag.  The more sleek a vehicle is, the more efficient a path it can carve out of the atmosphere.  Modern vehicles are pretty good in this regard.  38 year old tractors aren’t.  I have no design excuse here except that,  aerodynamics don’t really play a part unil faster speeds since aerodynamic drag increases squared to speed.  I takes substantially more energy to push through air at 55 mph than 35 mph, and exponentially more to go 65 or 75 mph.  If I needed an EV that could do highway speeds, I wouldn’t choose the Land Rover.

Mercedes-Benz SLS AMG in the wind tunnel.  An EV version is planned for 2015.

Mercedes-Benz SLS AMG in the wind tunnel. An EV version is planned for 2015.

That’s not to say that my projections for my Land Rover’s performance don’t include highway speeds.  On the countrary, I should be able to hit very illegal speed limits with ease… but I’ll burn through juice doing it for too long.  My design considerations only required an average of about 40mph and for that, while slightly bloated, the Land Rover should do fine.

While not the most ideal EV candidate for efficiency reasons, the Land Rover becomes moderately reasonable.  It more that makes up for it with its classic appeal, the fact that I already owned one, its ability to soak up EV components with ease, and utility.

As with most other products that can be represented by a triangle of competing attributes, the three points on the EV triangle are cost, performance, and range.  You can have amazing range and performance, but it will cost you.  Or you can have a low cost conversion with decent performance but low range.  The trick is finding out a middle ground that works for your situation.  Start with one point on the triangle and work from there.

So what makes a practical EV?  It all depends on how much work you want to do, how involved you want to be in the building and designing of your EV.

The easiest way is to purchase someone else’s EV, or find a factory built EV.  There aren’t a lot.  Many of you may know the demise of the GM Impact (as documented in the film “Who Killed The Electric Car”), but another factory built EV that continues to survive to day is the Toyota Rav-4 EV.  They’re hard to find, but by all accounts are great EVs.  You can also find Ford Range EVs, which was actually produced by FoMoCo for four years.  Most were sold to fleets.

Toyota RAV-4 EV.  Look Ma! No Engine!

Toyota RAV-4 EV. Look Ma! No Engine!

But where’s the fun in that?  This is about building an electric car!  For those that want the excitement of building their own, but without the stress of figuring it all out, you may opt for an EV kit.  Several EV companies produce plug-and-play kits for a few platforms; the VW Beetle, Porsche 914, and some universal small car or small truck kits.

Rebirth Auto's VW Beetle EV Conversion kit.

Rebirth Auto's VW Beetle EV Conversion kit.

By far the most challenging and satisfying method has to be building your own, from scratch.  At least I hope so.  We’ll see when I get to the end.  For the most part, though, pick any smaller car from around the late 80s to the late 90s.  Those seem to be cars that aren’t necessarily too old, but still maintain a reasonable weight, degree of safety, and functionality.  The slipperier the better!

You can certainly choose a late-model car, but the conversion will probably be more expensive and more difficult, namely due to weight gain and the complex electronics of today’s cars.  Things like older Honda Civics, VW Rabbits, Passats, Jettas, Toyota Tercels, Corollas, Geo Metros, Ford Rangers, GM S-10 small pickups, Miatas, Toyota MR2s, all would fit the bill.  I like the idea of converting a mid-90s Subaru Impreza or Legacy Wagon if you live in the snowbelt.  They suffer from a full-time 4wd drivetrain, but the losses may not be too bad.

It all comes down to what you desire… and how much you desire it.

More on how I came up with the component selection for my conversion in a later post…

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Article written by jeffg

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