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newguyintown 08-04-2011 10:30 PM

Help, What car for E.V. Conversion?
Hi everybody,
I'm looking for a car that would make a good candidate for an E.V. conversion. So far I've looked on eBay and Craigslist. I've found some old Honda CRX's which seem to have fairly low weight and drag. (on the wiki it had one of the lowest drag areas Vehicle Coefficient of Drag List - EcoModder)

I've also looked at some old sports cars. I thought that they might be aerodynamic enough to make a good E.V. but I'm not sure. I've found a 1978 Chevy Monza, and a 1980 Pontiac Sunbird which both seem to have a kammback type shape. My worry is that the roof slopes down too quickly, and there may just be turbulence all along it. Does anybody here know what the drag coefficients are for these vehicles?

I was also thinking of possibly using a diesel generator as a range extender. I could use biodiesel or normal diesel if need be. That would allow me to take the vehicle as far as I want. I was thinking if I make the generator's output a few kilowatts higher than the energy needed to maintain 60mph, the excess energy could go into the batteries to help on hills and things.

I also think that some aeromods are necessary to help reduce the energy required to run the car. I was thinking of a belly pan, rear wheel skirts, and smooth hubcaps, but I'm not sure what else to do. Could you guys help me out with ideas?


Daox 08-05-2011 08:25 AM

I'm not real sure why you need help. It sounds like you have a bunch of good ideas already.

Ryland 08-05-2011 10:14 AM

My worry with a car like a CRX is first off that they do not handle having extra weight in them very well and are really sensitive to how well they are balanced.
My other worry with the CRX is that you could be taking a gasoline car that gets some of the best gas mileage off the road, so if you do convert a CRX please start with one that someone already pulled the engine on, not with a nice running stock CRX HF

Patrick 08-05-2011 12:38 PM

Volkswagen Rabbit. There are many EVs based on it out there, kits are available, and even a book telling you how to do it. Check out Convert It: Convert It! (9781879857940): Michael P. Brown, Shari Prange: Books. There is also a kit available for the Porsche 914, but they are rarer and more expensive. You can make a great sports EV out of one, though.

newguyintown 08-05-2011 04:41 PM

Thanks for all your replies,

Daox, thank you, but I need help with some of the finer details, as well as some other areas which I am not very familiar with. I've learned a lot from research and calculations, but I have never actually built an E.V. so there are bound to be tricks of the trade I don't know.

Ryland, I'm hoping to find one with the engine and transmission shot or gone, but with the brakes, body, etc still good. I wouldn't want to take a good thing and mess it up. Plus it should be cheaper to get one that has the engine broken or missing. Do you know how the CRX is balanced, because I was thinking that if I keep track of what weight I take from where, and replace it with batteries, motors, generators, etc that the vehicle could end up the same weight distribution and it wouldn't affect the suspension. Would it be ok if the vehicle ended up lighter as long as the balance was the same? Or should it end up pretty much the same weight? I'm going to do some research myself on the weight distribution, as well as the weight of parts I would like to remove.

Patrick, thanks for the advice. I'll be sure to keep a lookout for Volkswagen Rabbit with a broken or missing engine. On the sports car idea, the ones I was looking at are quite a bit under 1k, and look like they'd be fairly aerodynamic, at least at low Reynolds numbers. My problem with them is that I don't know how aerodynamic they'd be at 60-70mph. I would really like to have an old sports car as an E.V. because it would be really cool, but it has to be practical too. I tried searching the internet, but the only figures I got were 0.5 for the coefficient of drag on both of them, which seems very unlikely.

Daox 08-05-2011 04:50 PM

Have you seen BenNelson's DVD on how to build your own electric car? He sells them and it'll give you a very good idea of what you'll need to do with your own car.

Electric Car Instructional DVDs

For issues you need help with along the way, we're always here to give help. :)

Frank Lee 08-05-2011 04:56 PM

We don't have enough info. For example, how much range are you seeking? That leads to what battery tech you plan to use and if it's lead acid and longer range then you need a base vehicle with a higher load capacity, like a small pickup, but then if you need four seats you'd want something else and....

newguyintown 08-05-2011 05:18 PM

Thanks Daox, I'll check that out.

Frank Lee, I'm thinking of getting about 40mi of city range in the vehicle. I was able to use the "Aerodynamic & rolling resistance, power & MPG calculator" found here Aerodynamic & rolling resistance, power & MPG calculator - to estimate how many kwh I need for the range. I found that with the CRX it doesn't take much energy to go 45mph compared to other vehicles. I think I'm going to try to wire my own Li-ion battery pack.(i know it'll probably take a few thousand cells and it'll be difficult but that will get me the pack i want) If I build the pack/packs myself, I can make them lay flat. I was thinking I could mount them on the underside of the car, placed correctly to get the balance how I want, so that the front to back weight distribution stays the same as stock, but I may possibly lower the center of gravity. I'd then put a belly pan over them to protect them. I was thinking of using a diesel generator as a range extender because I could use biodiesel or normal diesel if need be. The all electric range would cover my normal city driving, and the range extender would allow me to go as far as I want in it. I was thinking the generator could go where the engine normally goes, to try to keep the weight distribution similar, the weight down, and allow lots of airflow through. All the generators I was looking at are air cooled, so I think it would be advantageous to place it in a high airflow area. I also used the calculator to find out the amount of energy needed to maintain 60mph in a stock CRX. I was thinking that if I make the generator a few kilowatts more powerful the extra energy could go into the battery pack, and drawn out if winds pick up, or you have to go on hills, or just need to go faster. I then calculated the amount of gallons per hour to find out kWh per gallon, and in turn miles per gallon. It looks pretty promising so far, but has to be tested to find out for sure. I think the Honda CRX, or an old 70's-80's sports car would work well for me because if they take less energy to run, I don't need as many batteries, and in turn not as much weight is put onto the frame. I will have to research the weight of all the particular parts of the car to find out how much weight is taken from where, and how to replace it to keep the weight distribution the same. There isn't as much information available on the old sports cars as there is on the CRX, so if anybody knows anything about them, please let me know.

Frank Lee 08-05-2011 05:26 PM

I think you should look at our discussion on "range extending generators".

I've had the notion that if the base vehicle doesn't have the capacity for carrying a heavy battery pack, that it certainly would still be able to pull a battery trailer...

Daox 08-05-2011 05:29 PM

40 miles is going to require a pretty decent battery pack. You're looking at at least 16 kWh. Lithium is definitely the way to go though. I don't think I'd recommend doing a lead acid car unless you can get free/cheap batteries and don't need much range.

newguyintown 08-05-2011 05:39 PM

Thanks for your quick replies everybody,

Frank Lee, I actually did look at that discussion quite in depth before posting this. There's a lot of good info there.
I was thinking of mounting the batteries on the bottom of the car. They would weigh a few hundred pounds, but if placed properly could offset the weight of not having a transmission, and the reduction in weight from a "large" engine to a small generator.

Daox, yeah it's going to be a pretty hefty battery pack. I don't think it's going to be quite 16kwh though because the chevy volt has a 16kwh battery pack for 40mi of range, and the CRX is lighter and has a lower drag area. According to the calculator on this website, it should take about 4.3kw to maintain 45mph, so if I were going at 45mph for an hour it would take 4.3kwh of battery. But I have to deal with start and stop, acceleration etc.(regenerative braking should help but not eliminate this problem) So in order to get 40mi of city range(less than 45mph top speed) I'm thinking I need to make my battery pack around 6kwh. If I get around 30-35mi of range it should still be ok, and I can always add more later if need be.

Daox 08-05-2011 11:43 PM

So your setup is going to have regenerative braking? Many DIY conversions don't due to the added cost of going with an AC system. Still, I think a 16kWh pack is prudent for 40 mile range. 6kWh is definitely not enough. Even with the streamlined CRX you will still use at the very least 200Wh/mile. This is perfectly flat ground, ideal road conditions, etc. Take a look at and see if anyone has posted Wh/mile ratings for their CRX, I found 240 Wh/mlie. That will give you an idea of how much power it will take you to get your 40 mile range. Keep in mind that you also do not want to cycle your batteries to fully discharged. For lithium you should definitely not take them lower than 80% depth of discharge (DOD), and 70% will give you much better life out of the batteries. So, you need to add an additional buffer there. Now, as the batteries age they will also loose capacity, so you need to account for that as well. All these things add up and in the end a larger battery pack is what is needed.

For the EV I'm planning (if I ever get around to it), I am using 500Wh/mile as a worst case scenario for winter driving (cold bearings/trans oil/heater/etc). This is with a Toyota Paseo which isn't a whole lot bigger or less aerodynamic than a CRX. With these conservative numbers I know I'll be ok after a few years and still be able to make my commute. Don't skimp on the battery pack.

Ryland 08-06-2011 12:16 AM

The weight of a CRX's engine is less then 200 pounds, light enough that I can pick it up off the ground and carry it by my self to the back of a vehicle, you might have another 100 pounds to remove but things like the radiator are small and light, exhaust is pretty light.

Also remember that with lead acid batteries you don't want to discharge them below 50% unless you like buying new batteries and in the winter the useful energy in the battery drops so you need to over size your pack unless you are not easily frustrated.

newguyintown 08-06-2011 04:44 AM

Daox, thanks so much for the site. There were quite a few conversions for the Honda CRX there, but I could only find one with wh/mi figures or a battery that I could find specs for. From what I read he had a total capacity of 7.9kwh (72V*110ah), which allowed him to get 25-50 mi depending upon his driving style, so he got about 240wh/mi (he doesn't say how far he took the batteries down though) . Thanks for the advice on the battery pack, I'll make sure to make it bigger so it lasts longer. I was thinking of using two 10kw continuous, 20kw peak brushless DC motors found here hub motor, brushless motor, BLDC motor, bike conversion kit, brushless controller,electric bike,wheelchair controller,electric wheelchair,dual drive ebike, electric motorcycle,EV battery,electric unicycle ,brushless joystick controller, regenerative which can do regenerative braking.

Ryland, thanks for the info on the weight. I was thinking the transmission must weight at least 100lbs too, right? That along with the engine, and exhaust, etc should be at least 300lbs right? That would give me some room to work with as far as the generator, batteries, and motors go.

Patrick 08-06-2011 08:35 AM

Here is a good example of a CRX conversion with AC drive: Advanced electric vehicle conversion project

Ryland 08-06-2011 08:38 AM


Originally Posted by newguyintown (Post 254580)
I was thinking the transmission must weight at least 100lbs too, right? That along with the engine, and exhaust, etc should be at least 300lbs right? That would give me some room to work with as far as the generator, batteries, and motors go.

The transmission weighs about 75 pounds and is tiny, the only reason that I had some help me carry the engine and the transmission while they were bolted together for my CRX was that there wasn't any good hand holds, but if you are wearing cloths that can get dirty picking up and carrying the two bolted together is not a problem, so altho I didn't put them on a scale, I know that they are not heavy, nor big.

You said you plan to bolt the batteries to the underside of the car??? figure that you need around a foot of height for batteries and that again, unless you like buying batteries, you want to be able to get to them to check them every few months to check your cable connections and your water level, last time I looked at my Honda fuel tank, it looked to be about 6" thick and was pressed up to the under side of the car the other 6" is needed for ground clearance.

As for hub motors, do you have fabrication skills? a machine shop? I do and I would still stay away from hub motors because they are going to take a lot of work to make the rest of the parts you need to fit it to an existing vehicle.
Before you order from Golden Motors, you might take a look at what vehicles use their motors Motor Brand: Golden Motors and notice that if you manage to put together a car using one of their motors you will be the first and it's not because that company is new, I started looking at their products over ten years ago.

newguyintown 08-06-2011 05:40 PM

Thanks for the site Patrick!

Ryland, Thanks for the info on the weight. The total weight of engine, transmission, exhaust, etc should still probably be between 250-300lbs right?

I was thinking of wiring my own Li-ion pack that would just be a few inches thick.

I was just thinking of using the brushless 10kw dc motor they have, not the hub motor. Has anyone had any success with those? They look like fairly good motors, but I have no experience with them. Do you have any other recommendations for motors? I'd like to get something with a high efficiency so that I don't need to make the battery pack as big, or the generator. The price looked fairly good for them, and they had a controller matched for them, so that's why I was interested. Do you have a recommendation on an efficient motor/controller setup for a good price? I'd like to have at least 40kw peak power, but I could probably do less.

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