Daniel's EVPorsche 911 conversion

[daniel]

Today, using about the last of my "safe" battery charge (until the cable comes, promised for Monday) I drove very gently to the library, the store, and home, and then to the drugstore and home. If my calculations are correct I got 373 wh per mile.

I'd say that's acceptable performance for a car that was not engineered: I am sure Paul uses off-the-shelf components. And its roughly what my much lighter Zap Xebra gets.

The problem is that I was supposed to have a 225 v system and 125 miles of range. I'm thinking he just could not fit enough batteries in the car.

Note: I think I misspoke before: I may have said he promised a 256 volt system. The contract says 225 v. I apologize for my error.

Re-gen can make a big difference in hilly terrain, or when there are lots of stops and starts. On the freeway (which is where I am concerned about range) it does no good at all.

Another problem with re-gen is getting it to work the way I would want it: I think re-gen on the go-pedal is a very bad system. I want my car to coast, not brake, when I take my foot off the go pedal. I want a car to use re-gen only when I step on the brake pedal, instead of using the brake pads. But that requires a very sophisticated system with computer control and assurance that the brake pads will be used in a panic stop or if the re-gen or the computer fails. The Prius has such sophistication (though it also uses re-gen when you lift off the go-pedal).

Given my flat terrain, my dislike of re-gen on the go pedal, and my need for range only on the freeway trip to Coeur d'Alene, I think Paul was right in recommending against re-gen.

Today I figured out how to use the radio, which sounds VERY nice, and the moon roof, which is cool.

[mpt]

If I remember rightly; the 911 has rear tires like road-rollers on account of it having its engine where the tail-lights should be

I wonder what the weight balance is now with it having batteries (heavy) and motor (lightish - the 911 motor is light for an ICE).

Would it make sense to down-size the rear tires to reduce rolling resistance?

I also wonder how the ride height is; the standard 911 rear suspension expects a certain weight to position the rear tires; the slightest change can quickly tear up the insides of the tread; I've seen 5mm rubber on the outside and cloth on the inside before on a friends 911 S4.

Just musing; Elon Musk said that converting an existng car was tricky because everything had to be changed. What else was changed above the ICE swap?

MPT

[daniel]

If I remember rightly; the 911 has rear tires like road-rollers on account of it having its engine where the tail-lights should be

I wonder what the weight balance is now with it having batteries (heavy) and motor (lightish - the 911 motor is light for an ICE).

Would it make sense to down-size the rear tires to reduce rolling resistance?

I also wonder how the ride height is; the standard 911 rear suspension expects a certain weight to position the rear tires; the slightest change can quickly tear up the insides of the tread; I've seen 5mm rubber on the outside and cloth on the inside before on a friends 911 S4.

Just musing; Elon Musk said that converting an existng car was tricky because everything had to be changed. What else was changed above the ICE swap?

MPT

I cannot answer any of your questions. Paul has been converting Porsches (mostly --maybe exclusively-- using lead batteries) for years, so he probably understands the weight issues, and has probably adjusted the suspension if that's necessary. But I'd imagine he has not changed much other than swapping the engine and associated components for the motor and batteries and associated components. His web site may have technical information that I would not understand. It's CoolGreenCar dot net.

I am not a car enthusiast or mechanic. I'm just a guy who hates everything about gasoline, from its stink to the economic impact of importing our energy. I bought a Prius in 2004. I liked the brief moments of electric operation, and two years ago bought a Zap Xebra after a visit to Tesla Motors left me disappointed with the comfort of the Roadster, and unwilling to wait the year-plus they then estimated. The Porsche seemed the solution to the Xebra's inadequate range, but as much as I enjoy driving it, its range falls far short of what was promised.

[TEG]

You are a doubly perfect customer for them... You aren't too concerned with acceleration performance, and you don't want regen. Sounds like you may end up having a conversation with them about the range though.

[JRP3]

For all intents and purposes regen is not possible with series DC motors, so it was never an option in this case. A Warp 11 motor retails around $3k, a Zilla 1000 amp high voltage controller, what you would have gotten with the promised 225 volt system, used to sell around $2500. A comparable AC system with regen would run you more than $8K. Regen does not give back all that much anyway unless there is a lot of stop and go and hills involved.
The higher voltage system you were supposed to get would have been more efficient due to lower amp draw and therefor less waste energy as heat. It also would have given much better acceleration. Essentially it's like paying for a V6 and getting a 4cylinder, but with the mileage of a V8. OK, not quite that bad but that's the basic idea. I don't know if I buy the lack of room for more batteries, he could have used more smaller cells to get the voltage up. Either way he should have been aware of what he was doing and credited you for the lower specs or told you ahead of time about the change. My guess is he had to go with the lower voltage because he could no longer get the Zilla HV controller and the replacement couldn't go above 144.
If you can get under 400 wh/mi with normal driving and closer to 300 by taking it easy you're probably not doing too badly with that vehicle. I'm not positive but I would think the efficiency range for that motor would be between 2500-3500 rpms. Lower rpms will draw more amps, higher rpms you loose torque and efficiency due to increased back EMF.

If it is assumed that a motor is 100% efficient with no friction or windage losses, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord.

Counter-electromotive force - Wikipedia, the free encyclopedia

[daniel]

... Sounds like you may end up having a conversation with them about the range though.

Yep. Definitely.

On the positive side, the car handles beautifully. I can take corners much faster than in the Xebra or even the Prius. The Prius does not handle all that well, though it does what it was designed to do admirably. The Xebra handles great considering that it has only three wheels. The Porsche is a pleasure to drive.

Life is full of disappointments, and in the grand scheme of things, my problems fade into utter insignificance. There are people who do not have enough food; people who are losing their homes; people sleeping under bridges; people who cannot afford to see a doctor; people being blown to bits or disfigured in wars. I'm complaining because my too-expensive car does not have the range that was promised to me. I need to get some perspective!

I calculate that I have about 7 safe miles left before I hit 20% SoC, so tomorrow the car will stay in the garage. If the cable arrives as promised, the car will be charged up Tuesday morning, and I'll take it on the freeway until it's down about 30% so I return home with 40% remaining. Wednesday I'll do the same on rural surface roads (steady slow speed, not much stopping) and on Thursday I'll do the same on city streets. I'll post my numbers. Once I know what the car really does, I'll talk to Paul and see what he says. Also, there's a guy 30 miles from here who builds EVs. I'm not sure I'd want him to work on my car, but I'll see what he has to say. He'd probably enjoy seeing it anyway.

Perspective. I have too much money. I've foolishly spent some of it on a car that does not do what it was promised to do. I still have too much money. And a car that does half what I wanted, anyway.

Daniel

[JRP3]

Perspective. I have too much money. I've foolishly spent some of it on a car that does not do what it was promised to do. I still have too much money. And a car that does half what I wanted, anyway.

Daniel

But you're also learning much more than you ever wanted to know about EV's , and that's priceless
I don't think you got terribly ripped off or anything, we'll have to see what kind of range and performance you actually end up with. I'm sure it's a great vehicle, it's just too bad that the builder seems to have over promised and under delivered. If he had said "I can't get the same controller so the performance will be slower and the range will be less, and I'll knock $5K off the price", or something like that, it wouldn't be as bad. If I were the builder I'd much rather promise less and deliver more, guaranteeing happy customers, but it's probably harder to make money that way.

[TEG]

JRP3s last two posts covered a bunch of stuff I was thinking about, but now I don't have to say. Thanks, JRP3.

[daniel]

If he had told me that due to whatever he would only be able to give me 60 freeway miles, for a "safe" freeway range of 50 miles, I'd have canceled the project and asked for my money back, and either looked for another conversion shop that could give me 100 miles, or I'd have sold the car back to the used car dealer. I might have considered paying for more batteries to get the range I wanted, but I don't see how any more batteries could be made to fit. Maybe by taking out the back seat.

Remember that my Zap Xebra goes 40 miles, and I'm willing to take it 35. The high cost of this car is justified of I can drive it 75 miles with, say, 40% remaining for pack deterioration in future years. It is not justified if I can only go 15 miles farther than my Xebra goes safely.

But as I said before: PERSPECTIVE! I have a beautiful car that handles wonderfully. I'd never have bought it if I'd known it would only go 60 miles to empty on the freeway. But life offers worse things than this. Maybe the bottom line is that the Roadster and the eBox are the only EVs there are that can go 100 miles or more on the freeway, and I was asking the impossible. Maybe I need to get my deposit in on a Tesla Model S. Sigh!

[TEG]

You probably do want to charge it and drive it some more before concluding what it can really do range wise. Range gauges in EVs can frequently be inaccurate, and you probably can't make an accurate assessment based on extrapolations from going through one cycle of a partial charge.