GM's new electric car: the Volt!

let's talk about the Tesla Roadster instead

I prefer the Frank Hannon Band Jalopy

From Physics for Future Presidents Chapter 1 (course at Berkeley):

OK. Let's try that again. From there (http://muller.lbl.gov/teaching/Physics10/PffP_textbook/Ch01cust.htm)

A student drew my attention to an article that appeared in a magazine about an all-electric car. The article says the car, called the "Tesla Roadster", is powered by 6,831 rechargeable lithium-ion batteries, similar to those found in laptop computers. The car range is 250 miles. They claim, if you charge the batteries from your home power plug, that driving the car costs 1 to 2 cents per mile. Top speed: 130 miles per hour. Wow! Can't wait to get one? The cars will be built in a factor in England and available (they say) summer 2007.

But wait a minute. Let's calculate some numbers. My laptop battery costs $135. For 6,831 batteries, I would have to pay $922,000. That is far above the cost of the car – so they probably mean "cells", not batteries. A typical laptop battery has 10 cells inside it, so the cost of the batteries would only be $92,000. Only?

They say the range is 250 miles. That means the batteries have to be recharged every 250 miles. The batteries they use can be charged 500 times before being replaced. (That's why your old laptops need new batteries.) So that means replace the $92,000 worth of batteries after 250 x 500 miles = 125,000 miles. That's $0.73 per mile. So true, the "fuel" doesn't cost much, if you don't include replacing the batteries.

Let's see what the horsepower of the car really is. It goes 250 miles on 683 laptop batteries. Assume you are only going 50 miles per hour. Then you can drive for 5 hours. My laptop battery has an energy of 60 Watt-hours. With 683 batteries, the energy would be 60x683 = 41,000 watt-hours = 55 horsepower hours. That means the batteries in the Tesla Roadster can deliver 55 horsepower for one hour, or 11 horsepower for 5 hours.

11 horsepower? Is that what you imagined when you heard this car has a top speed of 130 mph and can go for 250 miles? Actually, at the top speed, it would probably require over 200 horsepower, and it could be driven for less than 15 minutes before running out of energy.

(The whole course is great, by the way, and can be watched on Google Video or iTunes, among others. In the first lecture, which covers this stuff, he mentions the weight and size problem too -- 700 (or 7000) batteries are very bulky and very heavy. Goes on to say hybrids are the future though.)

There are some pretty ridiculous assumptions being made there.

I mean the thing is not actually powered by retail laptop batteries.

Yeah. The real cost per cell is probably under $5. They are charging big bucks for the Tesla Roadster (I think around $100K).

As for the horsepower stuff - you only need the big horsepower while you accelerate! And that's where the magic happens - it goes 0-60mph in like 3 seconds.

A buddy of mine recently left the iPod group at Apple to go work for Tesla Motors. He "promised" me a test drive at some point. If it actually happens, I will report back.

Really, the lame thing about battery-powered cars is that they take 3.5 hours to charge. This means no long roadtrips without overnight stops every 250 miles. When you think about how much energy that is (3.5 hours on a 20amp 220V circuit, I think), it makes you appreciate the energy density of gasoline.

The horsepower during acceleration most likely comes from big capacitors. Your camera's flash doesn't flash because of the battery, but rather the charge a capacitor gains from it.

I mean the thing is not actually powered by retail laptop batteries.

Not literally of course, but retail laptop batteries are the cutting edge of commercially available batteries, with a huge commercial incentive to be developed as small and light as possible, and sold in an enormous market so their price is not likely to get lower any time soon. The idea that the Tesla Roadster will be driven by some smaller, cheaper lithium ion battery that Apple, Sony, IBM, etc. don't know about or don't want is unlikely.

Fair point in re: capacitors being used for acceleration, but unless you're driving on an ice rink you do always need a powerful engine to maintain speed. Your foot remains on the accelerator on motorways.

The point is that an all-electric (rather than hybrid) is so heavy and lacking in power that you have to make compromises elsewhere in the design to accommodate this while still having something that looks like a car. Sadly, they are not the future.

Unless battery technology, hor high energy density capacitor technology gets better. I wouldn't rule out all electric, but it is going to be primarily an urban technology. Petrol, Ethanol, Diesel, Biodiesel, Hydrogen IC and Fuel cell hybrid are going to provide long distance motoring in the future. Biodiesel hybrid has the best near term carbon load. But no car manufacturer is producing one as they don't think anyone will pay the diesel premium and the hybrid premium.

Interestingly the Golf Bluemotion has lower emissions than the Prius whilst running a non-hybrid fossil diesel motor (similar size of car, golf a little smaller). Stick Biodiesel in that and you have a fairly low carbon car. Combine the technology and you have something stunning, make it a plug-in with a renewable energy source .....

One thing no one ever considers is the amount of energy that goes into manufacturing and recycling cars. Batteries are very costly in this regard.

Yes, recycling 7000 Lithium cells from the Tesla when they stop working after ~3 years is not going to be cheap. Nor is replacing them.

You guys do understand that horsepower has approx. squat to do with acceleration, right?

An electric engine delivers its peak torque at zero rpm - this changes the mental shorthand we use to think about automotive performance.

This means no long roadtrips without overnight stops every 250 miles.

thing is, how many long roadtrips are going to be taken with this particular car? the vast majority of these things are used as commuter rides, so you need like 50-100/day for range on average.