Monday, September 28, 2009

When we're not driving autorickshaws

It's been a while since I've paid any attention to this blog and haven't really figured out a good reason to justify the time it takes to blog until I talked to Eric Ries who writes the blog at He said it's a great way to get feedback on your ideas, and immediately learn what was communicated clearly and what is up for debate. He mentioned he'd learned a lot from writing the blog and getting feedback.

Anyway, I'll try it and see what happens, or if anyone cares. Maybe I'll care in 10 years when I can go back and see what I was thinking and where I went wrong.

After riding around in an Autorickshaw for nearly 1000 miles, I good transition would be to talk about the Tesla, a car that's on the opposite end of the spectrum from a performance and pollution standpoint. When I first learned about the battery pack, I had trouble relating the battery consumption to things I was familiar with, so I did a few calculations that made it more clear to me, at least.

Like all cars, Tesla’s mileage varies by how hard you drive; “normal driving” (ie, the way the EPA measures gas mileage for cars), the Tesla requires 334 watt hours to go a mile. That means that one kilowatt hour will propel the Tesla about 3 miles. Hair dryers are generally 1000 – 1500 watts, so you have a choice: you can dry your hair for an hour, are drive the Tesla for 3-5 miles.

My Dell laptop battery says it stores 43 watt hours. That’s enough energy to move the Tesla about 700 feet, or, if I’m going 60mph, power the engine for about 8.5 seconds.

The Tesla battery pack holds 57 kilowatt hours, so it’s equivalent to about 1300 Dell laptop batteries. Cross checking, that says that we can power the car for 1300*8.5 seconds or 11,200 seconds, or just over 3 hours, for a battery pack life of about 200 miles.

Some other interesting data points: a rechargeable AA battery is rated at about 2000mAh. These rechargeable batteries are typically about 1.3 volts (slightly less than the standard 1.5 volts), for a total capacity of 2.6 watt hours. So about 18 rechargeable AA are equivalent to a Dell laptop battery, which seems about right judging by the relative sizes. And that AA battery will power the Tesla for about .46 seconds (just under ½ second) at 60 mph, moving it about 44.5 feet; it takes about 128 AA batteries to move the Tesla a mile.

Or, in dollar terms:

  • Off peak charging is 5 cents/kilowatt hour which is about 3 miles, just under 2 cents per mile for the energy cost. NOTE: the battery cost is additional. The Tesla battery lasts about 100,000 miles and currently costs $25,000 for an additional 25 cents per mile in battery cost.
  • AA batteries are about 50 cents, which means it would cost about $60 to go a mile (including battery costs) if you bought your energy at a convenience store.