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Future Batteries, by the Numbers

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Re: Comparison with gasoline
Cadman-LT   10/14/2012 8:01:16 PM
You guys make good points. I never stopped to think about the weight of the gas compared to the weight of the batteries before. Makes total sense.

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Re: Comparison with gasoline
naperlou   10/13/2012 11:50:39 AM
Chuck, that's a good point.  I like to let my car run down to empty, if I am in an area with lot's of gas stations, becuase it will get slightly better mileage toward the end of the tank.  Another thing to note is the difference weight of the engines.  I don't know what the Tesla S engine weighs, but I was told that the Tesla Roadster engine weighs only 70 lbs.  Add to that the fact that there is no transmission, and the S engine has to weigh a lot less than the 550i engine. 

One thing this makes clear is that when comparing two very different technology systems one has to consider more than the core part of the system.  In this case the motors are vastly different and just comparing them would leave you to believe that the electric car should be lighter.  Add in the energy storage system, and you come up with a very different story.

Nice new picture, by the way, but they cut off the top of your head. 

Charles Murray
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Re: Comparison with gasoline
Charles Murray   10/12/2012 6:25:00 PM
You're right, Naperlou. You are essentially saying that the specific energy of gasoline is far higher than that of a lithium-ion battery. And, yes, that's a drawback. It's also a drawback when the battery's charge is depleted. The 900 lb battery still weighs the same. It still has to carry its own dead weight, whereas the weight of the gasoline in your example has gone from 73 lbs to zero when the gas tank is depleted.

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Comparison with gasoline
naperlou   10/12/2012 10:35:01 AM
Cap'n, I read this article with interest.  While doing so I looked up a number of things.  What I found was suprising.  One comment you made was the search for a $5K battery pack that got 300 miles of range.  Tha is a long way off, as you mention. 

With a gasoline engine at 23 mpg (see below), the 300 mile range translates to 13.04 gallons.  The weight of the gasoline is 72.88 lbs.  Tesla claims that range for their Tesla S with the 85 kW-h battery.  The weight seems to be about what the roadster's is at a minimum.  That would be 900 lbs.  The ICE gets 0.26 lbs./mile, while the Tesla S has a rating of 3 lbs./mile.  So, the first thing that needs to be addressed is the weight of the battery.  The Tesla S battery has density of 94.44 W-h/lb.  This is at the high end of the range you quote for current batteries (note the unit difference). 

What is even more interesting is that the curb weight for a Tesla S is 4,647.3 lbs.  That's a lot.  The roughly equivalent BMW 5 series sedan (which Tesla is targeting with the S) is the 550i sedan.  The power output is similar, The BMW weighs 4,365 lbs.  I find this very interesting for both given all the articles and discussion on weight saving, etc. that the car manufacturers are supposedly working on. 


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