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Battery guys, keep on chargin’!

A company called Envia Systems announced it has built a record-setting lithium-ion battery with an energy storage density of 400 watt-hours per kilogram (Wh/Kg).  A more typical Li-ion battery has about half that density.  Converting this to standard engineering units gives 1.44 megajoules per kilogram (Mj/Kg).

Compare the energy content of gasoline.  Published sources say the energy content of gasoline is around 35 megajoules per liter (Mj/L) or roughly 40 Mj/Kg.  So, gasoline carries almost 28 times as much energy as the same weight in a battery.

That’s a big disadvantage for a battery, but even that doesn’t tell the whole story.  Let’s say you have an electric car with a (very large) 1000-lb battery.  That’s 454 kg, so the total energy in the battery is 654 Mj.  A car with a smallish 12-gallon gas tank is carrying roughly 78 lbs of gasoline, or 35 kg.  The total energy in the gas tank is therefore 1400 Mj.  So a car with a relatively small gas tank has over twice the energy on board as a car with a very large version of the world’s best battery.

But not even that tells the whole story.  Whereas a car can keep going till you burn every last drop of gasoline, you can’t suck all the electrons out of a battery.  And whereas gas can keep you at full speed till it’s all gone, battery performance drops off sharply as it nears the end of its charge.

So I need less than 40 lbs. of gasoline to drive as far as a half-ton battery, and my gas-powered car won’t get wimpy as the tank gets low, nor will its overall performance decline as the gas tank gets old.

To the savvy engineers, chemists, and physicists at Envia, my hearty congratulations on your notable achievement.  It didn’t come easy, and your work holds promise of changing the landscape of the energy world.  I wish you the best of success and hope your efforts eventually contribute to the production of a practical electric car.

But please keep on working, because battery powered cars are just not competitive.  Yet.

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