Two recent questions strangely converge, what is a good rocket fuel and what is a good battery for a laptop PC. It got me interested in how much energy can be stored in a given mass.

At Bell Labs, I remember a great talk given in 1985 by the head of the Physics department. He talked about the exponential growth in network bandwidth, memory density and computing speed. But he noted, battery lifetime is limited by chemistry and is only asyptotically approaching a modest limit. The latest seems to be the Lithium Ion Polymer battery, which is finding use in mobile devices. The recent stories of exploding laptop PCs are a clue to just how much the energy density of batteries is increasing.


Let’s look at the specific energy density (kiloJoules per kilogram) of some technologies:

Ultracapacitor ............... 50 kJ/kg
Lead Acid Battery ........... 100 kJ/kg
Ni-Cd Battery ............... 200 kJ/kg
Alkaline-Manganese .......... 300 kJ/kg
Lithium ion Polymer ......... 600 kJ/kg
TNT ....................... 4,000 kJ/kg
LOX/LH2 .................. 13,000 kJ/kg
Nuclear Fission .. 90,000,000,000 kJ/kg

We can see that chemical batteries still have a long ways to go before approaching the energy density of chemical fuels and explosives. Liquid oxygen and liquid hydrogen (LOX/LH2) is the about as good as it gets from chemical energy density, which is why they were chosen as the ideal rocket fuel by K.E. Tsiolkovsky over a century ago. And wouldn’t it to use a laptop for one week instead of just six hours, per charge?


Ultra capacitors are interesting. Although they are just now catching up to batteries, in theory they might someday rival the density of any chemical energy source. One advantage they already have over batteries is almost infinite lifetime. The D-cell compatable capaciters seen above are rated for 500,000 duty cycles.

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