Energy Density Primer
Perhaps it is helpful to understand the significant
technical challenge electric flight still faces by comparing
the energy densities of gasoline and batteries.
Jim Lawrence (also see “Winning the Berblinger Prize
and LEAP” sidebar) wrote, “Lead-acid batteries like those
in boats and cars have been around for 150 years. They’re
cheap and relatively environmentally friendly to fabricate
and recycle, compared to lithium-based (Li-Ion-lithium-ion,
and LiPo – lithium-polymer) batteries. Lead-acids store
around 6% the energy density of gasoline. Running the
numbers tells us it takes 167 pounds of batteries to hold the
same energy as a single pound of gas.
“Lithium batteries as typically used in cameras, cell
phones and radio-control models store around four times as
much energy as a lead-acid battery, but cost several times
more per watt-hour. That still means an airplane needs to
carry more than 40 pounds of batteries for the equivalent
energy of a single pound of gas. That’s less than one quart!
Looked at another way, that 25 gallons you fill up a typical
LSA’s wings with would require 6,000 pounds of lithium-
technology batteries, and more than 10 tons of leadacids.
“No wonder research into improving battery energy density
is feverish around the world; even a doubling of storage
capacity would deliver multiple-hour flights. Many observers
predict we’ll have an affordable, mass-produced battery
technology with just that – two times current capacity
– by 2015.
“Meanwhile, many R&D groups worldwide have already
demonstrated up to 4X increases in energy density in exotic
“There’s another factor: The best internal combustion engine
only converts 25 to 30% of the energy potential of gasoline,
whereas brushless electric motors operate at 80% and
even higher efficiencies. Even so, we need more, and dramatic,
breakthroughs in battery capacity to close the gap.”