Electriflying

Jim Logajan wrote:

wrote:

100LL Avgas is 44 MJ/kg.

Batteries need to be able to produce better than 20 MJ/kg to be
generally usefull for transportation.

The Avgas energy density is not comparable to battery energy density
because it doesn't factor in thermal to mechanical conversion. Current
automobile engines extract only about 20% of that 44 MJ/kg. Electric
motors are anywhere from 75% to 95% efficient in converting electrical
power to mechanical power.

So a battery with ~11 MJ/kg powering an 80% efficient electric motor
has the same usable energy density as Avgas powering an internal
combustion engine.

A Lycoming is about 27% to 29% percent efficient at 70% power.

Setting the goal for battery energy density at 20 MJ/kg would simply
be wrong because it uses the wrong numbers.

There is also more to battery selection and technology than simply the
energy density of a fully-charged battery pack. Consider that a deep
discharge battery of current technology, such is used in a laptop
computer or a Prius, is rarely discharged below about 20% of capacity to
help extend its life. This is perhaps analagous to the unusable fuel in a
fuel tank of say 5%. That difference needs to be taken into account in
any comparison.

By the same token, to extend life, batteries are also rarely fully
charged, with maybe 10 to 15 percent of theoretical capacity unused.
That makes the usable capacity of a battery in the range of 65 to 70
percent of its theoretical capacity.

Further, there are other realities like the fact that all batteries lose
capacity when they are cold, as they age, and when (not if) cells in the
battery pack fail. That means a reserve calculation is a bit more
difficult, and would likely mean a traditional 5 or 10 percent reserve
would have to be increased to ensure there are no surprises.

Finally, you can also increase your payload with a conventional engine by
reducing the amount of fuel being carried, thereby sacrificing range.
That would not be possible with a battery, since more than likely you
would not be able to reduce cells to reduce weight. The packs would tend
to be a package deal to economize in overall design weight.

Therefore, any calculation using the theoretical energy density of a full
battery pack is wildly optimistic.