It's a dumb saying. Let's say Bill Gates stopped making any more money tomorrow. How long would it take for you to catch up to his wealth?

Lithium-Ion battery energy density has about doubled in the last 25 years: http://futurist.typepad.com/my_weblog/images/2008/03/11/batt.... Even if gas technology stops improving tomorrow, it'll take 100-200 years to catch up on the energy density front.

Battery propulsion doesn't need to get anywhere near the energy density of fossil fuels in order to have equal utility, due to the huge difference in efficiency (both theoretical and practical). Ignoring utility and just thinking pure kWh for a moment, do you have a source for your 100-200 year claim, assuming maximum Carnot efficiency for fossil and 100% efficiency in electric motor conversion?

From my armchair perspective, the Tesla Model S would be equally useful to any gasoline car for all but a handful of edge use cases, with only the minor change of adding a metal-air battery as a range extender for excessively long trips. And that's today, not 100 years from now.

Arguably, Bill Gates has more than "a lot" of y intercept.

The saying also deserves an addendum: "... if you've got the time."

But there is probably a hard ceiling to the efficiency of safe and cost effective Li-ion batteries. We can't be entirely sure because we don't fully understand how Li-ion batteries work, but my best layman's educated guess is that those are close to tapped out as well.

The strength of the battery+motor electric drivetrain is that it's modular. Come up with another energy storage method, build it into a power pack, then plug it right into existing Tesla S cars, get the over the air software update, and you're back in business. There's also all of that frunk space.

Obviously batteries will not surpass 100% efficiency and are already reasonably close to that so yes, batteries are nearing a hard limit on efficiency. Energy density on the other hand is no where near its theoretical limits.

> Obviously batteries will not surpass 100% efficiency and are already reasonably close to that so yes, batteries are nearing a hard limit on efficiency. Energy density on the other hand

Energy density is what I meant. I found some graphs to look at. You are probably right about there being lots more room for improving energy density.

>my best layman's educated guess...

Can you elaborate on your reasoning?

Only that we've been trying for awhile now. I just looked at some graphs online, and maybe they'll get 50 to 100% times better in the next decade or so. That's pretty significant.

Gasoline and diesel engines have actually improved a lot in the last ten years.

I don't disagree (at least w.r.t electric cars), but to play devil's advocate: a factor of 50-100 is A LOT. We're talking about two orders of magnitude here, which would require a fundamental breakthrough in battery technology to achieve parity with gasoline. Even if we make batteries twice as efficient, we're still 25-50 worse than gasoline. And breakthroughs in battery technology are hard to come by. So if Toyota wants to hitch their wagon to fuel-cell technology instead, all the power to them! Batteries are not necessarily the future of portable energy.

If you now consider that the only way to use that energy is through combustion, and that an combustion engine is only 1/4th as efficient as an electric one, that factor suddenly shrinks to 12.5 - 25 already.

Frankly, energy density is a bit of an distraction. If we go by energy density alone, we would presumably all be cruising on Uranium, which energy density is... well off the charts.

But there a plenty of good reasons we don't fuel transport with Uranium, and there are just as many good reasons why we shouldn't continue to use gasoline.

But of course you're not just worrying about fuel/battery energy density difference. You should look at energy deliverable to the road per pound of fuel + engine + exhaust system versus electric motors + batteries, which I would guess is nowhere near a factor of 50-100.