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Comment by mda | Hacker News Reader

As far as I remember all battery breakthroughs in last 20 years were sad stories.

That's maybe a bit pessimistic. We have come a long way (NiCd 40-60 Wh/kg, NiMH 30-80 Wh/kg, LiIon 150-250 Wh/kg), we're just still a long way from fossil fuels (~12,000 Wh/kg for gasoline). I think the biggest trick is to set expectations properly and wait it out as battery technology continues to evolve.

http://en.wikipedia.org/wiki/Energy_density http://en.wikipedia.org/wiki/Rechargeable_battery

Woah, you can't compare those numbers directly. If you could then Tesla cars would be travelling just 10s of miles per charge. Obviously, there are many other factors: weight and bulk of the power train, power train efficiency, regenerative breaking, driving conditions, etc.

Oh, totally agree! I wasn't necessarily referring to electric cars specifically, more just comparing how much energy different things store, although it does highlight why building long-range and high-performance electric cars is a Hard Problem.

There's a lot of things that are different. The 50L of fuel in my car is going to weigh about 45kg and will get me about 800km, and the batteries in a Tesla weigh 300-500kg (not sure exactly, found a few different numbers). But like you say, the power train is considerably different, etc etc.

Poking around, on the Lithium-air battery wiki page, they have this quote:

> The energy density of gasoline is approximately 13 kW·h/kg, which corresponds to 1.7 kW·h/kg of energy provided to the wheels after losses.

Any clue what the energy provided to the wheels is for electric cars like a Tesla? I'm wonder what the energy loss is for electric cars from stored form to actual power at the wheel.

The Tesla Roadster was 88% efficient. I don't remember the exact value for the Model S, but it's in the same general range. This huge difference is the main thing that makes electric cars viable.

Also, electric cars can be powered by coal, nuclear, hydroelectric, etc... or whatever the grid is powered by.

Good electric motors typically have 90%-95% efficiency. However tesla is using AC motors driven by DC batteries, so they have to convert.

That's only part of the truth, though. There has been a steady increase in battery capacity per kg, dollar and volume over many years, it just hasn't been a revolutionary process.

I think this is mostly a reflection of tech/science reporting. Batteries are hard. They're complex with many factors into what goes into a good battery. But the moment a lab creates one with good energy density, headlines just say "Scientists create batteries 20x greater than your iPhone" without paying attention to the fact that it has 20 charging cycles before it's useless.

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