If you look at energy in a capacitor - Joules = 1/2 ce^2, most of the energy is at the top of charge, and you'd have to be a real swithing supply guru (which I happen to be) to get most of it out, even, at some usable range of output voltage - those parts would be costly, and you might need more than one set to cover the range of inputs.
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Li batteries are eminently recyclable, probably easier than lead-acids which are already recycled in large %. I know this as I use them in my off-grid solar system, and frankly, would kill to get the Volt system for my home (or several of them, as that's what it would take to replace the truckloads of submarine batteries I now use).
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Sorry I didn't quote numbers above - someone else (Xiph) thankfully did that for me - they are so far off, no matter who's been spinning them - we can assume that's the case like with most numbers (noticed what you actually pay for groceries vs the official inflation rate for example?) - that's my point - even worst case and post-spin we're not in the same order of magnitude, much less "near" or "better" and won't be soon, if ever. With only a bit of physics knowledge, you'll understand why. It's true that we are also not that far from limits on battery energy density, this is one thing that definitely does not follow "Moore's law" as the periodic table is already full. But that doesn't mean that even with single atomic layer high D dielectrics and very expensive single layer epitaxy, we're going to beat that with caps. In fact, the chances are, we have a higher failure rate in practice than with batteries unless no cosmic ray ever hits your single layer insulator...and so on.
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And oh, if you hit the webpage in my sig and pass the turing test by finding it - my number IS on there.
No other gas even comes close...the guy who provides my welding gasses, for example, even acetylyene which has to be dissolved in acetone to be "safe" at any pressure over 15-20 psi - it self-explodes otherwise (those unsatisfied carbon bonds) - can't even get the license to sell hydrogen, it's far too much a hazmat.
Now you want to let joe sixpack work with the stuff in quanity, all over the world? Yeah, it'll solve the population problem anyway. Along with the other stuff mentioned, like embrittlement, no way to liquify it at normal temperatures, a continuous explosive range with any air mixture...inefficient production, energy-wise...long list.
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The Volt excells on twisty mountain roads, at least with the right driver. It's even more fun to smoke ricky rice racer with the Volt than it was with the 2010 Camaro SS I traded in for it. That low CG rocks on hairpins, this car leans less than just about any other I've driven. Maybe it's ricky who needs to learn how to get around a course faster, but this does just fine on the roads where I live.
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I'd love a Tesla too - if I could afford all that, but that's not reality. I don't go on road trips anymore, but I sometimes do need the range extender when going two major cities over, and it's nice to have. Putting this drivetrain into a caddy seems like an excercise in the ridiculous, though. I'm guessing GM did it to finally put it into a car they could charge more for (and actually make money), and because a huge fraction of Volt owners traded in a Caddilac, a beemer, or an audi for it - hint, they are run by bean counter marketing types.
Old programmers never die, they just hit account block limit.