NiMh is still probably the lightest per unit energy system, and it is not an environmental nightmare, but it can't produce the juice as it were. NiMh just doesn't have the ability to supply sufficient current for motor type applications. That's why high drain RC cars and power tools etc. still rely on NiCd, but for portable electronics, NiMh work great because they can deliver a reasonable current for a hell of a long time relatively speaking. The typically destroy carbon, alkiline, and NiCds by as much as a factor of 5 or more in low-medium current applications.
Li-ion on the other hand handles high current as did NiCd and Lead Acid, but Nick is right, their lifecycle sucked compared to the other technologies.
And we still have the big challenge... Supply and demand... Everyone is making small cells for notebooks etc. There is no monster market to do the R&D required to make car battery size cells, and since there are no car battery sized cells, all the industries that might use them don't. Somebody like toyota, and FHI can drive that supply/demand model from both ends and hopefully make progress on larger cells.
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I am a Commodore PET --- Now get off my lawn you kids...
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