Yeah, I think these have been around for a little bit, though you don't hear about them too much. Braille was another lightweight brand I remember back in my college days. We ran one on our Formula SAE car for the lightweight aspect of it. Doesn't necessarily have the neat "reserve charge" for the jump start like the Antigravity, but lightweight nonetheless. I put a random Li-Ion battery in one of my motorcycles recently as it was supposed to last the winter better. Long story short it wasn't really that different, but it was crazy how light it was compared to an old lead acid. It felt like a mock display batter with no guts in it like one of those plastic engine blocks. It would zing the starter over nice and quick when fully charged though.