[ben_w]

Low energy density though — a fully charged 1kg LiIon battery has enough energy to raise its own weight by a height of about 50 km.

Wikipedia says energy density of 0.36-0.875 MJ/kg; assuming 0.5 MJ/kg and putting it into PE = mgh -> 0.5 MJ/kg * 1kg = 1kg * g * h -> h = (0.5 MJ/kg)/g = ~51 km

http://www.wolframalpha.com/input/?i=%280.5%20MJ%2Fkg%29%2Fg...

[epistasis]

I think that ultimately this limits how cheap gravity batteries can get, there’s just too much material and those materials are already well-understood components with little chance for falling costs. We would likely need entirely new and innovative materials or designs A gravity battery that used completely new materials or design to come up with something cheap.

Where as lithium ion batteries generally get cheaper the more energy dense we make them (in the same chemistries, at least) because fewer resources go into making that same kWh of storage.

[Budabellly]

Admittedly, I know very little of the battery industry, and not much more about gravity battery viability.

That said, naive me would would ask why we can't just fill ballasts with waste water or other dense waste materials on-site?

Energy density over a gravity battery's working lifetime is a different metric than "per-charge", and maybe more relevant for supplying energy back to a grid (compared to, e.g. a home).

[epistasis]

Most people guess wrong about what technologies end up winning, so me as an internet rando could definitely be wrong about gravity batteries! Certainly the idea that lithium ion batteries would be cheap/high-specific-energy enough for cars would have seemed crazy 20 years ago. And even more so that lithium ion would make it to the grid!

My thought about them are mostly about learning curves, however. We have a long ways to go while we make lithium ion batteries cheaper, probably decades, and we improve a lot each year. Gravel, rock, stone, etc. and however we contain and move that around? That seems to be something that humans have been optimizing since our first days, and there doesn't seem to be much room for improvement at the moment.

My other assumption, which could be false, is that gravity batteries are not currently economical on our current grid with current electricity prices and price swings, an assumption I make because there are no installs I know of, and none planned. Instead of being uneconomic, they could just be untried, because the electricity industry is fairly innovative. If gravity batteries are economical today then I think there could be future for them.