What are we defining as a battery?

If it's purely energy storage I think the hydro energy storage facilities (Like Dinorwig in the UK) definitely qualify: https://en.wikipedia.org/wiki/Dinorwig_Power_Station which has a capacity of 1,800 MW

Tom Scott did a video on this here: https://www.youtube.com/watch?v=6Jx_bJgIFhI