A Google and Xcel Energy project in Pine Island, Minnesota, has been reported as the "world's largest battery," capable of 100 hours of continuous discharge. The claim is real, but the technology behind it is the interesting part most coverage skips.

What it actually is: an iron-air battery, built by Form Energy, not the lithium-ion battery most people picture when they hear "battery." The project is rated at 300 megawatts of power and 30 gigawatt-hours of total stored energy, with up to 100 hours of continuous discharge.

Why it isn't lithium: 100-hour discharge simply isn't practical with today's lithium-ion economics, you'd need an enormous, expensive amount of battery cells to store that much energy for that long. Iron-air batteries trade off some efficiency (roughly 40-50% round-trip, well below lithium's ~90%+) in exchange for being far cheaper to build at very long durations.

The one qualifier worth keeping: "world's largest" holds up for total stored energy and discharge duration. Other grid batteries elsewhere output more raw power at any given moment, so this is the largest by energy and duration, not an unqualified biggest-battery-on-Earth claim.

The bottom line: a genuine, well-documented project, and a good example of battery technology being more varied than the lithium-ion batteries in EVs and phones.