A grid-scale battery in the Scottish Highlands got a chance to prove its mettle in March when, 11 days after it started up, a massive wood-burning generator in Northern England shut down unexpectedly. Suddenly 1,877 megawatts of supply was missing, causing the 50-hertz frequency of the grid’s alternating current to crash below its 49.8-Hz operating limit in just 8 seconds. But the new 200-MW battery station leapt into action within milliseconds, releasing extra power to help arrest the frequency...