The controllers typically accept writes in a log structure that can be replayed once power is restored. Because the battery is only powering some SRAM (might be

The controllers typically accept writes in a log structure that can be replayed once power is restored. Because the battery is only powering some SRAM (might be more DRAM these days) and a small ASIC or whatever, not even the whole controller, a small-laptop sized battery can keep that up for sometimes days. Lots of variability in how those are implemented depending on how enterprise-y and expensive they are.