This article is decent: https://modoenergy.com/research/ercot-battery-energy-storage... (no affiliation, looked like based on disclosure data).

Look at the revenue: it's 6% (now 12%) from energy arb and 58% (now 34%) from RRS-PFR (primary frequency response). Fully discharging and charging a battery is expensive from a lifecycle perspective (I think, I would love a reference), and if you can get paid a lot of money to charge/discharge by 5%, or not to do anything at all but sit charged (ECRS), you'd take that before damaging your expensive installation.

There isn't a lot of generation like this, primarily because getting a thermal generator to scale up or down is expensive, even for the nimblest of combined cycle plants.