At sufficient scale (many MWs) batteries can charge or discharge rapidly and so provide a service which helps with RoCoF (rate of change of frequency) if there’s a loss on the system. This was traditionally provided by the inertia inherent in rotating turbo-generators, but will probably be a useful feature of large battery installations. On the other hand, as described in the T&D World article, such a battery needs to be maintained at around half-change just in case this service is needed.

I’m not surprised that arbitrage isn’t sufficient for the economics. And even if it is, it obviously becomes rapidly less so for each additional battery installation.