"sacrifice a part of their battery capacity to support the power grid, and get paid"
I suppose it's another brilliant MBA scheme. I'd love to see the discussion on that.
"Hey, we have all this power doing nothing, we could monetize it by supporting the power grid."
"Great idea, what happens if the grid goes down though ?"
"That virtually never happens, the grid has its own securities."
"Then why did we buy all those batteries in the first place ?"
From my point of view, those batteries are not just sitting there, they are poised to be available if the power grid goes down for any reason. That is their only job.
To give them another job is to ignore why they are in place and, in doing so, diminish their ability to respond to their primary duty.
But hey, I don't get a bonus either way, so have fun.