Interestingly, just yesterday I found out 4 already exists, but it's inverse of what you're thinking: top up excess battery capacity overnight, when grid prices are low, and then resell during the day when purchase prices are high.

I think the economic problem is that while there /may/ be overproduction during the day, the day is the only time other than early-mid evening when there is significant demand.

You would effectively be targeting the early-mid evening demand, assuming there was overproduction during the day, and with the current cost of batteries, their installation, and their replacement, I can't see the numbers working out.

I would love for battery prices to come down enough to make something like this possible though.