Batteries dont need to discharge back to the grid to time shift energy demand.
Australia has heated water overnight att off peak times for years with ripple signals because otherwise the coal plants wouldn't run efficiently.
They don't need to use that domestic hot water to feed electric energy back into the grid, they just need to use it to avoid calling on the grid when they would otherwise have added to peak load.
EVs are batteries on wheels, with computer chips and internet connections to tell them the best time to charge. All the pieces are there and in actual use for decades.
The problem we're discussing, and the biggest problem for renewable, is that we need massive amounts of storage to handle the cases where production is unexpectedly low for long periods of time - months of low wind and cloud coverage. Car batteries don't help in any way with this problem, which is the biggest risk for a renewable-only grid.
Now, let's assume this problem were solved, and now we start replacing all fossil fuel power plants with renewables + storage. Unfortunately, we are also replacing ICE vehicles with BEVs at the same time, which require massive amounts of extra electrical power to replace the power they were getting from oil. Suddenly, we can no longer close all of our fossil fuel plants, because all of the growth in renewables is consumed by BEV charging.
The problem you're discussing - handling variable load - is somewhat secondary. It's a significant practical problem if you're adding new solar and wind to the grid without closing down old fossil fuel generators, which is what everyone is mostly doing currently, so it is a very discussed problem. The problem of ensuring reliability from a renewable-only grid is so complicated, no one is really tackling it yet, especially since they can barely construct enough new renewables to handle all the extra new load from growing industry + BEVs.
Australia has heated water overnight att off peak times for years with ripple signals because otherwise the coal plants wouldn't run efficiently.
They don't need to use that domestic hot water to feed electric energy back into the grid, they just need to use it to avoid calling on the grid when they would otherwise have added to peak load.
EVs are batteries on wheels, with computer chips and internet connections to tell them the best time to charge. All the pieces are there and in actual use for decades.