I'd need to see real numbers on using EV batteries.
Today's price was between €60 to €100 per MWh
ie. 6c to 10c a Unit but the 10c price was during peak demand when people come home from work and presumably charge the car.
The other peak price was when people started work in the morning, again I'd imagine they'd be charging then too.
But if we pretend that people who have just finished driving are OK with the grid draining their batteries. Then you have a max differential of 4c in the price of a unit. A Leaf has a 40KWh battery so could hold €1.60 of electricity, not including energy conversion losses. Add in the reduced demand with so may EV's chasing that fee, and it's probably only a euro worth. And there's the depreciation on the batteries as they have finite life cycles and cost thousands to replace.
It's just a subsidy for battery makers.
Using car batteries to charge other car batteries over the grid at peak demand is a waste of energy, ie charging low batteries after the commute.
Load shedding would be a much cheaper way of doing the same thing.
Smart storage and water heaters. Pumped storage. Water pumps.