Not less efficient, useless. Imagine instead of directly powering the car by combustion engine, you first use a combustion engine to generate electricity to fill the battery.

How would the car benefit the environment?

Coal generation favours long run times and is inefficient at being ramped up and down quickly. Using battery systems to soak surplus power time shifts a commodity which otherwise has to be negatively priced. The ideal input for a battery is of course renewable energy but there is a case for batteries as energy soak during times of low consumption, and to help stabilise demand in the network. This isn't an individual car efficiency statement, it's about socialised costs.