The battery charging in a Toyota Synergy drive happens as a result of "Gearing down" the "Transmission" in the driveline. In simple terms, the synergy drive is similar to a CVT or whatever it is called in a tractor. The tractor CVT uses hydraulic pumps and motors to speed up or slow components of a planetary gearset. In that process, some hydraulic fluid is back fed to the motor driving the machine. Heat is generated and typical hydraulic inefficiencies come into play but at least some of the energy is used instead of being lost as heat. The synergy drives use electric to drive or slow parts of the gearset to make the desired gear ratio. Some can be stored in a battery and some is back fed to the planetary but in the end it's more efficient than using hydraulic fluid to do the same. Watching the display on a Toyota hybrid, you can see times that both ICE and electric are providing power while the battery is adding charge. Still burning gas to do this though. And again, I've never seen the car standing still with the motor running and adding charge to the battery. It is not now, and never has been a paying proposition to run an engine solely to charge a battery that will solely be used to  propel a car.