When out o charge does gas engine drive wheels or charge battery which drives wheels?

I see conflicting answers on this question. Some say that when you run out of battery (and driving in normal mode), the gas engine charges the battery, which drives the wheels. Others say the gas engine drives the wheels directly without charging the battery.

Does anyone know for sure?

This is important because if it's the latter, the battery is basically untouched (and thus will have longer life) when in gasoline engine mode.