Regular hybrids charge their batteries by regeneration, harvesting energy used to slow the car down during braking or going down hill. Basically the electric motor becomes a generator. All the electrical power comes from the gas engine but some of the energy is recouped instead of just being turned into heat by the vehicles brakes. Plug in hybrids do the same thing but usually have larger batteries that can be charged by an external source. Most have an electric only range of around 50 km. The only real downside compared to EV’s is you have the same maintenance issues (oil changes, tune ups etc) as other ICE vehicles.