Portable oxygen systems. Any recommendations?

My mother has COPD and requires oxygen. When she goes away from home she has to bring 2-3 tanks of oxygen with her. I am interested if any one has had experience with Portable Oxygen Systems that would provide continuous flow of air instead of the pulse air flow systems. I have read some of the new systems on the market can even be used or charged while riding in the car by means of the car battery, much like charging a cellphone. She has a homebase unit covered by medicare. Wondering if medicare would also cover the portable smaller unit.