I have 2 InogenOne G2 (24 cell battery) portable concentrators for sale. They were bought in 27th Feb 2015 from Intermedical. They have 6 levels of pulsed oxygen, which in reality delivers a max of 4 litres per minute. The battery lasts for approx 5 hours at max delivery,though we rarely used the battery for that long because it has both mains and car chargers which can be used when sitting to save the battery for when moving about.
It comes with carry bag, hand strap, shoulder strap, trolley, car charger and mains charger. I also adapted a small ruck sack (black) to use with it.
I have the invoice for one of them which validates the warranty for 3 years.
My husband found them very easy to use, particularly with the trolley, and helped him a great deal until he needed continuous flow oxygen.