I bought a Respironics Simply Go concentrator about 4 years ago. At 200 hours it completely quit charging the batteries and I had to go thru a medical supply  to return it for repairs. You cannot even talk to the manufacturer.  With the original price of 2800 ( the price has dropped since then), 1 new battery @185 and a repair bill of 600 it cost me $17.92 per hour to use it for the first 2 years. If I was going to buy another concentrator, I definitely would not buy this one.  I realize that others have had better luck with it but this is my own personal experience