I have two InogenOne G2 portable concentrators for sale.
They are just over a year old. They have 6 variable flow rates but because the flow is pulsed the top rate equates to about 4 litre per minute. On the highest setting you get 4 - 5 hours using just the battery, but if at home, or in the car, it runs off the mains or the car batteries.
They come with carry bag, hand strap, shoulder strap, trolley, car charger and mains charger.
I also have one spare charger so that you can just trolley from one place to another without worrying about carrying the mains kit.
They are lightweight to trolley about and can help you to do considerably more every day.
I live in Hythe Kent.