Most important characteristics:

Inogen One G3 Battery (8 cell)

One of the most important aspects of a portable oxygen concentrator is the battery. After all, without the battery you're not very mobile! The Inogen One oxygen concentrators are very energy efficient, so they last long on one battery. However you might need an extra battery, for example if you're planning on taking a long flight. In that case you can choose between this small 8 cell battery (BA-300) or the large 16 cell battery (BA-316). You have to consider that the 8 cell battery is lighter, but has a shorter running time. The 16 cell battery lasts longer, but is also heavier. 

Charge and running time:

To charge an extra battery more easily, you could consider buying an external battery charger.