Inogen One batteries are charged by the concentrator. If you own multiple batteries, this can make it difficult to make sure they are all fully charged. That is why the external battery charger was introduced. The external charger is connected to the mains with its own power supply. If you slide the charger over the battery, it will start charging automatically. Now you never have to worry about a depleted battery again!
On top of that, the external battery charger also charges your batteries quicker. These are the recharge times: