Charge one battery at a time using this desktop battery charger. This charger connects to an AC power source (wall outlet) and will charge a fully depleted batteries in less than 5 hours.
Inogen One G3 Double (16 Cell) Battery
Inogen One G3 External Battery Charger with Power Supply
Inogen One G3 Single (8 Cell) Battery
Inogen One G4 4 Cell (Single) Battery
Inogen One G4 8 Cell (Double) Battery