If you've made the decision to purchase an additional, backup battery for extended portable operating of your Lifestyle Rhythm P2 oxygen machine, to truly maximize your experience you will absolutely want to consider the Lifestyle Rhythm P2 external battery charger. Unlike the AC power supply and the DC car charger, you will not have to wait for each individual battery to charge when you have the external battery charger. The Lifestyle Rhythm external battery charger enables you to charge one battery on the side with the use of any wall outlet with the included power supply, allowing you to enjoy usage of your Lifestyle Rhythm P2 with 1 battery while your extra battery is simultaneously charging on the side. As long as you always have your extra Lifestyle Rhythm P2 battery charging on the external battery charger while using your Lifestyle Rhythm P2 oxygen machine, you won't need to sit around for hours waiting for your battery to charge. A fully depleted Lifestyle Rhythm P2 battery will take up to 2.5 hours to fully recharge. When the battery is in the correct position, a solid red light will indicate that the battery is charging.