If you're thinking about adding a backup battery to ensure you can use your Rhythm P2-E7 for longer periods, then you'll definitely want to consider the Lifestyle Rhythm P2-E7 external battery charger. With your Lifestyle Rhythm P2-E7 portable oxygen concentrator, there's no need to wait around for batteries to charge like you would with an AC power supply or a DC car charger. By using the external battery charger, you can charge one battery while still enjoying the freedom of using another, just plug it into any wall outlet with the included power supply. This handy setup means you can use your Lifestyle Rhythm P2-E7 with one battery while the other is charging, so as long as you have at least two batteries on hand, you'll never have to go without your essential medical-grade oxygen. If your Rhythm P2-E7 battery runs empty, it'll take up to 2.5 hours to get back to full power, and you won't be using both batteries at the same time like you do with the AC or DC options. When the battery is in the correct position, a solid red light will indicate that the battery is charging.