If you've made the decision to purchase an additional Rhythm P2-S4 battery, you will want to consider the Lifestyle Rhythm P2-S4 external battery charger. When using 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 for your Lifestyle Rhythm P2-S4 portable oxygen concentrator. The Lifestyle Rhythm P2-S4 external battery charger gives you the ability to charge one battery on the side with the use of any wall outlet with the included power supply. Now you can enjoy usage of your Lifestyle Rhythm P2-S4 with 1 battery, while your extra battery is simultaneously charging on the side. The external battery charger works with the single, double, and triple battery, so if you have 2 batteries, you'll never have to worry about going a second without your much needed medical grade oxygen. When you have 2 or more batteries, you'll want to be using one to operate your concentrator while having your second battery charging on the external battery charger, meaning you won't need to sit around for hours waiting for your battery to charge.