... larger volume, better sound quality.The product's function and usage as follows.1.Button section:Volume +, Volume -, MODE buttonVolume +:Press this ... for the backup battery, you could not use the synchronization function.3.When using the "ipod" software to listen to music, go ... than 25% , even though in the state of charging, it also can not play music. charger & speaker & case 3 in 1 for iPhone 4