4 IN 1 CHARGING SOCKET STRIP - This 12V power strip is equipped with a 12V USB socket + a voltmeter with LED digital display and a 12V cigarette lighter socket + a LED rocker switch. Convenient for installation and operation. Also removable to replace other charging ports or anything else that fits.Waterproof Double USB Charging Port - This 12V USB car socket features two built-in USB charging ports, two output ports: 5V/2.1A and 2.1A (Total 4.2A), detects your devices automatically and simultaneously charge 2 devices at optimal speedReal-time LED voltmeter with - With the large digital voltmeter display that displays a voltage of 5-48V, you can monitor the condition of the battery in your car or boat when the voltage is in normal condition.LED on/off rocker switch - designed with rocker style, easy to use. Long service life up to 10000 cycles. Additional cigarette lighter socket is portable for other devices in your vehicle.Overcharge Protection - Overheated over-current overcharge protection; This dual USB charger socket is also smart to determine the most suitable current for your device.Hands-free calls:"One button" - answer/reject/hang up/redial phone calls.It easily switches between incoming calls and music to ensure your driving safety.Wide compatibility:Universally compatible: Optimises the fast charging of all Android and Apple devices such as iPhone 8X / 7Plus / 6S / 5S / Samsung Galaxy S8 Plus / S7 Edge / S6 / Google Pixel 2 XL / Nexus 6 / Motorola / Sony and much more.QC 3.0 USB port:Quick Charge 3.0 fast charge significantly shortens charging time. At maximum performance, the efficiency of the charger is 80% faster than a normal charger.The smart output current pairs your smart device and charges it safely without damaging the battery.Below are the usage tips:1. Find an empty channel on the car radio. Then plug this FM transmitter into the cigarette lighter of your vehicle.2. Turn on the phone's Bluetooth function, search for "BT_C27A", press it to connect.3. Adjust the frequency on the FM as same as radio. Our recommendation is: Search for empty channels near you on Google for optimal results.4. Open the music player and enjoy your music.How to adjust the volume to avoid interference?The volume of the car radio can reach 1/3 or 1/2, but not 2/3.The volume of the FM transmitter can reach 2/3 or 4/5, but do not try to reach the full volume.The volume of your phone could reach 2/3 or 4/5. However, do not try to reach the full volume.