Waterproof DC12V Marine Car Boat Cigarette Power Socket and DC Digital Voltmeter Included DC12V Power Socket and DC Voltmeter The socket will accept any standard accessory plug however when matched with the corresponding accessory plug the unit will be watertight and vibration resistant when in use. Twist-and-Lock system, allows a suitable power plug (not included) to lock into place This is a high quality voltmeter for checking the state of your battery and battery charger. It mounts in a 1-1/8 hole in panels up  thick, a nylon retaining nut holds the meter in place.