[Digital Voltmeter display] If two batteries are connected, then the voltage displayed on the voltmeter is the battery that has higher voltage. Only connect the battery that you would like test the voltage and disconnect the other battery. When the voltmeter is not working, press the button once, the voltmeter will display the value for 2 seconds. [Manual state use function]--The blue indicator light is ON, press the button each time, you can complete the circuit connection and disconnection. The circuit switched on the red indicator, the voltmeter lights up at the same time. The circuit disconnect red indicator light, the voltmeter goes out at the same time [Automatic state use Function]--When the input voltage exceeds 13.3V, the circuit will be connected automatically. The voltmeter and red (LED) indicator will also be ON. When the input voltage drops to 12.7V, the circuit will be disconnected automatically. The voltmeter and red (LED) indicator will also be OFF [Voltmeter display]-- If two batteries are connected, then the voltage displayed on the voltmeter is the battery that has higher voltage. [High & low pressure protection]-- When the input voltage reaches lower than 11V or more than 17V, the circuit will be disconnected automatically to protect the main battery from over discharging or protect the secondary battery from high voltage impact [Compatibility]: This battery isolators are small and suitable for almost anywhere, for ATV, UTV, Boats, RV's,marine vehicle, truck, caravan Colour: Black Material: ABS+metal Package Contents: 2 x Digital Display Dual Battery Smart Isolator With Screw mounting accessories 2 x English manual Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.