The battery magic eye, also known as a hydrometer eye or state-of-charge indicator, is a small device typically found on automotive batteries. It provides a visual indication of the battery's state of charge (SOC) or condition. Here's how it works:

Mainly divided into three states (the color of the battery eyes of different manufacturers may be different): look at the color of the battery "eye", for example: green or blue means it is in good condition; black or red means it needs to be charged; white means the battery needs to be replaced.