An automatic charger is designed to power the transformer when you first plug it into a receptacle. The automatic charger needs to "see" at least 4 volts in the battery to power up the circuitry. This initializes the charge sequence. When a completely discharged battery, or one with less than 4 volts charge remaining, is attached to the charger, the charger never "sees" this battery and will not begin the charging cycle. It will also cancel out the Engine Start function. The yellow charging light will also not come on. The meter (where used) will remain in the default "100%" position.

In order to get past this mode, you can "jump start" the battery so to speak, by momentarily connecting another fully charged battery in parallel. This will allow the charger to "see" a higher initial voltage enabling it to begin the charging sequence.

Three reasons:
1. The electronic control circuit board and LED lights use a minor amount of battery power to operate and to determine the battery's state-of-charge.  This results in the battery charger having to periodically charge the battery back to full charge.  Consequently, the green light goes on and off as this process occurs.
2. All batteries have some internal power losses, which the charger is replenishing similar to #1.
3. The length of time that the green light is on is dependent on the battery type, its rated capacity, the degree of discharge, its age, and temperature.

The type of charger you are using more accurately determines this, but generally your battery charger is safe to leave powered up and connected to your battery until the battery has reached a full charge. Manual chargers should be removed as soon as the battery has reached a full charge. These chargers will not cycle or shut down by themselves. They will continue to put a current through the battery until they are disconnected. The use of a hydrometer or voltmeter to monitor state of charge is recommended for use with manual battery chargers.

Automatic chargers are more forgiving on your battery than manual chargers, but they are not designed for indefinite use. They will begin a cycling process, as described above, to prevent overcharging of the battery. Continued cycling will cause premature wear on the electromechanical parts of the charger, causing the charger to fail prematurely. The only charger recommended for extended or a storage application is the 1.5 amp 12-volt charger specifically designed for long term, slow trickle charging/maintenance applications.