These have a voltage comparator circuit (bistable with hystersis) which works like this:
When you first connect a battery, a bistable is set. When set, the voltage limit is set to a charge voltage of ~14.2V. The charger supplies current limited by the transformer/charger (~6A) until the battery terminal voltage comes up to the higher limit. When the battery voltage exceeds the higher limit, the bistable is reset, which lowers the voltage limit to a float voltage of ~13.0V, (and turns on one of the LEDs), which is where charger holds the battery voltage as long as it remains plugged in.
What are the LEDs doing? What are the LED labeled?