All automatic battery chargers in the automotive market today have some sort of voltage regulation to prevent overcharging the battery. The rate of the flashing is dependent on the battery type, its rated capacity, the degree of discharge, its age, the temperature, as well as the amperage rating of the battery charger.

If the charger’s green LED begins blinking when you connect the charger clips to the battery, the amperage in-rush current to the battery is reaching the pre-set shut-off voltage and the charger is shutting down. The voltage will drop as soon as the charger shuts down, turning the charger back on. This may also mean the charger has detected a battery problem - for instance, its ability to take or hold a charge.  Sometimes a battery may be sulfated (accumulation of lead sulfate on the battery plates), and the sulfation is creating a high resistance to the current flow (ability to accept a charge).  Or, perhaps the battery is deeply discharged (below 8-volts).