I have a very old garage type battery charger which has the following little circuit in it besides the main charging stuff. Now I don't know what the circuit is for because I don't understand how it works. I assume it is either a "battery full" circuit or a reverse polarity circuit. The 12v relay activates on switch on and the charger works. There are no lights or led's to indicate anything. It's old, built before the led era. Gives me a few ideas for modifications. Can someone explain what this circuit does?