Trickle charger.
Looking at the circuit you see a 2n3055 transistor handle the output, on the base is a 15v lm7815 positive regulator. In this configuration a very small relay can be used and will last forever due to the low current needs of the transistor base. Also the small relay is much less noise from on to off state, and less current is needed to operate.
The trouble shooting for this device is easy, one of two component will fail, the transformer or the transistor on the output and both are cheap and easy to replace.
The 500mf cap simulate a battery being charged and the lamp loads the battery to show the range of charge min to max.
Change the 20ohm resistor to change the charge current.
