Basic principle: The robot reaches the charging station via tracking the perimeter wire loop. When the robot reached the charging station, it will recharge. The robot will track the perimeter loop (clockwise) until it detects a voltage at its charging pins (charging voltage). Then it will stop for charging.
The charger is build into the charging station (or it is connected to it). The charger must be able to handle several aspects (here: Lithium-Ion, lead-acid battery is similar):
-Charge battery via roboter charging pins
-Limit charging voltage
-Limit charging current
If you are going to re-use your charging station, it will probably handle these aspects for your specific battery type.
Inside the robot, the battery voltage is monitored (see schematics below). This allows the robot to detect a low battery state so it knows when it will have to drive home to the charging station. Also, the charging voltage and current are monitored. This allows the robot to detect if it has reached the charging station and when the battery has been fully recharged.
charging station circuit:
Power supply => Charger 24V (+)----- Charging station charger pin(+)
Charger 24V (+)----- DC-DC converter 5V => Perimeter sender Arduino Nano
robot charging circuit:
Charging station (+)---+------+-- relais ---- current sensor ----- battery (+)
Charge plotting etc.
Via pfodApp (Android) you can get a plot of the charging process.
Charging station - Ideas