How do inductive chargers detect a load?
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The transmitter transmits bursts and detects the resulting current flow in the transmit coil. Depending on the value/shape of that current the transmitter can detect if there is a load present, nothing present or a foreign object present.

When a load is present the voltage and current in the transmit can be influenced (modulated) by the load. That way communication can take place. It is almost identical to how RFID and NFC work.