A method of operating an access control system comprising an electronic lock cylinder movable between an unlocked position in which it allows access to a building and, alternatively, a locked position in which it prohibits access to this building, this method comprising the following steps: - upon insertion of a key into the electronic cylinder, mechanically and electrically contacting first and second electrical contacts of the electronic cylinder to, respectively, first and second electrical contacts of the key, the first and second electrical contacts of the key being electrically connected to respective electrodes of a battery housed within the key, - electrically connecting first and second electrodes of a capacitor of the electronic cylinder, respectively, to first and second electrical contacts of the electronic cylinder to charge said capacitor from the battery housed within the key when said key is inserted into the electronic cylinder, - evaluating the validity of access rights of the key introduced into that electronic cylinder and, only if the access rights are evaluated as valid, generating a command to unlock the electronic cylinder, - only in response to the unlock command and only when the capacitor is charged, powering an electrical actuator of the electronic cylinder from that capacitor, and then - when the electric actuator is powered, it moves: - from an active position, in which it holds the electronic cylinder in its locked position, to - an inactive position in which it allows the cylinder to move to its unlocked position, characterised in that the method comprises: - closing a first switch which electrically connects the first electrode of the capacitor to the first electrical contact of the electronic cylinder and thus starts charging the capacitor from the battery housed in the key, then - measuring the state of charge of the capacitor, then - after the measured state of charge has exceeded a predetermined threshold and prior to triggering the supply of power to the electrical actuator from the charged capacitor, controlling the opening of the first switch to electrically isolate the first electrode of the capacitor from the first electrical contact of the electronic cylinder and thus electrically isolate this first electrode from the battery housed within the key, and then - triggering the supply of power to the electrical actuator of the electronic cylinder from the capacitor while the first and second electrical contacts of the electronic cylinder are still in mechanical and electrical contact with, respectively, the first and second electrical contacts of the key. - upon insertion of a key into the electronic cylinder, mechanically and electrically contacting first and second electrical contacts of the electronic cylinder to, respectively, first and second electrical contacts of the key, the first and second electrical contacts of the key being electrically connected to respective electrodes of a battery housed within the key, - electrically connecting first and second electrodes of a capacitor of the electronic cylinder, respectively, to first and second electrical contacts of the electronic cylinder to charge said capacitor from the battery housed within the key when said key is inserted into the electronic cylinder, - evaluating the validity of access rights of the key introduced into that electronic cylinder and, only if the access rights are evaluated as valid, generating a command to unlock the electronic cylinder, - only in response to the unlock command and only when the capacitor is charged, powering an electrical actuator of the electronic cylinder from that capacitor, and then - when the electric actuator is powered, it moves: - from an active position, in which it holds the electronic cylinder in its locked position, to - an inactive position in which it allows the cylinder to move to its unlocked position, - from an active position, in which it holds the electronic cylinder in its locked position, to - an inactive position in which it allows the cylinder to move to its unlocked position, - closing a first switch which electrically connects the first electrode of the capacitor to the first electrical contact of the electronic cylinder and thus starts charging the capacitor from the battery housed in the key, then - measuring the state of charge of the capacitor, then - after the measured state of charge has exceeded a predetermined threshold and prior to triggering the supply of power to the electrical actuator from the charged capacitor, controlling the opening of the first switch to electrically isolate the first electrode of the capacitor from the first electrical contact of the electronic cylinder and thus electrically isolate this first electrode from the battery housed within the key, and then - triggering the supply of power to the electrical actuator of the electronic cylinder from the capacitor while the first and second electrical contacts of the electronic cylinder are still in mechanical and electrical contact with, respectively, the first and second electrical contacts of the key.