Closure system for vehicles, which can be actuated by an electronically acting transmitter/key which outputs a code and which when actuated, in particular when a vehicle door is opened, acts on an immobiliser - e.g. on a switch of the starter and/or the engine ignition - a recording device being attached which determines at least approximately and at least indirectly the instantaneous voltage of the energy source of the transmitter/key and triggers a display which warns the user if the voltage of the energy source has significantly decreased or at least threatened to decrease. <IMAGE>
