The disclosure relates to a battery charger for one or more hearing devices, comprising: a test sound generator configured to produce testing sound; an input configured to obtain first information regarding a first microphone output of a first hearing device in response to the testing sound and obtain second information regarding a second microphone output in response to the testing sound. A processing unit is configured to detect a clogging of a filter such as a wax filter based at least in part on the first information regarding the first microphone output of the first hearing device, and based at least in part on the second information regarding the second microphone output. An output is configured to provide a signal indicating the clogging of the filter.