A battery-powered hearing aid (1) is programmed via a programming device (2), the hearing aid (1) having a battery (11) and a line connection (3, 4) for connecting the battery (11) to the programming device (2). The programming device uses a variable voltage source (5) and an input comparator (6) with a first and a second input terminal and an output terminal to output a variable voltage to one input of the input comparator (6) and determining the voltage of the hearing aid battery (11) by using the second input of the input comparator (6) of the programming device (2). By ramping the output of the variable voltage source (5) used as a reference, the voltage of the battery (11) can be determined with great accuracy using existing hearing aid programming circuitry.