The present invention relates to a battery charger for charging a battery. The present invention especially relates to a battery charger comprising a switching power supply controlled on the primary side and a charge monitoring circuit for monitoring the charge state of the battery. For providing an improved battery charger which allows a secure and reliable charging of batteries with a reduction of the circuit complexity, the costs for appliances and the construction size, the charge monitoring circuit is--according to the invention--disposed in the switching circuit on the primary side of the battery charger.