Why does a twin charger only charge one battery at a time?

Why does a twin charger only charge one battery at a time? trimble charger dual battery slot Power supply and cord only, Trimble Dual Slot Battery Charging Cradle not included Related Products trimble dual slot battery charger manual Dual Slot 2 Slot Battery Charger 109000 Input of Charger：DC19V Output of Charger： 2A Input of Adapter：100-240~5060Hz MAX

trimble dual slot battery charger manual NEW TRIMLE DUAL Slots Charger for Trimble R10 S8 GPS, Total Station

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