Supply mode allows the charger to be used as if it was a battery power source. What that means it just like a battery when you connect a light it illuminates, in power supply mode if you attached a light it would illuminate. What that also means is that in power supply mode if you incorrectly attach it to a device with the positive and the negative around the wrong way you may damage the equipment. It also means the unit will spark if you touch the alligator clips together which will damage the charger.