I'd like to add a small plug to the existing battery pack of my Petzl Iko in order to have the ability to plug in a battery pack containing 3 AA (as opposed to AAA batteries). I have such a battery pack from an old Petzl Myobelt headlamp. I see no issues with this (apart from voiding the warranty) as long as I remove the Core battery before plugging in the 4.5V battery pack. My question is what would happen if I were to leave the Core battery in and (either inadvertently or not) and apply 4.5V to the output terminals of the Core battery? I measured the voltage of the output terminals of my Core battery and got +4V when no charger is connected and about -0.1V when a USB charger is connected, something I would not have expected.