Most batteries come with some form of a charge on them, li-ion batteries do not have a memory feature so it is safe to use them so long as they do have a charge. With the passthrough plugged in it is charging and the passthrough opperates on the battery, not the straight voltage from the USB port. And the battery is charged in turn from the usb port.
The passthroughs that do not have a battery are mostly 5 volt while the ones that have a battery box are 3.7 volt.
Sorry to hear about the sharing, I have done that before and it is very nice of you to let it happen
Hope that gives you a bit of information.
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