|
The only other device I own that uses a add-on external power supply discharges the external battery first. This is a deliberate behaviour based on the idea that if you decide to reduce the weight by removing the external power supply which is heavier than the base device, you have full charge on the internal battery. When the external supply is attached and discharged to ~10%, it automatically switches to the device's internal battery. At any time, the external power pack can be connected to a charger to be recharged (if attached, both batteries will charge since it blocks the device USB charging port when attached).
|