It's really weird. The external battery with a 2.1A output is switched off automatically after some time when nothing is conected. When I plug the device to it, nothing happen during ~1min then the red led is on during around ~5min but finally turned off and the external battery too 2 minutes after. If I manually restart the external power source as soon as the red led turned off, nothing happen during 2 minutes and the external battery finally turn off. I immediatly restart the external battery, and after ~1min, the red led turns on. It's like the device need a 2/3 min break after each 5 minutes of power. If I wait 10 minutes before restart the external battery, the red led turn on immediatly.
I'm finally not sure the battery is charging as I've never been able to have a fix red led