I have a friend who has a Microsoft Lumia 640 Lte, Windows 8.1, and last night he allowed the battery to go completely dead to where it cut at 0%, and afterwards, he plugged the charger in, and it took 3 hours to get to 20% of charge. Do anyone happen to know what could be causing this charging problem he's having? Could he have damaged his phone or battery by letting it die all the way out?