I've been trying to charge my 640 XL LTE DS phone (RM-1096) with a (good quality) 2.1A output usb charger, but get the error message that states the phone is charging slowly. A little googling highlights that many of the lumia phones are fussy about chargers, but also that the Qualcomm SnapDragon 400 processor is compatible with Qualcomm? Quick Charge? 2.0 technology. Does anyone know if such chargers are genuinely compatible with this phone ? In the UK the Motorola or HTC Quick Charge 2 chargers are not cheap (about ?25 or so), but if they are compatible they'd be a great improvement on the 750mA out put charger that the phone came with.