I have this issue on my 640's as well. I have a couple of usb outlets on my wall sockets that run at 2 amp output. I plug a 640 into them and I get the same message about charging slowly, on 2 amp!!
With the 640 it appears that you have to use the charger it came with. I have tried several chargers, with varying amp outputs, and still got the message. When I use the correct charger, all is well. I can't remember the output on the charger, but I'll have a look when I return to the office.
05-09-2016 05:03 AM