I had the same problem: very slowly charging, no data transfer. My fairphone 3 is one year old. I first tried the solutions to clean the usb-port, to try another cable, to connect to a PC instead of socket. No result.
The very nice supporter of fairphone offered to repair it without costs within the warranty, but this would have lasted two weeks inclusive shipping time. Too long 
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As the data transfer did not work it was clear for me that it was not the battery but the bottom module.
I bought a bottom module over amazon from i-sell everything went fine and easy at cost of 28,83 EUR. Original part!
I learned from the video how to change the bottom module - very easy. 
And everything works again as it should do. Great relief!!
It is good to have a phone that can be repaired easily!!
BUT:
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Problems with a central module of a fairphone only after one year of use: not very satisfiying!
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No refund of my costs by fairphone, although I dif the rapiring work by myself: Not very satisfying.