It should charge at least at 20W from what i've seen so far you have a problem with the charging circuit on your motherboard.
[image|1138806]
Since is still charging there is a low probability that the SMC is fried so the most probable cause of your issue is the Q7055 Gate that is not doing the right job or the U7000 but my bet is on the Q7055. (here I supposed that your motherboard number is 820 2915, they are almost the same all of the 15" from the same year)
[image|1138805]
You should look for a shop near you that is doing this type of micro soldering repairs.
Just to be sure, did you try a SMC Reset?