I succeeded in adding the charts
by moving them to the directory /storage/emulated/0/Android/data/org.opencpn.opencpn_free/files
A few months ago I tried to load cm93 charts
on my dad's tablet (Huawei something). I tried placing them on an SD card as well as on the internal storage
of the tablet in a folder I created called /storage/emulated/0/Charts.Nothing worked for me - the app would only display the default background chart. I recently installed OpenCPN
on my own phone
(Fairphone 3) and ran into the same problem. I have seen a few posts around with various fixes that did not work
for me, so I thought I would share my solution.
I now have storage/emulated/0/Android/data/org.opencpn.opencpn_free/files/Cm93 as the only directory in my chart files tab and the charts show perfectly well.