I always observe 4.0xxV from the li-po battery attached after when thingy52 stopped charging.
I think this problem maybe because of output current from the battery charger (XC6804) is consumed by battery charging and system load at the same time.
AFAIK, XC6804 doesn't have power path feature so, it will be problem if there is no circuit to isolate current consumption for battery charging and system load. (I can't find related circuit to isolate and switch power source between VBUS and VLi-Ion in the schematic.)
Please take a look this issue and post possible solution for this.
I tried reproducing the issue, but without success. My Thingy52 was initially at about 70% charge, and after charging it for about 3-4 hours, it reached 4.20 Volt.
Thank you for testing.
Now I also see 4.2V after waiting few hours.
I'm still not sure why battery charging has stopped around 4.0xx Volt and started again after few hours.
Anyhow, I should not argue that the battery is not fully charged anymore.
But I still concern that current sharing from the battery charger to Li-Po battery and system load is fine.
I did some more tests where I enabled all the different sensors, the Thingy-LED was set to max intensity, the battery service was on, etc. And I still had no problem charging the battery to 4.20 V. At the infocenter it's noted that the max charge current is set to 0.5 C, so the current sharing should not be any problem.
My experience is also that the Thingy52 battery sits at 80% and 4.00 V for up to 12 hours during charging, then finally goes up. I believe this should be expected: the discharge curves for lithium ion batteries I've seen tend to have a sharp drop-off from fully-charged (4.17 V for me) down to a base level that remains nearly flat for at least 75% of the cell capacity. The Thingy firmware estimates capacity based on a linear interpolation of voltage between 4.15 V (100%) and 3.1V (0%) which means it's going to read about 80% (4.0 V) until it's down to the last few percent.
So it's not that it isn't charging, it's that there's no measurable value that confirms charging is active.
This behavior does make it hard to estimate the true time-remaining at constant load, but having stable power is worth a little inconvenience.
is there a charging LED on the board? Then at least you'd be able to tell the difference between charging and no voltage difference or not charging. I use LIPOs in some of my devices and have a standard charge circuit based on a Microchip charging chip; I find that to be quite conservative especially when I plug in with the battery 'quite charged' it takes a while before it starts charging it again.