Charger il 97% done, I've lost some time with a differential amp with a soul on it's own, voltage trimming here and there and some issue with an old Lithium cell with an higher than expected resistance due to wearing that was causing trouble to the threashold system regulating chrge current.

At the moment the charger is:

_ precondition the cell @50ma if voltage detected is lower than 3.4V
_ Full charge current @500ma from 3.4v to 4.10v
_ Top charge @150ma from 4.1v to 4.2v
_ Finalize charge @50ma from 4.2v to 4.23v

Actually I was measuring cell voltage when applying current, but it turned out to give only false reading, so now I'm modifying the structure of the code - but then I hit the memory limit of the PIC16F690... so now I'm back at optimizing code before continuing :\

Update: I was getting false reading of the voltage due to too high input resistance seen by the PIC, lowering it solved the main problem.

Another issue is present here, the sensitivity: there's a 10bit ADC reading a 5V span: 5/1024 = 0,004mV step, so it's quite a guess distinguishing anything in between, 4,19..4,23 is a hit and miss job. Even with filtering, hardware and softeware, I end up slightly off target.