I am using the Ansmann Powerline Vario (without ‘X’). This charger seems to be identical to Vario X except that a wall plug and car charger was included.
My FP2 batteries won’t charge up to 100% as well - probably due to safety reasons:
This universal chargers only uses 2 connections to the battery - the inner 2 contacts of the battery are unused. The phone might use these contacts for data exchange with the charging controller inside the battery to get information about the charging level, battery temperature and other parameters to allow charging at higher currents or voltage.
Since these informations are missing, the Vario might check the battery voltage to detect a ‘full’ charge and use a lower charging current for safety reasons. Charging probably stops earlier to avoid battery damage.
This charger is perfect if you want to charge batteries abroad; it can be used with a power bank or a solar panel providing a stable current of min. 1A. I mainly use it to charge AA-batteries. It is not a 100% replacement for charging Li-Ion or Li-Po Batteries. Each device or battery-specific charger using all contacts of the battery will do a better job.