battay charging time until full dosent seam plausible

on my toughbook cf-20 mk1 the time until full while charging doesent go down sometimesit even increases if im like 75% after charging it from 0% it says 3 hours untill full wich doesent seam plausible this happen with both battarys the main one (the one in the screen)and the secondary one (the one in the key board)