What is the chemistry of your battery? Lead-acid, Ni-Cad, Ni-MH or Lithium? They all charge differently.
Why do you want to measure the voltage of a battery that is charging? It is a poor way to determine how much the battery is charged. You need to store the capacity number of the battery (or measure it then store it) then measure its charging current and charging duration. A small processor chip can do the calculation. Maybe a battery charger chip can do it.