Testing a battery charger
may be a bit tricky without known battery quality or specific testing equipment (current rate recorder) I suggest discharging the batteries to a known low voltage level (say 10-11 volts) and applying the charger to the terminals while watching the current delivery, voltage and time by using an inline ammeter or the meter on the charger. Current should tail off as battery voltage increases.
FInal voltage should be ~12.8-13.4. Obviously, if this is achieved, then the charger is OK. Sulfation, battery damage or poor electrolyte will,obviously interfere with proper performance.