starter cranks slow at first

for about a week now my starter has been sounding like i have a dead battery at first but it will allways build up the speed to start the engine after a few seconds. its really wierd, i havent seen this before. i checked my connections and they are all clean, the battery puts out 12v and with the car running 14.5

incase its any help to anyone it was the battery, some how it lost its cranking amps while still holding a full charge, just really wierd since ive never seen that happen before..