Battery Not Charging on stx 15f
 

My 2007 Kawasaki stx 15f battery will not charge. We can run the ski for a few hours but then the battery alarm comes on and the battery is down to 70%. When fully charged the voltage is 12.8, but when the ski is started the voltage drops to 12.3. Had the battery tested and it was fine. Not quite sure where to start trouble shooting the problem. Any ideas?