It depends on the size batteries you have and how much they have been discharged. My trolling batts take around 4 hrs. to charge at full discharge and they are group 29s. The cranking batt. takes about 1 /2 hr. with the on board charger because my motor keeps them charged while running. You could have a cell in the crank battery that is bad. You can use a multi-meter and check how much charge the on board charger is putting out. Disconnect the hot and ground wires from the charger to the batts., put red to red, black to black, plug in charger, and see how many volts are being put out from the charger to each batt. Should be 13 to 14 volts to each battery. If the charger is working correctly, take the batteries to an auto parts store and ask them to put a load test on them. I had one that they said was ok by their tester, and I ask them to do a load test. He said "that will take about 30 minutes". I said, OK. Came back and he said "It's bad". That will tell if they are good or not.