after a battery is fully discharged more than about 2 to three times the charge will deplete rapidly and wont hold much of a charge and this makes ur alternator work over time and could burn it up quick so take a multimeter or voltmeter and first check the battery and see what the charge is if its below 12.6 volts its been discharged and for every .2 volts below 12.6 is 25% depletion and if you want to find out if you have a bad cell you can take the multi/voltmeter and check each individual cell by putting one probe one the negative and the other in each cell this means pull the caps off the battery assuming its not a sealed/maintenance free battery each should come up 2.1v

well i hope this helps