When a battery has been charged the charger holts the charging process when it reaches stage 5 to see if the voltage drops below 12 volts within 3 minutes. This test would indicate that while the charger was able to reach 14.4 absorption voltage that something else may be wrong with the battery. The owners manual shows the stage here
Possible causes for not maintaining a voltage above 12 volts include a dead cell or sulphation. At this point, if it fails to hold 12 volt the battery will restart the charging process. If however after 20 hours the charge isn't complete the unit will show the caution lamp indicating there is a problem with the battery which requires user intervention. Its normally at this point you'd either check water levels, test with a hydrometer, check with a conductance tester, perform a discharge test with a cranking unit or deep cycle test to ascertain if the battery is fit for purpose and replace if required.
If the voltage does not drop below 12 volts at the end of 3 minutes the charger will continue to finish its last modes and assume the battery is OK. If you need or want to test the battery further you can test the batteries internal health with our CTEK Battery Analyser
