A fully discharged automotive battery may take up to 20 hours for full charging. The length of time is dependent upon the RC rating (reserve capacity). A battery with an RC of 60 and 2 amps could take 20 hours to charge, while a battery with an RC rating of 60 with 30 amps may take only one hour to fully charge. fully discharged automotive battery may take up to 20 hours for full charging. The length of time is dependent upon the RC rating (reserve capacity). A battery with an RC of 60 and 2 amps could take 20 hours to charge, while a battery with an RC rating of 60 with 30 amps may take only one hour to fully charge.