From a discharged state of 18 VDC, how many minutes of charging are required to achieve 30 minute backup capability?

• A. 15 minutes
• B. 30 minutes
• C. 25 minutes
• D. 23 minutes

Answer: (D)

The amount of charging time needed is governed by the battery’s energy state of charge and the charger’s profile, not by a simple linear relation to the backup duration. Reaching 30 minutes of backup requires the battery to attain a specific state of charge that provides the necessary stored energy to run the load for half an hour. From an 18 V discharged condition, the charger must bring the battery to that defined energy level, which for this system is achieved in about 23 minutes. The charging curve delivers high current up front and then tapers as the battery approaches the target charge, so the required time is a precise value rather than a simple proportional calculation. Charging for less time wouldn’t provide enough stored energy for the 30-minute backup, while charging longer isn’t needed to meet the spec.