So the Lifeline Technical Manual recommends RV and boat owners perform the following at the beginning and end of each season.
5.7 Capacity Testing
To determine the actual capacity of a Lifeline® AGM battery relative to its rated capacity, a full discharge test should be performed. Although there are various battery testers available on the market, such as carbon pile testers, impedance meters, conductance meters, and others, these testers are not reliable in determining the battery’s actual capacity. To determine the battery’s actual capacity relative to its rated capacity, use the following procedure:
1. Stabilize the battery at 68-86°F (20-30°C) for at least 24 hours.
2. Bring the battery to full charge as described in Sections 5.4, 5.5 or 5.6 as applicable.
3. Discharge the battery at a constant current of 25 amperes until the voltage falls to 10.5 volts (5.25 volts for a 6 Volt battery). Record the discharge time in minutes.
4. Compare the measured discharge time to the published 25A rating (reserve capacity minutes) for the battery.
5. If the battery delivers less than 80% of the rated capacity the conditioning procedure given in Section 5.5 should be attempted and the battery capacity should be retested.
6. If the battery delivers less than 50% of its rated capacity, it should be replaced. However, the user should determine the amount of capacity needed for their particular
application and adjust the pass/fail threshold accordingly.
They rate my 150Ah GPL-30HT for 315 minutes @ 25A.
Mine ran for 259 minutes, or 82%. Not too bad for a 66-month-old lead acid battery, especially one belonging to neglected by me.