The battery in this test is a very popular AGM, a Lifeline GPL-31T. It is rated at 105Ah at the 20 hour rate but this battery, off my brothers boat, was used and only capable of delivering 95.69 Ah's.


Despite the slightly diminished capacity, 91.3% of factory rated capacity, I kept the discharge rate at 5.25A at a battery temp of about 76.4F. Ideally I would have liked to have spent the time to figure out the new 20 hour discharge rate but this entails numerous tests to 10.5V and was not really that necessary for this experiment.


The 5.25A discharge rate is slightly higher than what this battery can deliver for 20 hours but close enough to make the points I wanted to make. Perhaps less than .02% of boat owners ever tests their batteries for actual capacity, so using the 20 hour rate on this battery is a bit more real world.


For a 20 hour test, or to see where your bank stands against how it is rated for capacity, a 20 hour discharge test is performed. The battery is charged to 100% SOC and then discharged at a 0.05C discharge rate. A discharge rate of 0.05C is equal to 5% of the batteries 20 hour rated capacity so 5% of 105Ah is 5.25A.


On a cruising boat the battery bank would be considerably larger and a 20 hour discharge rate would look like this:


200 Ah Bank = 10A Discharge Rate = 20 Hour Capacity


300 Ah Bank = 15A Discharge Rate = 20 Hour Capacity


400 Ah Bank = 20A Discharge Rate = 20 Hour Capacity


500 Ah Bank = 25A Discharge Rate = 20 Hour Capacity


600 Ah Bank = 30A Discharge Rate = 20 Hour Capacity