This test was a .4C charge for one hour and then discharged back to 50% SOC, after the 1 hour charge. The stored energy removed was 33.43Ah from a max charge rate of 42A.


SCALE IT UP: If we scale this test up, and it should scale well, a .4C charge rate on a 450Ah fairly typical cruising boats house bank would be a continuous 180A. This battery hit the absorption voltage point at approx 20 minutes when charged at .4C from 50% SOC. With a .4C charge rate your alternator or charger would only be in bulk, or running at its full output, for approx 20 minutes. While this certainly does not make the most efficient use of the alternator, or inverter/chargers, or generators it does charge the batteries to a higher SOC in a short 1 hour run.



Let's do the math:


Baseline Ah Capacity = 95.69Ah


Discharge to 50% = 47.98Ah (left in the battery after discharge)


1 Hour .4C charge then discharged and counted Ah's delivered back to 50% SOC = 33.43Ah


47.98Ah + 33.43Ah = 81.41Ah of stored energy


81.41Ah is approx 85% of the baseline Ah capacity of 95.69Ah's


BOTTOM LINE: The battery achieved approx 85% SOC in one hour at a .4C charge rate.


PERSPECTIVE: While .4C certainly does attain a higher level of SOC in 1 hour, approx 85%SOC, the gain makes ineffective use of a .4C charge source while only utilizing it at 100% for about 20 minutes. Most boaters charging AGM batteries would likely benefit from a compromise charge source current of about .25C to .35C for 1.5 - 2 hours +/- per day. Blended with solar you can get the bank to 85% SOC or so and let the solar take over.