This test was a .2C charge for one hour and then discharged back to 50% SOC, after the 1 hour charge. The stored energy removed was 20.46Ah from a max charge rate of 21A.


CHARGE EFFICIENCY: As I have mentioned before bulk charging, where the charge source provides its full current before the battery reaches the limiting voltage is nearly 100% efficient. Here we have a 1 hour recharge at a charge rate of 21A where the battery was able to store 20.46Ah of that 21A. A 21A charge for 60 minutes is 21Ah's of supplied energy. This means 97.4% of the energy supplied by the charger, or the 21A for exactly 1 hour, was removable as stored energy when we discharged back to 50% SOC. On this 1 hour .2C recharge the battery never attained the absorption voltage of 14.4V and was still in bulk when the charger turned off at the one hour mark.


SCALE IT UP: If we scale this test up, and it should scale well, a .2C charge rate on a 450Ah fairly typical cruising boats house bank would be a continuous 90A for 1 hour before your batteries even hit the absorption voltage set point. Yes, 90A continuously for 1 HOUR!! This is a metric $hit ton of work on a typical 80A - 120A alternator. This is why many a boater has burned up their alternators charging AGM batteries!


MYTH BUSTING: If you believe a battery monitor that counts Ah's, and supplies a fixed charge efficiency number to the returned Ah's (most all of them), can track your batteries accurately when you don't recharge to 100% SOC with each cycle, this myth & lore is blown out of the water right here. In bulk this battery had returned energy efficiency of 97.4% yet the last 4% of returned energy, to 100% SOC, takes 3.5 hours. There is a major difference in charge efficiency throughout the SOC curve. Charge efficiency is not linear throughout the SOC curve, but most all battery monitors are linear in their application of charge efficiency. They simply apply a negative count factor for charge efficiency as the Ah's are returned. For example if we return 10Ah's with a 90% charge efficiency setting the battery monitor will only show that as 9Ah's returned. If we were to stop charging in bulk we really do have closer to 10Ah's returned. Do this a few times........ Ouch! When you cycle more than a few times in the 50% to 90% SOC range there is no way for a typical Ah counter/battery monitor to accurately track your charge efficiency. For more info on this see my article on "Programming a Battery Monitor".


Let's do the math:


Baseline Ah Capacity = 95.69Ah


Discharge to 50% = 47.44Ah (left in the battery after discharge)


1 Hour .2C charge then discharged and counted Ah's delivered back to 50% SOC = 20.46Ah


47.44Ah + 20.46Ah = 67.90Ah of stored energy


67.90Ah is approx 71% of the baseline Ah capacity of 95.69Ah's


BOTTOM LINE: The battery achieved approx 71% SOC in one hour at a .2C charge rate.


PERSPECTIVE: It is pretty clear that a 1 hour charge at .2C is an inadequate charge rate for AGM batteries that are routinely discharged to 50% SOC, unless you really like hearing your motor or generator run.


NOTE: Lifeline Battery recommends a .2C charge rate as the bare minimum for these expensive AGM batteries. Odyssey TPPL AGM batteries are recommended to be charged at a minimum of .4C.