Here we are at 100% SOC at the .2C charge rate and it took 5:42 minutes.


Yes it took longer to attain 100% SOC (per Lifeline battery tech manual) but that time was only approx 12 minutes different with double the charge rate. Both charge rates took 5.5+ hours and I have repeated this test with AGM batteries is worse shape that exceeded 7 hours to attain 100% SOC likely due to sulfation.


IMPORTANT: Please understand that you will never likely charge this fast with a typical smart charger. Take for example the .4C charge rate. At 19 minutes it hit absorption voltage. Many so called "smart chargers" begin a timer, I call it an egg timer, once absorption voltage has been attained. If that clock started at 19 minutes, and was 2 hours long, the charger would have dropped to a float voltage of 13.4V at 2:19 minutes! OUCH!!!! Considering it took 5:11 minutes of absorption charging at the .4C charge rate your batteries would likely end up under absorbed, under charged and would not last very long. Once we reduce voltage, eg: dropping to float prematurely, we dramatically EXTEND charging times. In my shop I use adjustable power supplies to avoid dumb smart chargers. With a power supply I can control when the battery changes from absorption to float and not rely on an egg timer.



Is too much current bad for an AGM Battery?


For AGM batteries generally the more current the better. It helps with overall longevity to charge at high rates. Higher charge rates in AGM batteries actually yields longer life not shorter life.


Dave V. the lead engineer at Lifeline battery published a study supporting higher charge rates being good for AGM's.. Odyssey battery, thin plate pure lead AGM batteries, wants to see a minimum of .4C and Lifeline a minimum of .2C as recommended charge current.


This is from the conclusion section of Dave V's study:


"In order to achieve the maximum cycle life from sealed lead acid batteries, (AGM) not only should the DOD be kept as low as possible, but the charge current limit should be as high as possible."


The study then goes on to suggest that a balance needs to be met between equipment and optimal cycle life. Today Dave suggests a minimum charge current of .2C for Lifeline AGM's.


It is a pretty rare boat that can muster a .4C charge rate but some do. On a 450Ah bank that would be an alternator or larger inverter chargers that could sustain 180A when hot.


You as a boat owner will have to decide what it's worth in terms of equipment costs to charge at your AGM's at high charge rates. The eternal question of "Do AGM's charge faster with high charge rates applied?" seems to still be open for debate. This battery only saw an approx 12 minute difference in a 5.5hour charge period but yes it did charge "faster" at .4C than it did at .2C.