Here is another way to look at the charge process. This was a *Lifeline GPL-31XT battery (125Ah rated that was charged at .15C or about 15% of its rated Ah capacity. *NOTE: This was not the battery being tested in this article, and it was used. This image is just for just for illustrative purposes..


BULK: If we start at the top left of the chart we can see that the current held rock steady at 18.75A for *1:42 minutes. This current, 18.75A is .15C or a 15% charging current of a 125Ah battery.


*NOTE: 1:42 minutes of bulk charging (your alternator running at full bore) is more than enough to damage an alternator not otherwise protected for temperature or current limited to protect it. A charge rate of .15C would be an 80-90A alternator on a 400Ah bank, when accounting for the "hot" output, and not all that uncommon. I see lots of burned up alternators when they are under sized for the task.


VOLTAGE RISE: If we look at the left side of the chart we can see the voltage climbing to the absorption set point of 14.4V. It took 1:42 minutes at 18.75A for the battery voltage to attain 14.4V.


1/3 From Left: This is the point where the charge current and voltage flip-flop. Voltage stops climbing and is held steady, and current changes from being steady/constant to declining. Once the BULK/constant current charge has driven the battery voltage to the absorption voltage limit, the voltage is now limited or held steady and the charging current begins to decline.


CHARGE TIME: As current declines, out towards the lower right of the chart, the time it takes to get that last few % in takes significant time. At .15C this slightly used AGM took a bit over 6 hours to reach 100% SOC. I have had sulfated AGM batteries take 7+ hours even at a higher .2C. The health of your AGM battery can impact your time to full.




TESTING SUMMARY:


1- 50% SOC to 100% SOC at .2C = 5:42 - Exited Bulk Charge at 1:16:00
2- 50% SOC to 100% SOC at .4C = 5:30:00 - Exited Bulk Charge at 19 Minutes
3- 50% SOC Charged at .2C For Exactly 1 Hour = 71% SOC - Never Exited Bulk
4- 50% SOC Charged at .4C For Exactly 1 Hour = 85% SOC - Exited Bulk Charge at 19 Minutes
5- 50% SOC Charged at .2C For Exactly 2 Hours = 87% SOC - Exited Bulk Charge at 1:16:00
6- 50% SOC Charged at .4C For Exactly 2 Hours = 96% SOC - Exited Bulk Charge at 19 Minutes


DISCLAIMER: All batteries will test & perform differently based on age, type, chemistry and state of health. This battery and test are only really representative of this battery and test but should give some decent guidance as to what one may expect from a slightly used AGM battery. I purposely did not test a brand new AGM because that is not at all representative of what we as boaters have in use on-board our vessels for more than a short period of time. New batteries will perform differently but our batteries on boats rarely stay new for very long.










Good luck & happy boating!!