Rather than bore you with multiple images that all look the same I am going to sumarize the remaining three tests with the "stored energy" screen shot.
This test was a .4C charge for two hours and when discharged back to 50% SOC, after the 2 hour charge, the stored energy removed was 44.48Ah. While not a heck of a lot more than .2C it is the attained SOC that really makes .4C different.
Let's do the math:
Baseline Ah Capacity = 95.69Ah
Discharge to 50% = 47.64Ah (left in the battery after discharge)
2 Hour charge then discharged and counted Ah's delivered back to 50% SOC = 44.48Ah
47.64Ah + 44.48Ah = 92.12Ah of stored energy
92.12 is approx 96% of the baseline Ah capacity of 95.69Ah's
BOTTOM LINE: The battery achieved approx 96% SOC in two hours at a .4C charge rate
PERSPECTIVE: Charging to 100% SOC took 5:30 minutes so this means the last 4% of returned energy took 3.5 hours to complete at .4C and the first 46% of capacity could be returned in 2 hours at .4C. The problem still remains that the last 4% is critically important to AGM longevity.
BEGIN: .2C - 1 Hour Recharge Then Discharge
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.
BEGIN: .4C - 1 Hour Recharge Then Discharge
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.
Another View of The Charging Process
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!!
MAY-2006
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