1 Hour & 16 Minutes - .2C Recharge Absorption Attained
At 1:16 the battery finally attained 14.4V and the current had begun dropping.
If we assume that bulk was 100% efficient, and we will for round numbers, the energy returned to the battery at .2C during bulk charging looks like this:
21A X 1.27 Hours = 26.67 Ah
If we know the battery had 47.36 Ah left in it, at 49.5% SOC then:
47.36Ah + 26.67 Ah = 74.03 Ah stored in the battery
74.03 Ah as a % of 95.69 Ah (tested capacity) = 77.4% SOC
This battery, when charged at .2C from 49.5% SOC, stayed in BULK/CC for 1:16 minutes and entered ABSORPTION/CV charging, where current begins declining, at 77.4% SOC.
Once again this clearly shows how charge current affects the SOC transition point from bulk to absorption charging. At .4C the battery attained absorption voltage at approx 63.3% SOC and at a .2C charge rate it attained absorption voltage at approx 77.4% SOC.
2 Hours - .2C Recharge
At 2 hours the current is down to 11A..
3 Hours - .2C Recharge
At 3 hours the battery is accepting just 4.2A..
4 Hours - .2C Recharge
At 4 hours the battery is accepting just 1.6A..
Determining 100% SOC
The key piece here is how to determine you are at 100% SOC per the battery manufacturers instructions. Using net return amps, at absorption voltage, (not float voltage) is usually the easiest method when out cruising. Here is a screen capture of the Lifeline Battery Tech Manual. I have highlighted the important part for net return amps determining 100% SOC.
It is saying;
A voltage limit of 14.3V to 14.4V should be used when the battery temperature is at 77F, this battery was at about 75F. When voltage is steady at 14.3V to 14.4V the battery can be deemed 100% full when charging current declines to 0.5% of Ah capacity.
When 14.4V and 0.525A were attained is when I deemed this battery at 100% SOC. Technically the battery was only at 95.69Ah so I should have used 0.48A but close enough for this experiment.
100% SOC - .2C Recharge
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.
It's Important To get Back to 100% SOC!
The importance of getting back to 100% SOC, as often as possible, can not be over emphasized. Here Lifeline Battery is referring to the "walk down" effect of PSOC use (partial state of charge cycling). By not recharging to 100%, as often as possible, your Ah capacity will gradually "walk down" as sulfation becomes permanent and ruins them.
For more information on the effects of PSOC cycling AGM batteries see the May 2015 and August 2015 issues of Practical Sailor Magazine.
SUMMARY: In this test, on this AGM battery, the difference between a .4C and a .2C charge rate, from 50% SOC to 100% SOC, netted a 12 minute total charge time difference.
BEGIN: .2C - 2 Hour Recharge Then Discharge Test
In this second part of the testing I am replicating a few typical scenarios for recharging on a cruising boat. No one wants to run the engine or generator for very long so I wanted to illustrate what kind of energy can be stored in the AGM battery at a 2 hour time interval at a .2C charge rate.
This test started with a discharge from 100% SOC to 50% SOC with the battery delivering 47.85Ah at a 5.25A constant current load. The battery was then recharged for exactly 2 hours at .2C and then immediately discharged back to 50% SOC and the stored energy for that cycle was measured.
The charge rate for this test was 21A and the timer set for 2 hours. If the battery could stay in bulk for two hours our maximum potential energy into the battery would be 42A. Because the battery hit absorption voltage, before hitting the 2 hour mark, we simply can not get 42Ah of energy into the battery...
CHARGING - 2 Hour Charge Test .2C - The Two Minute Mark
At two minutes into the test at 21A the battery terminal voltage is at 13.1V.
MYTH BUSTING: Think about this snap shot if you use or are considering a battery combining relay for charge management and are concerned or have been scared off by one of the many myths surrounding the effective and highly reliable devices. The myth goes something like this: By using a battery combiner on AGM batteries and feeding the alternator or battery chargers charging current directly to the house battery bank first "will leave your start battery under charged". If you are practicing good battery management, and have the minimum suggested charge current for an AGM battery, this is a complete non-issue. In 2 minutes of charging, at 20% of Ah capacity from 50% SOC, the AGM battery voltage is already at the "combine level" for amazingly popular Blue Sea ACR. Battery voltage will rise pretty slowly from here but to get to an ACR's "combine level" is relatively quick and easy, especially if you have your system set up properly. The Echo Charger and Duo Chargers also turn on at similar voltages and those devices require all charge sources to be fed to the house bank.
CHARGING - 2 Hour Charge Test .2C - Two Hour Mark
In this image the charge source is about to turn off when it hits the 2 hour mark. Net accepted current at 14.4V @ 1:58 is down to 11.6A.
DISCHARGING - 2 Hour Charge/Discharge Test .2C - Two Minute Mark
Here the electronic load has turned on and has now begun discharging the battery back down to 50% SOC. I did not pause and let the battery rest between charging & discharging because this is not actually done on cruising boats.
The object with this test is to see how much usable energy we can store in the battery in two hours at a.2C charge rate..
STORED ENERGY - 2 Hour .2C Charge/Discharge Test
After charging the battery from 50% SOC for two hours at 21A we were able to remove 35.28Ah's and bring the battery back down to 50% SOC.
Let's do the math:
Baseline Ah Capacity = 95.69Ah
Discharge to 50% = 47.84Ah (left in the battery after discharge)
2 Hour charge then discharged and counted Ah's delivered back to 50% SOC = 35.28Ah
47.84Ah + 35.28Ah = 83.12Ah of stored energy
83.12 is 86.9% of the baseline Ah capacity of 95.69Ah's
BOTTOM LINE: The battery achieved approx 87% SOC in two hours at a .2C charge rate
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