Baseline Discharge 49.3% SOC
PREFACE: This article looks at the charging times & achieved state of charge of an AGM battery in multiple scenarios:
1- 50% SOC to 100% SOC at .2C
2- 50% SOC to 100% SOC at .4C
3- 50% SOC Charged at .2C For Exactly 1 Hour
4- 50% SOC Charged at .4C For Exactly 1 Hour
5- 50% SOC Charged at .2C For Exactly 2 Hours
6- 50% SOC Charged at .4C For Exactly 2 Hours
*DISCLAIMER: This test represents this slightly used AGM and may not be representative of a brand new AGM or a battery with more use and in a worse state of health.
.2C & .4C - Also called Point Two C and Point 4 C. This is just a percentage of Ah capacity in either charging or discharging current. For example .2C = 20% of Ah capacity or 20A for a 100Ah battery and .4C = 40% of Ah capacity or a 40A charge or discharge current for a 100Ah battery.
SOC - Sate of Charge
DOD - Depth of Discharge
Ah Capacity - The total ampere hours a battery can store and deliver at a discharge rate that yields a 20 hour run time at 77F before hitting a terminal voltage of 10.5V.
20 Hour Discharge Rate - Also the "20 hour rate". The discharge current at 77F that will yield a 20 hour run time before hitting 10.5V
How long does it take to charge from 50% SOC to 100% SOC?
I have long known that a higher charge rate, with AGM batteries, does not necessarily translate to *significantly faster charge times from 50% SOC to 100% SOC. For the first part of this test, 50% SOC to 100% SOC, I compared a charge rate of .2C with a charge rate of .4C on the same Lifeline GPL-31T battery.
"Significantly Faster" - Please understand, & take with a grain of salt, what I am comparing "significantly faster" to. I was once told by a self espoused expert on AGM batteries (expert defined here as an owner who had AGM batteries and whom had read the glossy marketing materials) that with his 150A alternator he could recharge his 440Ah bank of AGM batteries from 50% to 100% in less than an hour and a half. While the owner was fully capable of basic math, 1.5 hours times 150A = 225Ah's he failed to take into consideration that his .34C 150A alternator could not put out 150A for 1.5 hours and his bank, at anywhere close to .34C, would hit absorption voltage rapidly and begin limiting alternator current. Even if the alt could run at at a face value of 150A it is simply impossible to return 225Ah's to the bank in 1.5 hours from 50% SOC.
One can't really can't blame this owner for his rather misguided expertise because he had been rather grossly mislead by nearly everyone in the industry on the expected charge times from 50% SOC to 100% SOC with AGM batteries. Suffice it to say it is physically impossible to recharge a 440Ah bank of AGM batteries at 50% SOC to 100% SOC in 1.5 hours with a .34C charge current.
The actual tested capacity of the group 31 AGM battery was 95.69Ah's, its rated Ah capacity is 105Ah. For charge and discharge rates I kept them based on the "as new" capacity just as many boaters would do.
Charge Rate Explained:
0.2C = 20% Charge Rate of the Ah Capacity
0.4C = 40% Charge Rate of the Ah Capacity
In this image we have removed 48.54Ah from the battery which tested at 95.69Ah's of actual capacity. This leaves the battery at 49.3% SOC or just about 50% SOC. Voltage was used as the cut-off and 12.100V at 5.25A was the stop point for the discharge testing.
The Test Battery:
The battery used for this testing is a 2015 Lifeline Group 31 AGM Deep Cycle battery. It had been used for approx 5 months as part of a bank on a power boat (trawler type) that cruises Maine and resides on a mooring. Once here in my shop it was cycled to 80% DOD then back to 100% SOC twice. It was then equalized at 15.5V for 4 hours, (Lifeline calls this conditioning). The battery then underwent one 20 hour discharge capacity test and was then cycled to 50% SOC and back to 100% SOC, and another 20 hour capacity test was run. The average of the two 20 hour capacity tests was used as the baseline Ah capacity. The baseline Ah capacity of this battery was 95.69Ah. Throughout testing the ambient temp of the battery averaged about 75.2F. While not 77F, the 75F temp should not have negatively impacted the testing. All discharging was done at a constant current and at the 20 hour original factory rating not the as tested Ah capacity. The battery is rated at 105Ah so -5.25A was the discharge rate used. The test was set up to try and simulate real world use on cruising boats.
Charging - BK Precision 60A Variable Power Supply W/Dedicated Voltage Sensing
Discharging - Array DC Programmable DC Electronic Load W/Dedicated Voltage Sensing
Temperature - Fluke Infrared
Data Logging - Bogart Engineering Pentametric
Images - iPad Air with intervalometer software taking 1 picture every 2 minutes
2 Minutes - 50% SOC to 100% SOC .4C Charge Test
For the .4C test the timer was reset to 12:00 and an image was snapped using an intervalometer at 2 minute increments. At 2 minutes the voltage at .4C had already risen to 13.6V.
18 Minutes @ .4C Approaching Absorption Voltage
This shot shows the charger almost ready to exit bulk charging and enter absorption charging.
The term "bulk voltage" may sound fancy & technical but there really is no such thing as a "bulk voltage" because bulk is constant current charging. You can refer to a "bulk transition voltage" (also the absorption transition) where the charger ceases being in constant current but once voltage is held steady or becomes "voltage limited" the charger is no longer in BULK it is in constant voltage not constant current....
If you look closely you can see a little red light next to the letters CC. This means constant current or BULK charging.
Bulk charging is the most efficient part of the recharging process with efficiencies ranging close to 100% of energy delivered by the charger being usable and stored. Once the process switches from constant current/bulk to constant voltage/absorption charge efficiency continues dropping. In the high 90's we are converting a lot less of the supplied energy into stored energy.
Bulk & Absorption
If you read the highlighted part of the Lifeline Battery Technical Manual you will see that Lifeline correctly understands the difference between BULK and ABSORPTION charging. Many companies, including Balmar and many others, do not correctly understand using the word "bulk" correctly. When a company tells you that "bulk" is a voltage limited stage of charging this is PURE MARKETING BOVINE DUNG. Simply put bulk=constant current charging not constant voltage.
BULK - Bulk Charging is the constant current stage of charging where the charge source is limited only by what it can deliver in current. Bulk charging is not a voltage limited stage of charging despite many companies bastardizing the term bulk for apparent marketing purposes.
ABSORPTION - Absorption, float and equalization are all examples of constant voltage charging stages. Absorption or constant voltage is where the charge source holds voltage steady, hence the term "constant voltage". Once voltage is held steady, or it becomes voltage limited, current begins to decline and the charge efficiency worsens. Float charging is a further reduction in the constant voltage limit and an equalization voltage would be a further increase in the constant voltage limit of charging.
The charging devices we use on boats are all considered CC > CV charge sources, or constant current then to constant voltage. Simple stuff really. Please understand that BULK is not a voltage limited stage of charging, despite the marketing guru's vastly missing the mark on this one...
Kudo's to the guys at Lifeline Battery for properly understanding the difference between BULK and ABSORPTION. Shame on companies like Balmar for grossly misleading the consumer.. Words mean certain things and when we blur the definitions to sell $hit, it CONFUSES PEOPLE. (grin)
Let's take a look at a Balmar regulator and use their terminology then a more accurate terminology:
Balmar: Bulk, Bulk Voltage, Absorption Voltage, Float Voltage
What these really are: CC, CV, CV, CV
What it should read: Bulk/CC, Pre-Absorption/CV, Absorption/CV, Float/CV
Please remember that bulk is not a voltage limited stage of charging it is constant current.
20 Minutes @ .4C Absorption Voltage Attained
Based on this image, and the data logger, I know the battery attained 14.4V at around 19 minutes. Here at 20 minutes, with the voltage now held steady at 14.4V (green constant voltage light on) we can see that the charge current has already dropped from 42A to 41.5A or a .5A reduction in accepted charge current into the battery.
If we assume that bulk was 100% efficient, and we will for round numbers, the energy returned to the battery at .4C during bulk charging looks like this:
42A X .32 Hours = 13.44 Ah
If we know the battery had 47.15 Ah left in it, at 49.3% SOC then:
47.15Ah + 13.44Ah = 60.59 Ah stored in the battery
60.59 Ah as a % of 95.69 Ah (tested capacity) = 63.3% SOC
If you're paying attention this battery, when charged at .4C from 49.3% SOC, stayed in BULK/CC for 19 minutes and entered ABSORPTION/CV charging, where current begins declining, at 63.3% SOC.
This clearly shows how charge current affects the SOC transition point from bulk to absorption charging.
1 Hour - .4C Recharge
By 1 hour in our 42A / .4C charge current has already declined to 19.5A..
2 Hours -.4C Recharge
By two hours we are down to just 7.3A...
3 Hours - .4C Recharge
Here we are at 3 hours and current is now down to 2.8A.. That last few % takes the longest!
4 Hours - .4C Recharge
By 4 hours in we were down to 1.1A...
100% SOC - .4C Recharge
Here we are finally at 100% SOC. 100% SOC was deemed as 0.525A flowing into the battery at 14.4V per the Lifeline Battery Technical Manual.
At a .4C charge rate it took this fairly healthy, though not perfect, group 31 battery 5:30 to reach 100% SOC from 49.3% SOC.
2 Minutes - .2C Recharge
Beginning of .2C Recharge Test:
For this test 12.100V was also used as the cut off voltage after a full 100% recharge. The battery spent 32 hours of float at 13.4V and then had a 24 hour resting period. The battery delivered -48.33Ah or just about where it had on the previous test which ended at -48.54Ah.
As we can see in this image at 2 minutes the .2C charge rate only has the battery to 13.1V vs. 13.6V at a .4C charge rate. The voltage is climbing slower in bulk at .2C than it did at .4C.
1 Hour - .2C Recharge
At 1 hour the battery voltage has only risen to 13.8V at a .2C charge rate. The .4C charge rate had attained 14.4V at 19 minutes.
If this was your alternator it had better be robust enough to deliver its full output for at least 1 hour straight. In this case, as you see next, 1:16 minutes before it starts to catch a break. A charge rate of .2C on a 450Ah bank, a pretty typical bank on a coastal cruiser these days, is an alternator that can deliver 90A continuously even when hot.
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