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.


DEFINITIONS:


.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.


Testing Tools:
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