AGM Charging Voltages?

I installed one of those cigarette plug voltage monitors over a year ago to keep an eye on my 6+ yr old AGM battery voltage, in case it starts to drop off abruptly. I normally see it sitting around 12.4 to 12.8V before the car powers up (depending on what accessory/lights are fired up). My Main concern is the voltage when the inverter /charger kicks in- I routinely see 14.4 up to 15V on my meter ( and I would expect some losses from the battery to the cigarette plug point). Literature from Battery University says 2.25 to 2.3V per cell ( 13.5-13.8 V) is the optimal charging voltage for AGM, and other sites say the float voltage should be as low as 13.4V. I don't think the meter is that far off, since the 'sitting' voltage looks about right, if not on the low side. Do other owners see similar behavior?

With a voltmeter on the 12v battery posts (under the rear deck lid), if I unlock the car or open a door, the voltage drops from 12.9 to 12.54 because there is a drain on the 12v battery. This is also the measurement I get from the cigarette lighter/power plugs while they are active.
This continues for exactly 10 minutes until the 12v accessories shut off.

Then the voltmeter attached to the battery posts shows my 12v battery slowly going back up to 12.9v over a period of 2 minutes.

If I do the same thing but press the button to put the car in "Ready" state, the voltmeter shows 14.95v going to the 12v battery. This is what it reads when the HV dc-dc is apparently charging my battery.

14.5v is a number I'm used to seeing, and a quick search I found this from Century Batteries:

"AGM batteries should be charged using a charger
with a mode switch for AGM type batteries. It is recommended to use
a charger, current limited to 20% or 0.2C of the batteries capacity at a
charging voltage of between 14.6 – 14.8V following deep cycle use, or
13.6V – 13.8V for standby use. Charging should be paused if the battery
or ambient temperature exceeds 50°C."

I'm glad you guys are doing this as I thought this monitoring was spoiled by opening a door. I've read that 11.9v resting voltage is considered the end-of-life for AGM, so I'd love to see how close you get. I'll begin doing this myself soon for my 2017.

I have a new AUX 18L battery from East Penn sitting on my workbench - fully charged (with my charger's AGM setting), reading 100%. Shows 12.63 volts.

Have it ready for installation in my 2015 which is still running with the original 12v battery.

Mark, what voltage does your charger charge at in the AGM setting?

BTW I was browsing battery monitors and found a Bluetooth enabled unit that sends alerts if certain conditions are met. For $40, I'll give it a go.

https://www.amazon.com/dp/B07WCW49YM/

I also have an EAST PENN AUX18L on my bench that I bought new last month.
When I received it, the voltage was 12.5v

After overnight charging on my regular charger that puts out about 13.8 v I let it sit for a few hours and it read 13.06
Stayed that way for several days. Just checked it now about a week after taking it off the charger and it reads 12.96v

Edited to add: The 12.9v reading of the new battery on my bench matches the reading on the original battery that is still in my 2017 BEV. That makes me question how a reading of 12.63v for Mark’s new battery would be 100%.

Here is the reading of the battery:
View attachment Battery.jpg

My charger (Diehard Gold) will charge at 12 amps or 30 amps

I Just checked the newly installed battery (date mark 10/20) on my i3 this morning 62F temp.
float voltage - 14.4
charging voltage - 14.55

AGM Charging table shows (for a rollsbattery vrla agm battery) :

50F - 15.06V charge voltage; 14.16V float voltage
68F - 14.84 charge voltage; 13.94V float voltage

I'm guessing that the bmw i3 battery has a minor different characteristic than the rollsbattery


see link below:
http://support.rollsbattery.com/support/solutions/articles/4345-agm-charging