AGM Battery - Residual Voltage?

An AGM battery is a lead acid battery, it's best to not go too far down the rabbit hole of comparing a regular liquid electrolyte battery to AGM, there's no practical difference when you are measuring voltage/state of charge. There is tons of misinformation floating around the web once you start reading about AGM's.

Measuring the voltage of a battery only gives you an approximation of the state of charge, and it is not reliable if the battery is under load. A battery in a modern car is always under fairly significant load, with the various computers and wireless systems on standby. This is really important, as long as you are measuring voltage while under load, you aren't getting an useful window into state of charge.

To get an accurate sense of your batteries condition, go for a decent drive (to fully charge the battery) then disconnect the negative terminal shortly after parking. Leave it disconnected for at least 20 minutes, preferably longer, then measure the voltage. It should be holding above 12.7 fully charged with no load. At this point you could compare your reading to charts that indicate state of charge by voltage, but ultimately if it's not holding a full charge without load, it's no longer in good condition.

Be sure to check your owners manual or otherwise read up on the ramifications of disconnecting your battery if you don't know them offhand. You will probably need one or more codes to re-enable your radio and other electronic in-dash, alarm, etc.