[Mr V]

There are two common methods of determining the SOC of a lead acid battery. The most accurate is to measure the specific gravity of the electrolyte, unfortunately with a sealed AGM battery that is not possible. The other method is the resting voltage, that means a battery that has not recently been charged or had a load applied to it. The definition of recently varies depending on who you ask. Some say as little as 15min while others say it should be 24hrs. Most do agree on a fairly narrow range of around 11.8v as a 0% SOC. I'm pretty certain that the battery beak determines the SOC based on voltage particularly since the instructions I saw stated the battery needed to be "at rest" for an accurate reading.