Everybody calls that thing a battery monitor. Most people think it really is. Congratulations on being smart enough to realize it's not. More people should understand this. That device is more properly called a 12v system voltage gauge.
An actual battery gauge would involve a shunt placed into the main negative line at the battery. The shunt would measure current into and out of the battery over time. You program it to know what size your battery is and then it displays state of charge as a percentage.
These do exist, they cost a couple hundred bucks. Victron, Magnum, Outback, and other make them.
