Just to Be Annoying - What is a "Monitor"?
Volt meter: Shows the voltage of the battery. Accurate when the battery is at rest, that is about 30 minutes AFTER all charges and loads are removed. Will read high when the battery is on charge and low when there is a serious load on the battery. A resting battery and a chart will give you a guesstimate of the state of charge.
Ammeter: Shows amp flow in or out of the battery. Interesting for judging the power of your charging system and the size of your loads. Does not show state of charge.
True Battery Monitor: Most are "hour counters" that measure amp flow across a shunt and then try to calculate the state of charge from a known battery size, estimated battery condition, etc. One big exception is the "SmartGauge" which does the same thing without a shunt. It measures voltage and uses a mathematical model to compute state of charge.