Originally Posted by markopolo
The monitor is likely meant for use with lead acid batteries and not for use with lithium batteries. A lead acid battery at 12.8V would be seen as fully charged (100%) by a basic monitor but a LiFePO4 (lithium) battery at 12.8V (at rest) probably only has about 20% of its charge left.
As markopolo stated, the percentage SOC (State Of Charge) reading you see is based on the voltage levels of a lead acid battery (Wet Cell or AGM) to calculate the SOC, which are much lower than the voltage levels for Lithium batteries.
An AGM centric battery meter will show a Lithium battery to be at 100% charge pretty much 100% of the time. This is true of most battery meters meant for automotive use on the market today. They assume you have a standard automotive style Lead-Acid battery, Including meters mentioned in this thread that give a percentage amount for the SOC or display a Red, Yellow or Green LED to indicated battery status.
Best to just read the actual voltage level number, ie; 12.9vdc, and compare it to the chart below for a Lithium based battery's SOC.
I have a Roadtrek CS Adventurous with four EcoTrek 200 Lithium batteries and have found the chart to be close enough for determining the SOC of my Lithium based battery bank... I believe it will be pretty close to you voltage levels also for a close enough SOC...