Are you running it until the batteries die and the mod no longer works or are you just charging them because the battery indicator is telling you they are dead? You may be getting false readings.
When you install the fully charged cells, what does the mod tell you the voltages are when you turn it off and press & hold power & up buttons?
When you install the fully charged cells, what does the mod tell you the voltages are when you turn it off and press & hold power & up buttons?