For battery applications below 6V increase the capacitance of C1.
Currently C1 is a 4.7uF 0805 ceramic capacitor. It is on the 2.5V regulated supply for the microprocessor.
My recommendation if height is not a problem is to remove C1 and put it on top of C2 which would double the input supply capacitance and put a 22uF 0805 6.3V X5R capacitor for C1.
Symptons are the UI doesn't work when wired up properly with single cell rechargeble Li-Ion batteries like 18650 applications.