Here is an example of using voltage and trying to gauge SOC.
After my batteries charge to 14.1v, I'll float at 13.6v with solar. When the controller goes to sleep the only 12v loads running are the co/lp detector, inside fridge fans, little draw from my Senseboards on the batteries and the furnace when cycling through the night.
The lfp voltage curve is so flat that even at 91% SOC this morning at 0545 (solar controller still in sleep mode) batteries reading 13.32v when the furnace kicked on voltage dropped to 13.30v
Resting voltage for lfp is 13.2- 13.3v.
Best to have away to count amps going in and out. ;)