Understanding your state of charge estimate
Why resting voltage matters so much
A battery under active charge or discharge shows a voltage that reflects current flow through internal resistance on top of actual charge level, which can throw off a state of charge estimate by a meaningful margin. Waiting for the battery to rest removes that distortion and gets closer to the battery's true open-circuit voltage. Keep in mind this reading is a single snapshot, so if what you actually want is how long that charge will last under your loads, the backup runtime estimator is the more useful next step.
Why LFP is the hardest chemistry to estimate this way
LFP's voltage stays remarkably flat across roughly 20% to 80% state of charge, which means a small voltage difference can represent a large swing in actual charge level in that range. This is the main reason most LFP systems rely on a BMS with coulomb counting rather than voltage alone for accurate state of charge tracking, a limitation NREL's energy storage research has documented as a known challenge specific to iron-phosphate chemistry.
Why lead acid works better with this method
Lead acid's voltage curve slopes more steadily across its state of charge range, giving a clearer relationship between voltage and charge level than lithium chemistries typically offer. That's part of why simple voltage-based meters have remained common and reasonably useful on lead acid systems for decades, an approach covered further in the lithium vs lead acid comparison, and it's exactly the kind of simple voltage meter still common on boats, covered in the marine battery bank sizing calculator.
What to check next
If you're seeing voltage sag under load that seems steeper than expected, the voltage sag under load calculator covers that related but separate effect, and the state of health calculator is the better tool if what you actually want to track is capacity rather than momentary charge level. This same voltage-to-charge estimation challenge is part of how the DOE's Office of Electricity evaluates monitoring approaches for grid-scale storage systems.