The habit every lead-acid driver picks up, glance at the resting voltage to judge how full the pack is, quietly stops working on lithium iron phosphate. It stops working across the broad middle of the range, right where a straight answer would help.
An iron phosphate cell holds its voltage almost level for a long stretch of its discharge. From the high end down to nearly empty, the terminal voltage barely drifts, tracing a plateau rather than a slope. The energy is leaving the pack steadily, but the one number a simple gauge reads is sitting nearly still.
A few hundredths of a volt can cover a wide band of remaining charge.
Lead-acid behaves the opposite way, which is exactly why the voltage trick grew up around it in the first place. Its voltage falls in a fairly steady slope as it empties, so reading the terminal gives a rough but usable sense of fuel left.
On lithium the same gauge tells a comforting lie, reading near full through the bulk of the shift and then dropping through the floor in the last stretch with little warning.
