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