Section 3 · Topic 9
Capacity Metrics and Energy Delivery
Two quantities describe stored charge, and conflating them is a sizing error that surfaces only when the runtime falls short.
Capacity versus energy
Capacity in ampere-hours is the integral of current over time — the raw charge the cell can pass. Energy in watt-hours multiplies that charge by the voltage at which it is delivered — the actual work available. A 3.7 V cell rated 2 Ah holds 7.4 Wh, enough in principle to run a 1 W load for 7.4 hours. Two cells of equal capacity but different nominal voltage store different energy, and a system runs on energy.
Nominal versus usable
Datasheet capacity is not the capacity an engineer can use. Manufacturers list a typical nominal capacity and, separately, a lower minimum guaranteed value; pack sizing uses the minimum. Above that, usable capacity is bounded by the voltage cutoffs the BMS enforces, set inside the cell’s absolute limits to protect cycle life.
Voltage sag and the C-rate penalty
High discharge current reduces delivered capacity, but not through the true Peukert loss of lead-acid. The cell’s internal resistance produces a voltage drop proportional to current, so a cell under heavy load reaches its lower cutoff at a higher true state of charge — the BMS terminates discharge before the chemistry is exhausted.
Once the load is removed the voltage rebounds and the remaining capacity is still chemically present, merely inaccessible under that load. This is a voltage-sag and cutoff phenomenon, weakly rate-dependent in lithium-ion, rather than the steep capacity loss Peukert’s law describes for lead-acid.
Cold does not increase capacity. Low temperature raises viscosity and slows diffusion, increasing resistance and deepening the sag — stranding capacity below the cutoff until the cell warms and recovers it. High temperature does the reverse transiently (faster kinetics, more accessible capacity) while accelerating the irreversible reactions that permanently destroy the cell.
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