Formula: runtime (h) = capacity (mAh) × depth-of-discharge × efficiency ÷ load current (mA). Power-mode load current is derived as I = P ÷ V_nominal. Depth of discharge accounts for chemistries that shouldn't be fully drained (e.g. keep Li-ion above ~20% / 3.0V per cell for longevity); efficiency is a catch-all derating for DC-DC converter losses, voltage sag under load, and the Peukert effect (real capacity drops at high discharge rates). Defaults (85%/85%) are reasonable starting points — real numbers vary by cell and circuit.