Batteries, energy, and protection

A battery converts stored chemical energy into electrical energy. Voltage depends on chemistry, cell count, charge state, temperature, and load. Capacity in ampere-hours describes charge, not energy by itself. A useful first energy estimate is E ≈ V_nominal × capacity in Ah, expressed in watt-hours. Two packs with the same ampere-hour number but different voltages do not store the same energy.

Runtime is not simply the label divided by current when converters and changing loads are involved. Estimate load energy, include conversion efficiency, and check peak current separately. A 3.7 V nominal, 2 Ah lithium-ion pack holds about 7.4 Wh by this approximation. If a regulated system consumes 3 W and averages 85% conversion efficiency, modeled runtime is 7.4 Wh × 0.85 / 3 W ≈ 2.1 h. Real capacity, cutoff, aging, temperature, and duty cycle alter the result.

Lithium-ion and lithium-polymer cells require compatible charge control and protection against overcharge, over-discharge, short circuit, and excessive current. A protection board is not automatically a complete charger, and neither makes an unknown cell safe. Mechanical damage, swelling, heat, or a dubious source are stop conditions. For a desk robot, an enclosed protected pack or a reputable USB power bank is often a better introductory choice than loose pouch cells.

A common misconception is that cells may be placed in parallel merely because their labels match. Differences in voltage, age, and internal resistance can cause large equalization currents; pack construction needs suitable matched cells and engineering controls. Another is assuming a fuse protects against every battery hazard. Protection layers address different faults. Select a source only after checking voltage range, continuous and startup current, energy, connector polarity, charger compatibility, mounting, and end-of-discharge behavior. The power-budget model remains an estimate, not battery certification.