Batteries, energy, and protection

• Compare low-voltage battery options using energy and current requirements, and explain the roles of charging, cutoff, and fault protection.

• power-regulation

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.

Problem

A protected 7.4 V nominal, 2.5 Ah pack powers a robot averaging 8 W through a converter at 88% efficiency. Estimate energy delivered to loads and runtime.

Steps

1. Nominal pack energy is 7.4 V × 2.5 Ah = 18.5 Wh.
2. Modeled delivered energy is 18.5 Wh × 0.88 = 16.28 Wh.
3. Runtime is 16.28 Wh / 8 W = 2.04 h.
4. Treat 2.04 h as a planning estimate before cutoff, peak-current, temperature, and aging tests.

Result

The simple model predicts about 16.3 Wh delivered and roughly 2.0 h runtime.

Sanity check

At perfect efficiency runtime would be 18.5/8 = 2.31 h, so the loss-adjusted result must be lower.

Which statement best explains why a 3 Ah battery might still fail to start a motor?

A. Capacity does not guarantee the required peak current without excessive voltage sag or protection cutoff B. Ampere-hours are exactly the same as amperes C. A larger capacity always means lower voltage D. Motor startup consumes no current

Your answer: ______________________________

Reasoning:

Goal

Compare label-based runtime estimates with a safe external discharge observation from a protected USB power bank.

Equipment

• intact USB power bank
• USB inline energy meter
• USB LED light or fan
• timer
• manufacturer label or documentation
• notes sheet

Procedure

1. Record advertised capacity, the voltage basis if stated, output rating, and every conversion assumption; do not open the pack.
2. Measure actual output voltage, current, and power for ten minutes with the chosen load and calculate energy from average power × elapsed hours.
3. Extrapolate runtime as an explicitly provisional estimate and compare it with a label-based watt-hour estimate.
4. Deliberately disconnect and loosely reconnect only the external USB load cable to create a visible open-circuit fault; diagnose zero load current while pack voltage may remain available.
5. Reconnect firmly and confirm current returns; document any automatic shutoff behavior as an observation, not a defect assumption.

What to observe

• Expected: meter energy increases approximately with elapsed time under a steady load; actual power and any drift must be recorded.
• Expected fault clue: an open external load produces near-zero current, which is distinct from an empty pack or low-voltage cutoff.

Transfer challenge

Compare a reputable USB power bank and a protected rechargeable pack for a camera robot using voltage, peak current, energy, charging workflow, mounting, and fault containment.

Cautions

• Do not open, puncture, bend, short, modify, or bypass protection on any cell or power bank.
• Stop using and isolate from combustibles according to manufacturer guidance if a battery is swollen, damaged, leaking, unusually hot, or odorous; do not perform the lab with it.

Learner: ____________________ Date: ____________________

Evidence artifact or file: ____________________________________________

Notes: