Batteries, Storage and Power Sources
This domain tests primary and secondary cells, lead-acid and lithium-ion batteries, charging, capacity, series and parallel connections, and safety. The exam expects correct use of Ah, voltage, C-rate and energy calculations.
Core rules
- A primary cell is not rechargeable. A secondary cell, such as lead-acid or lithium-ion, is rechargeable.
- Battery capacity is rated in ampere-hours (Ah). Energy in watt-hours is Ah × voltage.
- Cells in series add voltage while capacity stays the same. Cells in parallel add capacity while voltage stays the same.
- Charging a lead-acid battery produces hydrogen gas. Keep the area ventilated and avoid sparks.
- Lithium-ion batteries need a compatible charger and protection against overcharge, over-discharge and short circuit.
Worked example
A 12 V, 100 Ah battery supplies a 5 A load. Ideal run time = 100/5 = 20 hours. At 80% usable capacity, practical run time is about 20 × 0.8 = 16 hours. The stored energy is 12 × 100 = 1,200 Wh, or 1.2 kWh.
Traps
Do not treat Ah as energy without multiplying by voltage. Do not add voltages when cells are paralleled. Do not charge a lead-acid battery in a sealed, spark-prone space. Do not reverse-charge a lithium-ion cell. Do not ignore the difference between ideal and usable capacity.