A generator converts motion into electrical energy

A generator does not create energy. It takes mechanical work at a shaft and delivers electrical energy to a circuit, with some energy becoming heat along the way.

A turbine is a rotating machine driven by a moving fluid. Steam, flowing water, wind, or hot combustion gases can turn its blades. The turbine turns the generator; the generator supplies electricity. An engine can also drive a generator without a turbine.

Supporting detail: Addendum 1 — How rotation produces electricity.

See Power and Efficiency for losses and AC and DC for the electrical output forms.

Different heat sources can feed the same steam cycle

Burning fossil fuels or biomass can heat water to make steam. Nuclear fission supplies heat without combustion. Geothermal systems use heat from the Earth, and concentrated solar systems use mirrors to collect sunlight as heat. The equipment differs, but a steam-based plant follows the same basic chain: heat, moving steam, shaft rotation, electricity.

Not every turbine uses steam. A hydroelectric turbine takes energy from water flowing downhill. A wind turbine takes energy from moving air. A combustion turbine uses hot gases directly; a combined-cycle plant also recovers exhaust heat for a steam cycle.

Photovoltaic cells skip the turbine

A photovoltaic (PV) cell converts light directly into electrical energy through the photovoltaic effect in a semiconductor. A connected load receives DC electricity. An inverter can convert that DC to AC.

Solar PV and concentrated solar thermal are different routes. PV produces electricity in the cell; solar thermal first collects heat. That distinction matters when considering thermal storage.

A generation mix is more than a list of percentages

A generation mix describes how much electricity comes from each source over a stated place and time. Installed capacity describes how much power equipment is rated to supply. Those are different measures: a large rated capacity does not mean a source supplied the same share of annual energy.

Compare cost, emissions, variability, and dispatchability — the ability to adjust output when requested. Wind and sunlight vary with weather and time. Fuelled generation and reservoir hydro can often be scheduled, but fuel, water, maintenance and operating limits still matter. Storage shifts energy in time; it is not an extra primary energy source.

No current percentage mix is supplied here. For dated U.S. generation data, consult the U.S. EIA's Electricity explained: electricity in the United States, and record the reporting year and whether the figures describe generation or capacity.

Pumped hydro stores energy by lifting water

Use electricity to pump water uphill, then let it flow downhill through a turbine and generator when electricity is needed. The stored energy is gravitational potential energy.

The cycle returns less electrical energy than it consumed because pumping, flow and generation all have losses. It needs suitable elevations, reservoirs and water management; it cannot be placed wherever a battery can.

For the energy and efficiency relationships, see Work and Energy and Power and Efficiency.

Thermal storage saves heat for later

Thermal storage holds energy in a hot material, such as a fluid or solid. A concentrated solar plant can collect heat while sunlight is available, store it, and later use it to produce steam and electricity. Stored heat can also serve heating loads directly.

Insulation slows heat loss; it does not eliminate it. Returning heat to electricity adds conversion losses. Choose the storage route for the energy form the load needs.

Rechargeable batteries store energy chemically

A battery supplies DC through electrochemical reactions. A primary battery is intended for a single discharge life and replacement. A secondary battery is designed to be recharged with a suitable charger, restoring chemical energy for another discharge.

Primary cells are useful portable energy supplies, but they are not reusable storage for electricity from a generator. Do not try to recharge a primary cell. Rechargeable does not mean unlimited life: cycling and aging reduce usable capacity.

See Battery Chemistries for the families and Battery Capacity and Packs for the arithmetic.

Choose storage for the load and the site

A small portable instrument needs a compact energy supply, so a battery is a practical fit. A site with suitable upper and lower reservoirs can consider pumped hydro for large stationary storage. A solar thermal plant needing heat after sunset can store heat without first converting it to electricity.

Compare the required power, the energy needed over time, the storage duration, available space and terrain, losses, and service life. These examples identify a good fit; they do not establish that one technology is always cheapest.

Capacity, self-discharge and aging are different

A cell is one electrochemical unit. A battery may contain one cell or several. Its chemistry helps determine voltage, mass, charging needs and how well it holds charge.

Self-discharge is loss of stored charge while no load is connected. Aging is loss of the ability to hold or deliver charge, even after recharging. Both depend on the cell design, temperature and storage conditions, so a chemistry name is not a shelf-life specification.

A capacity rating also depends on discharge current, temperature and the voltage at which the test stops. Use the particular cell's data, not a single capacity assigned to every AA or D cell.

Primary alkaline cells suit replaceable, modest loads

Common primary alkaline cells serve devices such as clocks, remote controls and flashlights. They are widely available and retain charge relatively well in storage. Their useful capacity and terminal voltage fall more under heavy loads and cold conditions.

They are not intended to be recharged. Leakage can damage equipment, especially when cells are exhausted or left in storage. Rechargeable-battery memory effect is not a useful concern for a primary cell.

NiCd and NiMH are rechargeable nickel batteries

Nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) use alkaline electrolytes, but they should not be confused with primary alkaline cells.

Use a charger designed for the chemistry and cell. Fast charging is appropriate only when the cell and charger support it. Do not deliberately drive a pack to zero volts to prevent memory effect: a weaker cell can be forced into reverse polarity by the others.

Supporting detail: Addendum 2 — Why a remote-control recommendation needs more than a chemistry name.

Lead-acid trades low mass efficiency for useful power and cost

Lead-acid batteries appear in engine starting and stationary backup supplies. They can deliver high starting currents and are relatively inexpensive, but they are heavy for the energy stored.

They self-discharge during storage. Prolonged storage while discharged promotes sulfation, which can reduce capacity; this is not the NiCd memory effect. Starting and deep-cycle designs serve different loads. Use the specified charging method: the electrolyte is acidic, lead is toxic, and charging can release gas. Flooded, absorbed-glass-mat and gel designs all exist; modern lead-acid does not always mean gel.

Lithium-ion needs protection; LiPo is not automatically better

Lithium-ion batteries power portable electronics, tools and electric vehicles. Their energy per unit mass is generally considerably higher than NiMH, with relatively slow self-discharge and no routine need for full discharge to prevent memory effect. They still lose capacity through calendar aging and cycling.

Protection and charge control must keep voltage, current and temperature within the cell's limits. A bare cell may not contain its own protection circuit. The pack design must provide the required protection; a protection circuit is not a substitute for a compatible charger.

Lithium-ion polymer, often called LiPo, commonly appears as a light pouch cell in portable devices and model aircraft. The packaging allows thin shapes; some designs support high discharge rates. It does not guarantee higher energy density, lower cost or better resistance to physical damage. Pouches need protection against puncture and crushing. Self-discharge and memory behaviour are broadly those of lithium-ion.

Nominal voltage and charging limits depend on the particular chemistry and cell. Do not use a single voltage for every lithium-ion battery, and do not confuse rechargeable lithium-ion with primary lithium-metal cells.

Battery history moves from a pile to portable rechargeable cells

The important sequence is the change in what batteries made possible.

• Volta, around 1800: the voltaic pile made a sustained electrochemical source practical to demonstrate.
• Gaston Planté, 1850s: lead-acid introduced a rechargeable battery.
• Carl Gassner, 1880s: dry-cell development made the electrolyte easier to contain for portable use. “Dry” means immobilized electrolyte, not absence of moisture.
• Waldemar Jungner, 1890s: nickel-cadmium added another rechargeable chemistry.
• Lewis Urry, 1950s: work on the modern alkaline cell improved portable primary batteries.
• Lithium-ion, 1990s: commercialization enabled rechargeable products with high energy per unit mass, building on earlier work by several researchers.

Supporting detail: Addendum 3 — Scope of this short chronology.

Ampere-hours measure charge; watt-hours measure energy

A 2 Ah cell and a 2 Ah battery can store very different amounts of energy if their voltages differ. Ah tells you charge. To estimate energy, you also need voltage.

Energy (Wh) ≈ capacity (Ah) × nominal voltage (V)
Energy (J) = energy (Wh) × 3600

For an ideal 2.4 Ah, 3.6 V cell: 2.4 × 3.6 = 8.64 Wh, or 8.64 × 3600 = 31,104 J. Convert mAh to Ah by dividing by 1000 before multiplying by voltage.

Supporting detail: Addendum 4 — Why the battery estimate uses an approximate sign.

See Work and Energy for energy units and Current, Voltage, and Resistance for voltage and charge.

C-rate sets a current scale; load current sets ideal runtime

A discharge rate of 1C means a current numerically equal to the capacity in Ah divided by one hour. The rate has units of h−1. Here C is a rate notation, not the coulomb symbol.

Rated maximum current (A) ≈ maximum discharge C-rate (h⁻¹) × capacity (Ah)
Ideal runtime (h) = capacity (Ah) ÷ load current (A)

A 2.4 Ah cell rated for continuous 3C discharge has a rated current estimate of 3 × 2.4 = 7.2 A. At a constant 1.2 A load, ideal runtime is 2.4 ÷ 1.2 = 2 h. At 3C, the ideal duration is one-third of an hour, or 20 minutes.

A C-rate may describe a test rate, a charge rate, a continuous limit or a short pulse limit. Only a specified maximum discharge rating belongs in the maximum-current equation. Actual pack limits may be lower because of wiring, temperature or protection hardware. A constant-power load also changes current as voltage changes.

Series adds voltage; parallel adds charge capacity

For matched cells, connect cells in series to increase voltage. Put identical series strings in parallel to increase Ah at the same voltage. 3s2p means three cells per series string and two strings in parallel: six cells altogether.

Pack V = series count × cell V
Pack Ah = parallel count × cell Ah
Pack Wh ≈ pack V × pack Ah

With 3.6 V, 2.4 Ah cells, a 3s2p pack is 10.8 V, 4.8 Ah and 51.84 Wh. At 3C it has an ideal rated-current estimate of 14.4 A. A 1.2 A pack load gives 4 h ideal runtime.

This is an electrical model, not a pack assembly procedure. It assumes matched cells, balanced series operation and equal current sharing between parallel strings. Real packs need appropriate cell matching, interconnections and protection. See Series and Parallel Circuits.