Lesson S3 · Special topics

How batteries work

A battery turns chemical energy into electrical energy. Open up a lithium-ion cell and watch lithium ions cross the electrolyte inside while electrons take the long way round through a lamp. Then read the numbers on a real cell (about 3.7 V, 3000 mAh, 11.1 Wh, C-rate), charge it with constant current and then constant voltage, see why heat and overcharging are dangerous, and join cells into series and parallel packs.

12 min11 steps5 questionsIntermediate

Fig S3 · Interactive

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5 quick questions

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01While a lithium-ion cell powers a lamp, how do the electrons get from the anode to the cathode?

The electrolyte lets ions through but blocks electrons, so the electrons must take the outside path. That flow through the wire and the lamp is the current that does the work.

02A cell is rated 3.7 V and 3000 mAh. About how much energy does it store?

Energy in watt-hours is voltage times capacity in amp-hours: 3.7 V × 3.0 Ah ≈ 11.1 Wh.

03What current is 2C for a 3000 mAh cell, and roughly how long would a full cell last at that rate?

1C is the current that empties a full cell in about an hour: 3.0 A here. 2C is twice that, 6.0 A, so it lasts about half as long.

04During lithium-ion charging, what happens once the cell reaches 4.2 V?

Charging is constant current first, then constant voltage at 4.2 V. In the constant-voltage stage the current falls, and charging ends when it drops to a few percent of 1C. Lithium-ion takes no trickle charge.

05Two 3.7 V, 3000 mAh cells are joined in series. What does the pack give?

In series (+ to −) the voltages add and the capacity stays the same: 7.4 V and 3000 mAh. In parallel the capacities would add instead. Either way the energy doubles.

Key terms

Glossary

Anode: The negative electrode of a cell while it discharges. In a lithium-ion cell it is graphite that holds the lithium when the cell is charged.
Cathode: The positive electrode while the cell discharges. In a lithium-ion cell it is a lithium metal oxide that takes the lithium back in.
Electrolyte: The material between the electrodes that lets ions pass but blocks electrons, so the electrons must flow through the outside circuit.
Ion: An atom with an electric charge because it has lost or gained electrons. A lithium ion (Li+) has lost one electron.
Capacity (mAh): How much charge a cell holds. 3000 mAh means 3000 milliamps for one hour, or 300 milliamps for ten hours.
Watt-hour (Wh): A unit of energy: volts times amp-hours. A 3.7 V, 3 Ah cell stores about 11.1 Wh.
C-rate: Current compared with capacity. 1C would empty a full cell in about an hour; 2C in about half an hour.
Thermal runaway: A loop in which heat speeds up the cell's reactions, which make more heat. It can end in fire, which is why packs carry protection circuits.

Fine print

Notes and sources

The cell is a teaching model: 3000 mAh, 0.1 Ω inside and an illustrative voltage curve between 3.0 V empty and 4.2 V full. Real cells vary with their chemistry and maker.
Lithium-ion nominal voltage is 3.6 V (some makers quote 3.7 V) and full charge is 4.20 V; an alkaline cell is 1.5 V (Battery University BU-303).
Charging is constant current, then constant voltage at 4.20 V, ending when the current falls to 3 to 5 percent of the Ah rating; lithium-ion takes no trickle charge (Battery University BU-409).
The dots on the wires are electrons, which flow from − to + outside the cell. Conventional current is drawn the other way.
The protection limits in the code example are illustrative; each protection chip and cell maker sets its own.

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