What Is a Lithium-Ion Battery and How Does It Work?

Detailed Explanation

Lithium-ion batteries dominate portable electronics, embedded IoT devices, electric vehicles, and grid storage because no other widely available rechargeable chemistry comes close to their combination of energy density, cycle life, and self-discharge rate.

Cell Chemistry and Structure

Every Li-ion cell contains four main components:

During charging, an external current forces lithium ions out of the cathode, through the electrolyte, and into the graphite anode where they intercalate (slot between graphite layers). During discharge, lithium ions move spontaneously back to the cathode — this ion movement generates the electron flow that powers the load through the external circuit.

Voltage Curve

A fully charged Li-ion cell sits at 4.2V (for standard NMC/LCO chemistry). As it discharges, the cell voltage falls along a characteristic curve:

The nominal voltage — the average over a full discharge cycle — is approximately 3.6–3.7V, which is why the label on most Li-ion cells reads 3.7V and battery packs are specified in multiples of that figure (7.4V = 2S, 11.1V = 3S).

Do not discharge below approximately 2.5–3.0V per cell. Copper from the current collector dissolves into the electrolyte at low voltages, permanently degrading the cell and creating internal short-circuit risk on recharge. A protection circuit or BMS must cut off discharge before this threshold.

Common Cathode Chemistry Variants

Different cathode materials tune the trade-off between energy density, power density, temperature range, and cycle life:

LFP deserves a note: its 3.2V nominal voltage is often a surprise — charging to 3.65V rather than 4.2V and discharging to 2.5V. An LFP cell in a system designed for NMC voltage levels will be undercharged or, worse, will have its protection circuit trigger too early. See LFP vs NMC vs NCA: choosing a battery chemistry for a full comparison of energy density, cycle life, thermal stability, and the circuit design implications of each chemistry.

Capacity, mAh, Wh, and C-Rate

Capacity is measured in milliampere-hours (mAh) or watt-hours (Wh):

• 1000 mAh = 1 Ah = a cell that can deliver 1A continuously for 1 hour, or 0.5A for 2 hours, before reaching its cutoff voltage.
• Wh = mAh × nominal voltage / 1000. A 2000 mAh, 3.7V cell stores approximately 7.4 Wh of energy.

C-rate is a normalised current expressed as a fraction or multiple of the cell's capacity:

• 1C means discharging the entire capacity in 1 hour. For a 2000 mAh cell, 1C = 2A.
• 2C = 4A (empties in 30 minutes); 0.5C = 1A (empties in 2 hours); 0.2C = 400 mA (empties in 5 hours).

C-rate matters for two reasons: the cell's internal resistance causes a voltage droop at high discharge rates (a 2000 mAh cell delivering 10A/5C will hit its cutoff voltage sooner than its mAh rating would suggest), and cells have maximum continuous discharge current ratings typically in the range of 1–3C for standard cells and up to 20–30C for high-drain types like 18650 INR cells used in power tools.

For charging, the standard rate is 0.5C–1C for the constant-current phase, with lower rates (0.1C) for top-off trickle charging. Charging faster than 1C increases heat generation and accelerates capacity degradation.

Self-Discharge and Storage

Li-ion self-discharges at approximately 2–3% per month — far lower than NiMH (~15–20%/month) or lead-acid. For long storage (months), the recommended state of charge is 40–60%, not 100% — storing at full charge increases stress on the cathode and accelerates capacity loss.

Design Considerations

• Always pair Li-ion with a protection circuit: overcharge above 4.2V, overdischarge below 2.5V, and overcurrent (short circuit) can all cause irreversible damage or thermal runaway. A protection IC (e.g. DW01A family) and back-to-back FETs costs a few cents and is non-negotiable in any unmonitored design.
• Plan the power conversion: Li-ion's 3.0–4.2V range is rarely what embedded circuits need directly. A buck converter steps it down to 3.3V or 1.8V; a boost converter can step it up to 5V. The LDO vs. switching regulator trade-off applies here — see linear vs switching regulator for the power-budget analysis.
• Temperature limits: standard Li-ion cells charge safely only between 0°C and 45°C. Charging below 0°C causes lithium plating on the anode — a dendrite risk that can short the cell internally. If a design must charge in sub-zero environments, the charging circuit must include temperature monitoring and must reduce or halt charging when the cell is too cold.
• Battery-powered PCB design: designing a complete battery-powered product involves charging circuit topology, protection, power path management, fuel gauging, and low-power rail architecture — a scope Zeus Design's hardware team handles from initial specification through to production BOM and PCB layout.

Common Mistakes

• Confusing cell voltage with pack voltage: a "7.4V Li-ion pack" is two cells in series (2S). Designing a charger or protection circuit for the pack voltage and applying it to a single cell at 3.7V — or vice versa — will overcharge or undercharge the cells.
• Ignoring the discharge cutoff: draining a cell to 0V in a discharged product that sits unused for months will cause copper dissolution and irreversible damage. Even a very low quiescent current leaking through the protection circuit will eventually take a stored device below its safe minimum. Design for very low standby current and consider a deep-sleep voltage lockout.
• Using C-rate specifications from maximum-drain cells in standard applications: high-drain cells (rated for 10C+) have different characteristics — lower internal impedance but often lower total energy density. Don't over-specify cell type based on a peak current event; the average current and total runtime budget matter more for most IoT designs.
• Leaving out the NTC thermistor input: many charger ICs have a TEMP pin expecting a 10kΩ NTC thermistor for temperature monitoring. Leaving it unconnected or incorrectly biased can either inhibit charging entirely or defeat the thermal safety feature — read the charger IC datasheet carefully for its default behaviour on that pin.