The fundamental principle: controlled chemistry
Every chemical reaction involves electrons moving between atoms. In a battery, we force those electrons to travel through an external circuit (your device) rather than directly between the chemicals. That controlled flow of electrons is electricity.
Two half-reactions happen simultaneously:
- At the anode: oxidation — atoms release electrons
- At the cathode: reduction — atoms accept electrons
The electrolyte between them allows ions (charged atoms) to flow internally to balance charge, but blocks electrons — forcing them to travel the long way around through your phone or lightbulb.
This separation of electron flow (external) and ion flow (internal) is the battery’s fundamental trick.
Lithium-ion: why it won
Modern rechargeable batteries — in phones, laptops, EVs, power tools — are almost all lithium-ion. Why lithium?
- Lightest metal element — high energy per kilogram (energy density)
- High voltage — lithium cells operate at ~3.6V vs 1.5V for alkaline
- Reversible reaction — lithium ions can shuttle back and forth thousands of times
In a lithium-ion cell:
- Anode: Graphite (carbon layers that store lithium ions between them)
- Cathode: Lithium metal oxide (commonly LiFePO₄, NMC, or NCA)
- Electrolyte: Liquid lithium salt in organic solvent (enables Li⁺ to move)
- Separator: Porous plastic membrane (prevents short circuit while allowing Li⁺)
Discharging: Li⁺ ions leave the graphite anode, travel through the electrolyte, and insert into the cathode structure (a process called intercalation). Simultaneously, electrons flow from anode to cathode through the external circuit — powering your device.
Charging: Apply external voltage, and the process reverses: Li⁺ is pulled out of the cathode and forced back into the graphite anode.
Why batteries lose capacity over time
Each charge/discharge cycle leaves the battery slightly worse than before. Several mechanisms contribute:
1. Lithium plating and dendrites If you charge too fast or at low temperatures, lithium ions can’t insert properly into the graphite and instead deposit as metallic lithium on the anode surface. These deposits form needle-like dendrites that can pierce the separator, causing a short circuit — the cause of some battery fires.
2. Solid Electrolyte Interphase (SEI) growth The first few charge cycles, lithium reacts with the electrolyte to form a thin coating on the anode called the SEI. This film is actually protective — it stabilises the interface — but it continues to grow slowly over time, consuming lithium and increasing internal resistance.
3. Cathode cracking The cathode material expands and contracts as lithium ions enter and leave. Repeated cycling causes microscopic cracks, reducing contact area and capacity.
4. Electrolyte decomposition Electrolyte breaks down at high temperatures and voltages, producing gases (causing swelling) and reducing ion mobility.
Temperature: the battery’s greatest enemy
Heat is the primary driver of degradation. The Arrhenius equation — a chemistry rule — tells us that reaction rates approximately double for every 10°C increase. Every high-temperature charging or storage event causes years of extra degradation.
Ideal storage and operating temperature: 15–25°C
Why your laptop battery degrades faster in summer, left in a hot car, or charged overnight under a duvet: the elevated temperature dramatically accelerates all the degradation mechanisms above.
Cold is the opposite problem: it slows ion movement, increasing internal resistance and temporarily reducing available capacity (your phone dies in winter, but the battery recovers at room temperature — the lithium was there all along).
EV batteries: same chemistry, enormous scale
An EV battery pack is essentially thousands of small cylindrical or pouch cells arranged in modules. A Tesla Model Y has ~4,400 lithium-ion cells in its 75 kWh pack.
The Battery Management System (BMS) is the engineering challenge:
- Monitors temperature, voltage, and state of charge of every cell
- Balances cells so no single cell gets over/undercharged
- Controls thermal management (liquid cooling circuits)
- Implements charge rate limits (reducing fast charge above 80% to protect the top-end cells)
This is why EVs recommend charging to 80% for daily use — the top 20% requires slower charging rates that stress cells, and the capacity in that range is rarely needed.
The mental model: a battery is a mountain of electrons waiting to roll downhill
Chemical energy is potential energy — electrons at the anode are at a “higher” energy state (like a ball at the top of a hill). The cathode is at a lower energy state.
When you complete the circuit, electrons flow “downhill” from anode to cathode through your device, doing useful work on the way. Charging is forcing the electrons back uphill against their natural inclination, using external energy to rebuild the potential difference.
Common misconceptions
“You should always fully charge and discharge a battery.” This is true for old nickel-cadmium (NiCd) batteries, which suffered from “memory effect.” Lithium-ion has no memory effect. Partial charges are perfectly fine and actually better for longevity. Keeping your phone between 20–80% is ideal for battery health.
“Leaving your phone plugged in overnight damages it.” Modern phone chargers and BMS cut off charging at 100%. What does cause slow degradation is sitting at 100% state of charge for extended periods at room temperature. iPhone’s “Optimised Battery Charging” addresses this by learning your wake-up time.
“You should remove your laptop battery to preserve it.” Not applicable to most modern laptops, which have integrated batteries. But charging to 80% via settings is genuinely helpful if the laptop is always plugged in.