How Do EV Batteries Work? A Plain English Guide

An EV battery pack stores energy as a chemical charge in thousands of lithium-ion cells, then releases it as electrical current to drive the motor. The pack is managed by a battery management system (BMS) that monitors every cell in real time, keeping temperatures safe, balancing charge levels and protecting against damage. Pack sizes in current UK models run from around 40 kWh in compact city cars to over 100 kWh in large SUVs, delivering roughly 150 to 400 miles of real-world range.

What is inside an EV battery pack?

An EV battery is not one large battery in the way a torch battery is. It is a carefully engineered stack of individual cells, grouped into modules, which are assembled into the complete pack.

Cells

Each cell is a small, sealed unit containing two electrodes (an anode and a cathode), a porous separator and a liquid electrolyte. During discharge, lithium ions move from the anode through the electrolyte to the cathode, and electrons travel around the external circuit to power the motor. During charging, the direction reverses. The chemistry of the electrodes determines the energy density, thermal stability and longevity of the cell.

Cells come in three physical formats:

• Cylindrical (the classic AA-style shape, used by Tesla and others)
• Prismatic (rectangular, used by BMW, Samsung SDI)
• Pouch (flat and flexible, used by Hyundai, Kia, LG Energy Solution)

Each format has trade-offs in energy density, manufacturing cost and thermal management.

Modules

Cells are grouped into modules, which include the local electrical connections, temperature sensors and sometimes basic balancing circuitry. A typical module might contain 20 to 100 cells depending on design. Modules make the pack easier to manufacture and, in theory, easier to service.

The pack

Modules slot into the complete battery pack, a flat, rigid structure usually mounted in the floor of the car (giving a low centre of gravity). The pack includes cooling channels, structural reinforcement and the main electrical connections. Some newer designs, notably Tesla’s 4680 cell approach and BYD’s Blade Battery, are moving toward a “cell-to-pack” architecture where cells slot directly into the pack without a separate module layer, saving weight and improving volumetric efficiency.

What the kWh figure means

The total capacity of the pack is measured in kilowatt-hours (kWh). One kWh is enough energy to drive a typical EV about 3 to 4 miles under mixed conditions, though this varies significantly by car, speed and weather. The “usable” capacity is always slightly less than the total: manufacturers reserve a buffer at the top and bottom of the charge range to protect the cells from the extremes that accelerate degradation.

Model	Total capacity	Usable capacity	Approximate real-world range (WLTP)
Renault Zoe (older model)	52 kWh	46 kWh	~245 miles
Volkswagen ID.3 Pro	58 kWh	52 kWh	~266 miles
Hyundai IONIQ 6 Long Range	77.4 kWh	74 kWh	~340 miles
Tesla Model 3 Long Range	82 kWh	75 kWh	~358 miles
BMW iX xDrive50	111.5 kWh	105.2 kWh	~380 miles

How does the battery management system work?

The battery management system (BMS) is the electronics brain of the pack. It monitors and controls every cell continuously, performing several critical functions.

Monitoring voltage and temperature

Each cell has a safe operating voltage range, typically 2.5 to 4.2 volts per cell for most lithium-ion chemistries. The BMS measures cell voltages thousands of times per second and shuts down the pack if any cell goes outside safe limits. Temperature sensors are distributed throughout the pack, and if temperatures rise too high, the BMS triggers active cooling.

State of Charge (SoC)

SoC is the equivalent of a fuel gauge, expressed as a percentage. The BMS calculates SoC by tracking current in and out of the pack (coulomb counting) combined with voltage measurements. Accurate SoC estimation is harder than it sounds because a battery’s voltage-to-charge relationship is not perfectly linear and changes as the battery ages.

State of Health (SoH)

SoH represents how much of the original capacity the battery retains. A new battery starts at 100% SoH. As it ages through charge cycles and temperature exposure, capacity gradually fades. At 80% SoH, a pack that started at 60 kWh now effectively holds 48 kWh. Most EV warranties cover the battery to at least 70% SoH for 8 years or around 100,000 miles, whichever comes first (terms vary by manufacturer).

UK models from Hyundai and Kia typically offer 8 years / 100,000 miles battery warranty. Renault, MG and others have similar commitments. Tesla guarantees 70% SoH retention for 8 years or 120,000 miles on the Model 3 Long Range.

Cell balancing

Cells within a pack never age at exactly the same rate. Some cells lose capacity faster due to microscopic manufacturing variations or local temperature differences in the pack. If one cell falls behind, the BMS can either discharge the stronger cells slightly to match (passive balancing, which wastes energy as heat) or actively move charge between cells (active balancing, which is more efficient but more complex). The goal is to ensure the weakest cell determines the usable capacity of the pack as little as possible.

Thermal management

Lithium-ion cells work best between roughly 15°C and 35°C. Below that range, internal resistance rises and effective capacity drops, which is why range is reduced in winter. Above 40°C, degradation accelerates, and beyond 60°C there is risk of thermal runaway, a self-sustaining exothermic reaction that can damage or destroy the pack.

Most modern EVs use liquid cooling: coolant flows through channels pressed against the cell surfaces or modules, carrying heat away to a radiator. In cold weather, the same loop can be used to warm the battery using a heat exchanger connected to the heating system. Preconditioning, which warms the battery before a fast-charge session or a cold drive, uses this thermal management system.

What causes battery degradation and how fast does it happen?

Battery degradation is normal and inevitable, but the rate varies significantly depending on how the car is used and charged.

The main causes of degradation are:

• High temperatures: heat is the primary accelerator of capacity fade
• High state of charge held for long periods: keeping the battery at 100% for days accelerates degradation, particularly in NMC chemistry
• Frequent fast charging: DC rapid charging generates heat and stresses cells more than AC slow charging
• Deep discharge: regularly running the battery to near zero is harder on cells than stopping at 10–15%

Real-world UK data from services such as Recurrent Auto suggests that most EVs retain around 90% of original capacity after five years of typical use. Tesla vehicles, with a larger buffer and sophisticated BMS, often show better figures.

The practical implication: if you charge at home using a 7 kW wallbox to 80% most nights and use rapid charging only for long trips, your battery should remain in good health for the life of the car. If you fast-charge daily to 100% and park in full sun, expect faster degradation.

What is LFP chemistry and is it different?

LFP (lithium iron phosphate) batteries, used in the Tesla Model 3 Standard Range, BYD Dolphin, BYD Atto 3 and Citroën ë-C3, have a fundamentally different cathode chemistry. The key practical differences for UK drivers:

• LFP cells can be charged to 100% every day without significant degradation; Tesla explicitly recommends this for LFP-equipped cars
• LFP has lower energy density per kg, so the pack is physically heavier for the same usable kWh
• LFP is more thermally stable and less susceptible to thermal runaway
• LFP cells have a flatter discharge curve, which can make SoC estimation less precise; BMS calibration charges (to 100%) are sometimes recommended

See our LFP vs NMC batteries guide for a full comparison.

What are the common myths about EV batteries?

“You should never charge above 80%”: This applies mainly to NMC chemistry for regular home charging. LFP cars are designed for 100% daily charging. For NMC, occasional full charges are fine; it is the habit of keeping an NMC pack at maximum state of charge for extended periods that accelerates wear.

“Cold weather permanently damages the battery”: Cold temporarily reduces available capacity but does not cause permanent damage unless you attempt to fast-charge a deeply cold (below about 5°C) battery without preconditioning. The BMS will normally limit charge rate to protect the cells in cold conditions.

“EV batteries are full of toxic metals that can’t be recycled”: Lithium-ion batteries are recyclable. The UK and EU regulatory framework requires manufacturers to ensure battery recycling routes exist. Processes for recovering lithium, cobalt, nickel and manganese are improving rapidly.

What is OTA battery software and why does it matter?

Because the BMS is software-controlled, manufacturers can push improvements over the air without any physical work on the car. Tesla routinely updates charging curves, thermal management algorithms and SoC estimation via OTA updates. Volkswagen, Hyundai and others do the same to a lesser degree.

Research published in May 2026 found that an AI-powered “health-aware” charging algorithm could extend battery life by approximately 23% by adapting the charge rate to the real-time state of the cells. This kind of capability can be delivered as a software update, meaning cars already on the road could benefit.

What should you look at when comparing EV batteries?

When shortlisting EVs, the battery questions worth asking are:

1. What is the usable capacity in kWh, and what real-world range does that deliver?
2. What chemistry is it (LFP or NMC) and what does that mean for daily charging habits?
3. What is the maximum AC and DC charge rate (kW)?
4. What is the warranty commitment (years, miles, minimum SoH)?
5. Does it support preconditioning for rapid charging?
6. Does it support bidirectional charging (V2H or V2G)?

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