How lithium batteries work in electric and hybrid cars

The basic principle is simple: lithium ions move back and forth between two electrodes, without being consumed along the way. The sources of wear are temperature, a high state of charge and high currents, rather than mileage in itself. A battery’s condition therefore depends more on how the vehicle has been used and on its cell chemistry than on the odometer reading.

The basic principle: charging moves lithium ions

A lithium-ion battery does not store electricity as such. It stores energy by moving lithium ions between two electrodes. During charging, the ions travel from the cathode to the anode, where they are embedded in a graphite material. During discharge, they travel back, while the electrons are forced to pass through the vehicle’s electric motor. Nothing is burned and nothing is consumed. In theory, this could continue indefinitely.

In practice, it does not. Each time the ions move, minor side reactions occur at the electrodes. Some lithium becomes permanently bound in the so-called SEI layer on the anode surface. This layer is essential to the battery’s operation, but it grows slowly throughout the battery’s life, and every increment removes lithium from circulation. This is the single most important cause of capacity loss over the years.

The rate of this process is governed primarily by temperature, how long the battery is held at a high state of charge, and the magnitude of the currents drawn. That is why the same battery can age twice as quickly for one owner as it does for another.

From cell to module to battery pack

A single cell provides only 3–4 volts. A vehicle requires hundreds of volts and tens of kilowatt-hours, so the cells are connected in two stages.

Cells are connected in series to increase voltage and in parallel to increase capacity. A typical passenger-car pack operates at 350–450 volts, while newer platforms use 800 volts. The higher voltage allows the same power to be transferred at a lower current, which produces less heat and enables faster charging without thicker cables.

Two systems surrounding the cells are at least as important as the cells themselves: thermal management and control electronics. Cooling channels or plates keep the cells within their ideal range of approximately 15–35 °C. In cold conditions, the pack is warmed before fast charging instead. This is what preconditioning means. A battery that frequently operates outside its temperature window ages noticeably faster.

Construction also differs between manufacturers. Traditionally, packs are made from replaceable modules. A growing number of new models instead use cell-to-pack designs, in which the cells are bonded directly into a load-bearing structure. This reduces weight and increases energy per litre, but makes module-level repairs difficult or impossible. For a used-vehicle buyer, this is a relevant question: can the battery be repaired in sections, or would the entire pack need to be replaced?

NMC or LFP: chemistry determines behaviour

Almost all the differences between battery types stem from the material used for the cathode. Two chemistries dominate the market today.

LFP has moved from a niche choice to accounting for roughly half of global EV battery capacity. For the used-vehicle market, this means the share of vehicles offering long cycle life and less sensitivity to a full charge is increasing year by year.

One practical consequence is less obvious but important: LFP has a very flat voltage curve. Between approximately 20% and 90% charge, cell voltage changes very little. This makes it harder for the vehicle’s systems to determine the battery’s actual state of charge, which is why LFP vehicles periodically request a full charge for calibration. The same characteristic makes battery-health measurement more demanding for LFP, because voltage alone reveals less.

BMS: the battery management system

A Battery Management System sits above the cells. The BMS is what turns a collection of connected cells into a vehicle battery.

• Cell monitoring. Voltage and temperature are measured continuously, generally for each cell group.
• Balancing. Cells age at different rates. The BMS evens out these differences; otherwise, the weakest cell limits the entire pack.
• Thermal management. Activates cooling or heating and limits power when the temperature is outside the safe operating window.
• Protection. Disconnects or limits the battery in the event of overvoltage, undervoltage, excessive current or excessive temperature.
• Estimation. Calculates state of charge (SoC) and battery health (SoH). Neither can be measured directly; both are calculated values.

The final point deserves closer attention. When the vehicle shows a 62% charge, this is an estimate based on voltage, current counting and a model of how that particular battery behaves. The same applies to battery health. The estimate may drift over time, particularly if the vehicle is rarely charged to the extremes, and manufacturers use different calculation methods. A health figure should therefore always be accompanied by information about how it was obtained.

What this means in practice

For drivers

• Avoid leaving the vehicle at 100% charge for long periods, particularly in hot conditions. For NMC, 20–80% is a sensible everyday range. LFP tolerates a full charge and occasionally requires one.
• Fast charging is not inherently harmful, but it should not be the default. The difference becomes apparent over years, not after isolated charging sessions.
• Use preconditioning in winter. Fast-charging a cold battery is one of the more demanding stresses it can experience.
• For extended periods of inactivity, park in a cool, shaded location where possible. Heat ages the battery even while the vehicle is stationary.

For vehicle dealers

• Chemistry affects valuation. An LFP vehicle with high mileage often has more of its service life remaining than an NMC vehicle with the same odometer reading.
• Ask about charging patterns, not just mileage. The proportion of fast charging explains much of the difference between seemingly identical vehicles.
• Find out whether the battery uses modules or a cell-to-pack design. This determines the realistic cost of a future repair.
• Document battery condition at trade-in and sale. A measurement removes the greatest uncertainty in the transaction for both you and the customer.

Frequently asked questions