A traction battery is a large rechargeable battery designed to power the electric motor that moves a vehicle or machine. Unlike the small 12-volt battery under your car’s hood, which exists only to start the engine and run the lights, a traction battery delivers sustained energy for propulsion. It’s the main power source in electric cars, hybrid vehicles, forklifts, warehouse pallet trucks, underground mining locomotives, and subway tunnel vehicles.
Traction vs. Starter Batteries
The battery most people are familiar with is the starter battery, also called an SLI (starting, lighting, ignition) battery. It delivers a short, intense burst of power to crank an engine, then gets recharged by the alternator while you drive. A traction battery does something fundamentally different: it provides a steady flow of energy over hours of operation, acting as the vehicle’s fuel tank rather than just its ignition switch.
This difference shapes everything about how traction batteries are built. They’re much larger, often weighing hundreds of kilograms. They store far more energy. And they’re engineered to be deeply discharged and recharged thousands of times without degrading quickly, something a starter battery would fail at within weeks.
How a Traction Battery Is Built
A traction battery pack is assembled in layers. The smallest unit is an individual battery cell, a single electrochemical container roughly the size of a small can or a flat pouch, depending on the design. Groups of cells are wired together into modules, and multiple modules are combined into the final battery pack, which is the large, flat slab bolted to the underside of most electric cars.
This modular structure exists for practical reasons. If a small number of cells fail, a single module can be replaced rather than the entire pack. The arrangement also allows engineers to configure the pack’s total voltage and capacity by adjusting how many cells are wired in series (to increase voltage) versus in parallel (to increase capacity).
Common Battery Chemistries
Most modern traction batteries use lithium-ion chemistry, but “lithium-ion” is a broad category. The two dominant types on the market today are LFP and NMC, and they represent a genuine trade-off.
LFP (lithium iron phosphate) batteries are the more durable and affordable option. They last beyond 2,000 charge cycles, cost roughly 30% less than comparable alternatives, and handle heat well, operating effectively up to 60°C. They also carry a smaller environmental footprint because they avoid cobalt and nickel mining. The downside is lower energy density, meaning you need a heavier battery to store the same amount of energy.
NMC (nickel manganese cobalt) batteries pack more energy into less space, reaching up to 260 Wh/kg. That makes them the preferred choice for vehicles where range and performance matter most. The trade-off is higher cost and greater environmental concern due to the cobalt and nickel content. NMC batteries also have a shorter cycle life than LFP.
In practice, many affordable EVs and commercial vehicles now use LFP packs, while performance and luxury models lean toward NMC.
Voltage: 400V and 800V Systems
Most electric cars on the road today, including the Tesla Model 3 and Volkswagen ID.3, use a 400-volt battery architecture. This has been the industry standard for years. A newer generation of vehicles, led by models like the Porsche Taycan and Audi e-tron GT, operates at around 800 volts.
Higher voltage allows the same amount of power to flow with less electrical current. That reduces energy lost as heat in the wiring and electronics, improving overall efficiency. The most noticeable benefit for drivers is charging speed: 800V systems can support ultra-rapid charging up to 350 kW, cutting charge times significantly compared to 400V vehicles on the same charger. As high-power charging infrastructure expands, 800V architecture is expected to become more common across all price segments.
The Battery Management System
No traction battery operates without a battery management system, or BMS. This is the electronic brain that continuously monitors every cell or cell group in the pack, tracking voltage, current, temperature, state of charge (how full the battery is), and state of health (how much capacity has degraded over time).
The BMS does more than just observe. It actively intervenes to prevent dangerous conditions. If a cell’s temperature climbs too high or its voltage drifts outside safe limits, the BMS can reduce charging or discharging rates, or cut off the battery entirely. It also handles cell balancing, making sure all cells in the pack charge and discharge evenly. Without this, some cells would wear out far faster than others, shortening the pack’s useful life.
Why Thermal Management Matters
Lithium-ion cells perform best between 25°C and 40°C. Outside that range, they lose capacity faster, deliver less power, and in extreme cases can enter thermal runaway, a chain reaction where overheating cells cause neighboring cells to overheat in turn. Temperature variation between modules also needs to stay under 5°C for the pack to age evenly.
To manage this, traction batteries include dedicated thermal management systems. These fall into a few categories. Passive systems use no external energy: natural airflow, heat-absorbing phase change materials, or heat pipes that transfer warmth away from cells. Active systems use an external power source to control temperatures more precisely, typically through liquid cooling loops (the most common approach in modern EVs), forced air, or refrigerant circuits. Many vehicles combine both approaches in a hybrid system.
Liquid cooling, where coolant flows through plates or channels in direct contact with the modules, is the dominant solution in passenger EVs because it removes heat more effectively than air while keeping the system compact. The trade-off is added weight, complexity, and a small parasitic drain on the battery itself, since the pumps and fans need power to run.
Beyond Electric Cars
Traction batteries have a long history outside the passenger vehicle market. Electric forklifts have relied on them for decades, traditionally using flooded lead-acid cells with tubular positive plates rated for 1,500 or more charge cycles, with capacities ranging from 100 to 1,800 amp-hours. These heavy-duty packs power warehouse operations, logistics fleets, underground mining locomotives, subway tunnel maintenance vehicles, and environmental cleaning equipment in coal mines.
Industrial traction batteries face different demands than EV packs. A forklift battery needs to deliver steady power over an 8-hour shift, survive being deeply discharged daily, and tolerate harsher environments including dust, vibration, and temperature swings. Many industrial operations still use lead-acid chemistry for its low upfront cost and proven reliability, though lithium-ion is steadily replacing it as prices fall.
What’s Coming Next
The most anticipated advancement in traction battery technology is the solid-state battery, which replaces the liquid electrolyte inside conventional lithium-ion cells with a solid material. This promises higher energy density, faster charging, and improved safety since solid electrolytes are far less flammable. The technology is real but still reaching production scale. One early milestone: the Karma Kaveya, a super coupe expected to ship with solid-state batteries starting in late 2027. Broader adoption across mainstream vehicles will take longer as manufacturing scales up and costs come down.