An electric car battery is a large, flat, rectangular slab that sits underneath the vehicle’s floor. It typically spans nearly the full width and length of the cabin, resembling a thick metal tray or platform. From the outside, you see a sealed metal enclosure, usually aluminum or steel, with bright orange high-voltage cables running from it. But inside that shell is a carefully organized hierarchy of smaller components, each with its own distinct appearance.
The Outer Shell: A Sealed Metal Box
The most visible part of an EV battery is its enclosure, sometimes called the battery box or battery tray. It consists of two main pieces: a bottom housing tray and a top cover plate. The bottom tray sits on the underside of the vehicle, directly exposed to the road, so it’s built to withstand impacts from rocks, debris, and bumps. The top cover adds structural stiffness and seals everything inside from moisture and contamination.
Most battery enclosures are made from either high-tensile-strength steel or 6000-series aluminum, with aluminum being the more common choice in newer vehicles because it saves weight. The aluminum pieces are typically extruded for the structural frame and stamped for the flat cover and tray surfaces. Once assembled, the whole box gets a powder coating and paint to resist corrosion, chemicals, fire, and weather. Steel enclosures go through a similar finishing process, often including shot peening followed by a corrosion-resistant coating. Some manufacturers are also experimenting with glass fiber composite enclosures to cut weight even further.
The result, from the outside, looks industrial and unremarkable. It’s a flat, dark-colored metal rectangle bolted to the vehicle’s undercarriage, roughly 4 to 6 inches thick depending on the vehicle. You’d never mistake it for anything dramatic. But that plain exterior hides a complex interior.
Inside the Pack: Cells, Modules, and Connections
Open up that sealed box and you’ll find the battery organized in three levels. The smallest unit is the individual battery cell. Groups of cells are wired together and housed in sturdy casings called modules. And those modules are connected together inside the outer enclosure to form the complete battery pack.
A single lithium-ion cell produces only about 1 to 6 volts on its own, so hundreds or even thousands of cells are connected in series and parallel to reach the voltage and energy capacity the vehicle needs. The modules that hold these cells include specialized collector plates that create robust electrical connections between cells and aggregate their power output. The module housings also protect cells from external shocks that could cause damage or trigger dangerous thermal events.
When you look inside a battery pack with the top cover removed, you’ll see rows of rectangular modules arranged in a grid pattern, separated by cooling hardware and wiring. Bright orange cables, required by safety standards, connect the modules and route high-voltage power out of the pack. These cables are colored orange specifically as a visual warning to technicians, and they often carry printed markings like “Caution! High Voltage 600V” or lightning bolt symbols.
Three Types of Battery Cells
Not all EV battery cells look the same. There are three main formats, and each has a distinct shape that affects how the battery pack is designed.
- Cylindrical cells look like oversized AA batteries. Inside, sheets of electrode material are tightly wound into a “jelly roll” and sealed in a rigid metal cylinder. Tesla popularized this format. The round shape means there are small gaps between cells when they’re packed together, so space utilization is the lowest of the three types. Flexible ribbon-shaped cooling tubes are typically woven between or around these cylinders to manage heat.
- Prismatic cells are rectangular blocks with aluminum or steel casings. The electrodes inside are either stacked flat or folded into layers. Their rectangular shape allows them to be packed tightly together with almost no wasted space, giving them the highest space utilization of any format. Flat aluminum cold plates with internal cooling channels are sandwiched between these cells, typically about 3 millimeters thick.
- Pouch cells skip the rigid metal casing entirely. Instead, the electrode layers are sealed inside a flexible aluminum-plastic film, similar to a vacuum-sealed food pouch. This makes them incredibly thin (as slim as 1 millimeter) and adaptable to different shapes. They offer high space utilization and are often used where packaging flexibility matters. Like prismatic cells, they’re typically cooled with flat cold plates pressed against their surfaces.
What’s Inside a Single Cell
If you could slice open an individual cell and look at its cross-section, you’d see a repeating sandwich of very thin layers. The basic structure is the same regardless of cell format. There’s an anode (the negative side), typically made of graphite coated onto thin copper foil. Then there’s a cathode (the positive side), made of a lithium-containing compound coated onto aluminum foil. Between every anode and cathode layer sits a thin polymer separator that keeps them from touching while allowing lithium ions to pass through.
In a pouch cell, these layers are stacked flat and folded in a Z-pattern. In a cylindrical cell, they’re rolled up like a spiral. Either way, the layers are soaked in a liquid electrolyte that carries lithium ions back and forth during charging and discharging. The whole assembly is remarkably thin and lightweight on its own, which is why so many layers need to be stacked or wound together to store meaningful amounts of energy.
The Cooling System You Can See
One of the most visually distinctive features inside a battery pack is the thermal management system. Keeping cells within a safe temperature range is critical, so most modern EVs use liquid cooling. The cooling hardware looks different depending on the cell type.
For cylindrical cells, manufacturers use flexible structures like flat ribbon-shaped tubes that snake between the rows of cylinders. For prismatic and pouch cells, the standard approach is cold plates: flat aluminum panels with tiny internal channels (called minichannels) through which coolant flows. These cold plates sit directly against the cell surfaces, sometimes sandwiching each cell between two plates. In some designs, a single large cold plate covers the entire bottom of the battery pack, creating a more even temperature distribution across all the cells.
The coolant lines, pumps, and heat exchangers connect to the vehicle’s broader thermal system, which may also manage cabin heating and cooling. From inside the pack, you’ll see aluminum plates, flexible tubing, and coolant fittings running alongside the electrical connections.
Scale and Placement in the Vehicle
The battery pack in a typical EV weighs between 800 and 1,200 pounds and can be roughly the size of a twin mattress, though only a few inches thick. Mounting it flat under the passenger cabin lowers the vehicle’s center of gravity, which improves handling and stability. It also means the battery essentially becomes a structural element of the car’s floor.
If you were to crawl underneath an EV, the battery tray is the large, flat panel covering most of the underside between the front and rear axles. It’s bolted to the vehicle frame and sealed against water intrusion. From above, inside the cabin, you’d never know it’s there. The floor feels and looks like any other car’s. The entire design philosophy is to make something enormously complex look like nothing at all.