Battery Chemistry and Construction

The HV battery pack is the single most expensive component in an EV. Understanding what is inside it — how the cells are built, how they are organized, and what chemistry they use — helps you diagnose concerns intelligently and explain them to customers accurately.

Not all lithium-ion batteries are the same. Three main chemistries dominate the EV market. NMC — Nickel Manganese Cobalt — offers high energy density, meaning more range per pound. Used in many European and Korean EVs. NCA — Nickel Cobalt Aluminum — similar high energy density, used in Tesla vehicles. Both NMC and NCA deliver excellent range but require careful thermal management and are more susceptible to thermal runaway if damaged or overcharged. LFP — Lithium Iron Phosphate — has lower energy density, meaning slightly less range per pound. But LFP is more thermally stable, lasts more charge cycles, tolerates being charged to 100 percent regularly, and costs less. Tesla and many Chinese manufacturers are adopting LFP for standard-range vehicles. When diagnosing a battery concern, knowing the chemistry tells you about its charging behavior, temperature sensitivity, and expected lifespan.

EV battery cells come in three physical shapes. Cylindrical — looks like a standard AA battery, just larger. Tesla has used 18650 and now 4680 cylindrical cells. Prismatic — flat rectangular cans, used by BMW, Volkswagen, and many others. Pouch — flat, flexible aluminum-laminate pouches, used by GM, Hyundai, and others. Each form factor has tradeoffs in energy density, cooling efficiency, manufacturing cost, and packaging flexibility. The form factor does not change diagnosis — but it changes how the pack is physically assembled and serviced.

Individual cells are grouped into modules. Each module typically contains 8 to 24 cells wired in series and parallel combinations to achieve the desired voltage and capacity. Modules are then stacked and wired in series to build the complete battery pack. A typical EV pack contains 8 to 30 modules. The total pack voltage is the sum of all cells in series — typically 350 to 400 volts for most EVs, up to 800 volts on some performance and newer vehicles. Some newer designs are moving to cell-to-pack architecture, eliminating the module level entirely and mounting cells directly into the pack structure for greater energy density.

Thermal runaway is the most dangerous battery failure mode. If a lithium-ion cell is overcharged, short-circuited, physically damaged, or exposed to extreme heat, it can enter an uncontrollable self-heating reaction. The cell temperature rises rapidly, the electrolyte decomposes, flammable gases are released, and the cell can catch fire. Heat from one cell in thermal runaway can trigger adjacent cells — creating a cascading chain reaction through the pack. Modern packs include barriers between cells and modules, thermal fuses, and BMS safety cutoffs to prevent and contain thermal runaway. As a technician, if you see a swollen cell, smell a sweet chemical odor from the pack, see smoke, or detect elevated pack temperatures on the scan tool — stop work, evacuate the area, and follow emergency procedures. Do not attempt to diagnose or repair an actively failing battery.