EV Battery Chemistry and Construction: Cell Types, Pack Architecture, and Thermal Runaway

Why Chemistry Matters for Diagnosis

Not all lithium-ion batteries behave the same. The chemistry of the cells determines the battery's energy density, operating temperature range, charge behavior, cycle life, and failure modes. When you are diagnosing a battery health complaint, a charging fault, or a thermal management concern, knowing which chemistry you are working with helps you understand what normal behavior looks like and what failure looks like.

A customer with an LFP battery who charges to 100% every night is doing exactly what the manufacturer recommends — LFP handles regular full charges well. A customer with an NMC battery who regularly charges to 100% may be accelerating degradation — NMC does better kept between 20% and 80% for daily charging. These are different recommendations for different chemistries, and mixing them up leads to wrong advice and confused customers.

Knowing cell form factor matters for understanding physical inspection findings and pack serviceability. Knowing pack construction helps you understand what the scan tool is telling you about module-level faults.

NMC — Nickel Manganese Cobalt

NMC (Nickel Manganese Cobalt Oxide) is the most widely used chemistry in European and Korean EV packs. Hyundai, Kia, BMW, Volkswagen, Mercedes, and others use NMC in their battery packs.

NMC's advantage is high energy density — more stored energy per kilogram of battery mass. Higher energy density means more range from a lighter, more compact pack. This is a significant engineering advantage in a vehicle where weight and space both matter.

NMC's tradeoffs: it is more sensitive to high temperatures than LFP. Prolonged exposure to heat accelerates degradation. Overcharging NMC chemistry is more dangerous than overcharging LFP — a significantly overcharged NMC cell is more prone to thermal runaway. The BMS must prevent cell overvoltage reliably. Maintaining NMC cells between 20% and 80% state of charge for regular daily charging (only charging to 100% before long trips when needed) extends cycle life compared to routine full charges.

NCA — Nickel Cobalt Aluminum

NCA (Nickel Cobalt Aluminum Oxide) is similar to NMC in energy density profile and is used primarily by Tesla in its older and higher-range vehicles. Tesla 18650 and 21700 cylindrical cells use NCA chemistry.

NCA offers slightly higher energy density than NMC, which contributed to Tesla's early range leadership. Like NMC, NCA requires active thermal management and is sensitive to high temperatures. Tesla invested heavily in battery cooling system design — the liquid cooling system that runs through the floor of the battery pack under every cell — because the chemistry demands it.

Diagnosing an NCA-based Tesla: the BMS, thermal management, and charging behavior are all calibrated for this specific chemistry. Tesla's diagnostic approach through their own systems is the primary path. Third-party scan tools with Tesla enhanced access are available but verify their capabilities for the specific model year.

LFP — Lithium Iron Phosphate

LFP (Lithium Iron Phosphate) has been used in commercial vehicles and stationary storage for years and is now appearing in mainstream EVs. Tesla's standard-range Model 3 and Model Y use LFP. BYD's entire lineup uses LFP. Many Chinese-market EVs use LFP, and that is carrying into vehicles sold in North America.

LFP's advantages: significantly more thermally stable than NMC or NCA. The chemical structure resists thermal runaway far better — LFP is considered among the safest lithium chemistries. LFP tolerates regular full charges to 100% without the degradation penalty that NMC incurs. LFP has a longer cycle life — more charge-discharge cycles before significant capacity loss. LFP is also less expensive because it does not require cobalt or nickel, which are scarce and expensive.

LFP's disadvantage: lower energy density than NMC or NCA. More mass and volume required for the same energy storage. This typically means a slightly shorter range for equivalent pack size, or a larger, heavier pack to achieve equivalent range.

For service and diagnosis: LFP manufacturers typically recommend charging to 100% for daily driving rather than stopping at 80%. The full-charge recommendation is intentional — LFP's SOC estimation is most accurate at the top and bottom of the charge range. A customer with an LFP vehicle who charges to 100% is following correct guidance for their chemistry.

Cell Form Factors

Lithium-ion cells come in three physical shapes. The form factor does not change the diagnostic approach, but it affects how the pack is physically assembled, how cooling is implemented, and how modules are serviced.

Cylindrical: Looks like a standard AA or C-size battery but larger. Tesla's 18650 cells are 18mm diameter by 65mm long. Tesla's newer 4680 cells are 46mm by 80mm. Cylindrical cells are robust, handle automated manufacturing well, and allow for predictable thermal management. Their round shape leaves gaps between cells that can be used for coolant flow. Tesla packages thousands of small cylindrical cells in a large pack — this means many cells in parallel, which reduces the impact of any single cell failing.

Prismatic: Flat rectangular metal cans. BMW, Volkswagen Group, and many others use prismatic cells. They pack more efficiently than cylindrical cells — no wasted space between round cells. Easier to integrate cooling plates between cells. The metal case provides structural support. Each cell contains more energy than a small cylindrical cell, so fewer cells are required per module.

Pouch: Flexible flat pouches made of aluminum laminate. GM (Ultium), Hyundai (Ioniq), and others use pouch cells. Pouch cells pack very efficiently and can be shaped to fit available space. They have no rigid outer case, so the module housing provides the structural support. Pouch cells can swell as they age or if damaged — a swollen pouch cell is an observable warning sign of a cell in distress.

From Cells to Modules to Packs

Individual cells are too small and too low-voltage to drive a vehicle directly. They are grouped into modules. A module typically contains 8 to 24 cells (or more, depending on design) wired in series-parallel combinations to achieve the module's target voltage and capacity. The module housing holds the cells in position, incorporates the bus bars that connect them, and may integrate temperature sensors and the local BMS electronics.

Modules are then wired in series to build the complete battery pack. The total pack voltage is the sum of all cell voltages in series. A 400-volt pack with 3.7-volt cells in series requires roughly 108 cells in series. A 800-volt pack requires roughly 216 cells in series. The total capacity depends on how many parallel strings of series cells are in the pack.

Pack voltage matters for safety and for system compatibility. An 800-volt system requires 800-volt-rated PPE and test equipment. A component rated for 500 volts is not safe on an 800-volt system. Verify the system voltage from service information before any HV work.

Cell-to-Pack Architecture

Traditional pack construction: cells in modules, modules in a pack structure. Some newer designs eliminate the module layer entirely. Cells mount directly to the pack floor and top structure, becoming structural elements of the pack itself. BYD's Blade battery uses this approach. Tesla's structural battery pack integrates the pack into the vehicle's floor structure.

Cell-to-pack increases energy density by removing the weight and volume of module housings. It also reduces cost. The tradeoff: individual cell or cell-group replacement becomes extremely difficult or impossible without replacing the entire pack. This is a significant serviceability concern as these vehicles age and enter the used market. Understand what you are working on before quoting a battery repair on a cell-to-pack design.

Thermal Runaway

Thermal runaway is the most dangerous failure mode in a lithium-ion battery. It is important to understand it — and to know when to stop diagnosing and start evacuating.

Thermal runaway is triggered by cell overcharge, internal short circuit (from separator failure or physical damage from a collision), external short circuit, or external heat exposure that exceeds the cell's thermal limits. Once triggered, the reaction is self-sustaining: the cell heats up, which accelerates the chemical decomposition of the electrolyte, which releases heat and flammable gases, which raises temperature further. The reaction is not self-limiting without external intervention.

A cell in thermal runaway can trigger adjacent cells through heat transfer. This chain reaction can spread through an entire module or pack. Modern packs include fire barriers between cells and modules, thermal fuses, and BMS cutoffs — these slow and contain the spread but do not guarantee containment in a severe event.

Warning signs you may encounter in the shop: a battery module that is significantly hotter than adjacent modules on the scan tool. A sweet chemical odor from the battery area — the electrolyte decomposing. A visually swollen module or cell (visible on opened packs or pouch cell configurations). Smoke from the battery area. Any one of these is a stop-work signal. Evacuate the area. Do not re-enter without confirming the event has fully subsided and the pack temperature has returned to normal — battery fires can reignite hours after an apparent stop because interior cells were not fully involved in the initial event.

The Bottom Line

Know which chemistry is in the vehicle you are working on — it affects charging recommendations, degradation expectations, and thermal behavior. LFP is stable and tolerates full charges. NMC and NCA have higher energy density but need more careful thermal management. Cell form factor does not change diagnosis but affects physical serviceability. Pack architecture determines whether module-level service is possible. Thermal runaway is a medical emergency and a fire emergency, not a diagnostic challenge — recognize the warning signs and evacuate first.