Your phone has one lithium-ion cell. It produces between 3.0V (empty) and 4.2V (full), stores about 15–20 Wh of energy, and fits in your palm. Your EV's battery pack produces 350–800V, stores 24–100 kWh, and weighs 200–600 kg. Scaling from one cell to a pack that can power a car is not a matter of making a bigger cell — it is a matter of connecting hundreds of smaller cells in precise geometric arrangements that multiply both voltage and energy storage to the required levels, then wrapping that arrangement in a system that monitors every cell, manages temperature, and prevents any failure from cascading.
Understanding the cell-module-pack hierarchy, why series connections multiply voltage and parallel connections multiply capacity, and what the BMS does at the pack level gives you a foundation for understanding everything from EV range specifications to battery replacement costs.
- The battery pack has three levels: individual cells → modules (groups of cells) → pack (all modules plus BMS, thermal management, and housing).
- Series connections multiply voltage: 100 cells in series × 3.7V per cell = 370V pack. Capacity stays the same as a single cell.
- Parallel connections multiply capacity: 3 cells in parallel × 50 Ah per cell = 150 Ah total. Voltage stays the same as a single cell.
- Most EV packs use a combination: series for voltage, parallel for capacity. "96S2P" means 96 series groups of 2 parallel cells = 96 × 3.7V = 355V, with 2× the cell capacity.
- The BMS monitors every cell's voltage and temperature, balances charge between cells, and controls the main contactors that connect the pack to the vehicle.
The Cell: Where Energy Storage Happens
Every EV battery starts with individual cells — typically 3–5 cm in size, containing the active electrode materials, electrolyte, and separator that do the electrochemical work of storing and releasing energy.
A typical lithium-ion cell for EV use:
- Nominal voltage: 3.2V (LFP) to 3.7V (NMC)
- Capacity: 20–300 Ah depending on cell size and format
- Energy: 70–300 Wh per cell
- Voltage range: 2.5–3.65V (LFP) or 3.0–4.2V (NMC)
The cell's voltage cannot be changed — it is determined by the chemistry. To build a pack that works with an EV's power electronics (which need hundreds of volts to operate efficiently), cells must be connected in series.
Series Connections: Building Voltage
When cells are connected in series (positive terminal of cell 1 to negative terminal of cell 2, and so on), the voltages add up. The capacity stays the same as a single cell.
Example: 10 NMC cells in series
- Pack voltage: 10 × 3.7V = 37V
- Pack capacity: same as one cell (e.g., 100 Ah)
- Pack energy: 37V × 100 Ah = 3,700 Wh = 3.7 kWh
For a full EV pack at 400V nominal, you need approximately 400V ÷ 3.7V ≈ 108 NMC cells in series. For an 800V pack: approximately 216 cells in series.
The weakest cell in a series string limits the entire string. In series, the same current flows through every cell. If one cell has less capacity than the others, it reaches its voltage limit (upper or lower) before the others — at which point the BMS must stop charging or discharging to protect that cell. The entire pack's usable energy is therefore determined by the worst-performing cell, not the average. This is why cell matching (selecting cells with similar capacity and impedance) during pack assembly is critical to pack performance.
Parallel Connections: Building Capacity
When cells are connected in parallel (positive to positive, negative to negative), the capacities add up. The voltage stays the same as a single cell.
Example: 3 NMC cells in parallel
- Pack voltage: same as one cell = 3.7V
- Pack capacity: 3 × 100 Ah = 300 Ah
- Pack energy: 3.7V × 300 Ah = 1,110 Wh = 1.11 kWh
Parallel connections also reduce internal resistance (three cells sharing the current means each cell carries one-third the current, and impedances add in parallel). This allows higher power output for the same heat generation.
The Combined Series-Parallel Architecture
Real EV packs combine both: series for voltage, parallel for energy/power. The notation "96S3P" means 96 series groups, each group containing 3 cells in parallel.
Target pack voltage ÷ nominal cell voltage = number of series groups needed. For 400V with NMC cells: 400V ÷ 3.7V ≈ 108 series groups.
Target pack energy ÷ (series voltage × cell capacity) = parallel strings. For 50 kWh pack with 108S, 100Ah cells: 50,000 Wh ÷ (400V × 100 Ah) = 1.25, round up to 2P → 108S2P.
108S2P = 216 cells. Pack voltage = 108 × 3.7V = 400V. Pack capacity = 2 × 100 Ah = 200 Ah. Pack energy = 400V × 200 Ah = 80 kWh.
For assembly and serviceability, series-parallel groups are physically packaged into modules. A 108S2P pack might use 9 modules of 12S2P each.
The Module: Between Cell and Pack
Modules are sub-assemblies containing a subset of the pack's cells. A module typically contains:
- A fixed number of cells in series and/or parallel
- Internal busbars connecting the cells
- Temperature sensors (thermistors) at key locations
- A module-level BMS chip (or harness connections to the central BMS)
- A structural housing protecting the cells
Modules exist for serviceability and thermal management. They allow a failed section of the pack to be replaced without disassembling the entire pack. They also allow thermal management channels to be designed at the module level — coolant passages running between cell rows within the module.
Some modern pack designs eliminate modules entirely — this is called Cell-to-Pack (CTP) architecture, pioneered by CATL (Contemporary Amperex Technology) and used in BYD's Blade Battery. In CTP, cells are arrayed directly in the pack housing without intermediate module packaging. This reduces the number of structural layers and improves pack energy density (more cells per volume). The trade-off is serviceability — a cell-level failure in a CTP pack is harder to isolate and repair. BYD's Blade battery in LFP chemistry uses cells as structural members of the pack housing itself, a further integration called Cell-to-Body (CTB) in their latest models.
The Pack: Everything Together
The complete battery pack integrates:
Cells and modules — the electrochemical energy storage Battery Management System (BMS) — monitors every cell, manages balancing, controls contactors Main contactors — high-voltage relays that connect/disconnect the pack from the vehicle Pre-charge circuit — limits inrush current when the pack is first connected Current sensor — measures pack current for SOC tracking Thermal management — cooling channels, heater elements, or both Structural housing — aluminium or composite tray, sealed against water and dust, often IP67 rated Manual Service Disconnect (MSD) — a high-voltage interlock that emergency services use to safely disconnect the pack
Never attempt to open or access an EV battery pack housing yourself. The pack interior carries high voltage (350–800V DC) even when the car is switched off — the main contactors are open, but cell voltage remains. DC at these voltages is extremely dangerous (higher current for the same body resistance than AC at equivalent voltage, no zero crossing to interrupt the arc). All service work on the high-voltage system must be performed by trained technicians with appropriate PPE (insulated gloves rated for the voltage, insulated tools, HV diagnostics). India's EV service network is developing HV safety training — when selecting an EV brand, consider whether the authorised service network in your area has certified HV technicians.
Why Pack Size and Vehicle Weight Matter Together
Larger packs give more range, but they also add weight. Weight increases energy consumption — particularly in city driving where frequent acceleration burns energy. This creates the "right-sizing" problem that EV manufacturers navigate:
- A Tata Tiago EV (19.2 kWh, 1178 kg) is optimised for Indian urban use — the small, light pack provides adequate urban range at minimum weight and cost penalty
- A Hyundai Ioniq 5 (72.6 kWh, 2165 kg) is optimised for long-distance capability — the large pack carries the weight penalty, which the aerodynamic body partially offsets
The ideal pack size for most Indian EV owners is one that covers their daily commute comfortably (charging at home overnight) with enough buffer for unplanned diversions — typically 25–40 kWh for 4W urban users. The additional range from a 60–80 kWh pack costs more and weighs more, benefits that are relevant only for highway travel.
- The cell-module-pack hierarchy is the physical architecture of every EV battery. Cells do the electrochemical work; modules group cells for thermal management and serviceability; the pack integrates everything with BMS, contactors, and housing.
- Series connections multiply voltage (same current flows through all cells). Parallel connections multiply capacity (current splits between cells). Most EV packs use series-parallel combinations to achieve target voltage and energy simultaneously.
- The weakest cell in a series string limits the entire string. Cell matching at manufacture and BMS balancing during operation both exist to manage this constraint.
- Pack cost is 30–40% of EV retail price in India. This is why battery replacement is so expensive — and why the battery's warranty terms (typically 8 years / 1.6 lakh km in India) matter significantly for ownership economics.
- Modern Cell-to-Pack (CTP) designs eliminate modules for higher energy density. BYD Blade battery and CATL CTP are the leading examples. The trade-off is serviceability — individual cell replacement becomes harder in highly integrated designs.