Battery Basics: From Cell Specifications to Pack Architecture

A cell is the fundamental electrochemical unit. Multiple cells can be connected in series and parallel to achieve the required voltage and capacity. A module is generally a mechanically and electrically organized group of cells, while a battery pack is the complete assembly that may include cells, interconnects, BMS, insulation, enclosure, connectors, thermal management and other protection components.

For engineering purposes, the distinction is important because a cell specification does not describe the performance of the finished pack. Interconnect resistance, BMS limits, temperature, mechanical construction and cell matching can all influence the final system.

In larger-format designs, modules exist as an intermediate assembly stage primarily for manufacturing, serviceability and safety-segmentation reasons: a module can be tested, qualified and replaced as a unit, and a fault contained within one module does not necessarily propagate to the entire pack. In smaller consumer or light-industrial packs, the module layer is often omitted and cells are wired directly into a single pack assembly.

When reviewing a supplier's datasheet or a competitor's product, it is worth explicitly asking whether the quoted specification describes the cell, the module or the finished pack — the same numerical voltage or capacity value can mean materially different things at each level.

Cells connected in series increase voltage while cells connected in parallel increase capacity and current capability. A pack described as 4S3P contains four cells or parallel groups in series and three cells in parallel within each group.

The nominal pack voltage is approximately the nominal cell voltage multiplied by the number of series groups. Pack amp-hour capacity is approximately the cell capacity multiplied by the number of cells in parallel.

It is important to distinguish between nominal voltage, minimum (cutoff) voltage, and maximum (full-charge) voltage. A 13S LFP pack, for example, has a nominal voltage in the region of 41.6 V but will reach a maximum charge voltage well above that figure and a minimum discharge voltage well below it. Any downstream electronics — inverters, motor controllers, chargers — must be rated for the full voltage window, not just the nominal figure.

Parallel groups should be built from cells that are closely matched in capacity, internal resistance and state of charge at the time of assembly. Poorly matched parallel groups can result in circulating currents between cells even at rest, accelerated aging of the weaker cells, and long-term capacity divergence within the group.

Amp-hours describe electrical charge capacity, while watt-hours describe energy. For a simplified calculation, energy can be estimated by multiplying nominal voltage by amp-hour capacity.

For example, a nominal 48 V, 30 Ah pack represents approximately 1,440 Wh of nominal energy before accounting for usable-depth-of-discharge limits, conversion losses, temperature effects and other system losses.

Because voltage is not constant across the discharge curve, the Wh figure calculated from nominal voltage is an approximation rather than a precise measurement. A more accurate energy figure can be obtained by integrating instantaneous voltage and current over a full discharge cycle, which is how most cell manufacturers report the Wh rating on a datasheet.

Ah and Wh are also useful for comparing batteries across different voltage classes. A 12 V, 100 Ah battery and a 48 V, 25 Ah battery both represent approximately 1,200 Wh of nominal energy, even though their Ah ratings differ by a factor of four — a common source of confusion during procurement and specification review.

C-rate expresses charge or discharge current relative to the cell's rated capacity. A 10 Ah cell delivering 20 A is operating at approximately 2C discharge.

The acceptable C-rate depends on the specific cell design, temperature, duration and manufacturer's specifications. A cell with high nominal capacity is not automatically suitable for a high-current application.

C-rate limits are typically specified separately for continuous and pulse (peak) operation, and pulse ratings are usually time-bound — for example, a 3C pulse for 10 seconds. Designers should confirm both figures and design the electrical and thermal system around the more restrictive continuous rating for sustained loads.

Operating a cell above its rated C-rate, even briefly and even if the BMS does not trip, can accelerate degradation and increase internal heat generation disproportionately. C-rate should therefore be treated as a hard design constraint rather than a target to be approached as closely as possible.

Internal resistance determines how much a cell's terminal voltage drops under load and how much heat is generated internally during charge and discharge. Two cells with identical nominal capacity but different internal resistance will behave differently under high current, even though their datasheet capacity figures look the same.

Internal resistance is not fixed — it increases with cell aging, decreases at higher temperature (within limits) and increases sharply at low temperature. This is why the same battery can deliver full power on a warm day and struggle to start a motor on a cold morning.

When comparing cells from different suppliers, internal resistance measured under a consistent, stated test method (frequency, temperature, state of charge) is one of the more meaningful figures for predicting real-world voltage sag and thermal performance under load.

A complete requirements definition should be captured in writing before any cell or BMS is selected. Treating this as a formal specification document — rather than an informal conversation — reduces the risk of late-stage redesign and makes it far easier to compare quotations from multiple suppliers on a like-for-like basis.