Introduction: Knowing what sits inside a CATL NMC module helps technicians judge current paths, monitoring wiring, insulation, and safe handling limits before repair work begins.
A Nissan Leaf pack is not one large battery. It is a string of individual modules, and each module carries its own cells, its own current path, and its own monitoring wiring. When a pack loses capacity, the module is the unit that gets tested, swapped, and wired back into the group, which makes module structure the practical starting point for anyone learning EV battery repair. The CATL 4S1P 177Ah NMC module is a good example to study because its published features cover every basic structural element: cells, welded busbars, sensor wiring, a sampling interface, insulation, terminals, and a housing. this guide explains what each of those parts does on the bench and why it matters when a Leaf pack is being rebuilt.
NMC cathodes use nickel, cobalt, and manganese, and that blend gives a high energy density for the weight carried. In a 4S1P layout, four cells sit in series to set the voltage while a single cell per parallel group sets the capacity. Four cells at roughly 3.7V nominal land at the 14.8V module rating, and one 177Ah cell per group gives the full 177Ah. Multiply voltage by capacity and the result is about 2.6kWh in a 12.5kg package. The cell-level internal resistance is published at about 0.28mΩ, which is low enough to support a 1C charge and a 3C discharge, and that is the number that shapes everything else inside the module. A 3C discharge on a 177Ah module means roughly 530A moving through the internal joints for short bursts. That is the reason the structure looks the way it does: thick conductors, fused joints, and a rigid housing are not decoration, they are what keeps the current path stable under load. Research on lithium-ion cell production also shows that C-rate capability, internal resistance, and thermal behavior are designed together, so a module that looks tidy on the outside can still hide a compromised joint inside. Repair work starts with understanding that the module is an active, high-current assembly rather than a passive box of cells.
The busbar is the conductor that ties the cells together and carries the module's current out to the terminals. In this module the connections are laser welded, meaning the joint is fused rather than clamped or bolted. A fused joint has very little contact resistance, so less energy turns into heat inside the module, and the flat, uniform weld keeps the current path predictable across all four series positions. From a repair standpoint the weld is the part of the current path to watch most closely. A raised or pitted weld, a bent bar, or a scorch mark points to mechanical or thermal stress that will reappear under load, so technicians generally run a finger along the bar and check flatness before installing a module.
Cells do not manage themselves; the battery management system does, and it needs data to work with. Pressure-welded sensor wires run from each cell group to a sampling interface on the module, where the pack harness connects the module to the BMS. Through those wires the BMS reads the voltage of each group and, in many designs, temperature as well, then uses that information to balance the groups, limit current draw, or trigger protection before a weak group drags the whole string out of range. On the bench, the sampling connector and the wire landing points deserve the same attention as the high-current parts. Bent pins, a loose connector, or a wire that has been pulled will send the BMS a wrong voltage reading, and that usually shows up as a fault that looks like a failing cell when the cell itself is fine. Handle the connector by its housing, never by the wires.
Insulation is what separates a working module from an accidental short circuit. This module uses the original PVC insulation wrap over the cell stack and its connections, keeping live parts away from a metal rack, a neighboring module, or a dropped tool. Terminals are the deliberate connection points, and the matte finish on factory terminals makes the contact surface easy to inspect and easy to tell apart from one that has been bolted and re-torqued many times. The housing does two jobs at once: it holds the cells mechanically and it defines the mounting envelope. The VDA 355 footprint of 355 × 150 × 110 mm is what lets the module drop into a pack built around that standard, and the housing absorbs the mechanical load so the cells do not. Electrical, mechanical, and thermal abuse concepts sit behind international test frameworks such as IEC TS 62271-210, which is why insulation, terminal condition, and housing integrity are usually treated as one topic in module handling rather than three separate ones. They interact: a scratched PVC wrap, a burred terminal, and a cracked housing edge all raise the chance of a fault during assembly. Public structure descriptions cover the typical published features of a module like this one, and exact internal spacing can vary slightly between production batches, so the inspection habit matters more than a memorized drawing. An EV battery manufacturer sets those details during assembly, and they are exactly what someone on the receiving end can check with their own eyes.
A 12.5kg module sounds manageable, and it is, with two hands, a level grip, and a plan for where it lands. The handling rules that matter in a workshop come straight from the structure. Keep the module resting on its housing rather than on the terminals or the sampling connector, set it on a non-conductive surface, use insulated tools near the terminals, and stop it from sliding against other modules so the PVC wrap stays intact. Before a module goes into a pack, technicians generally look at three things: busbar flatness, terminal condition, and sensor connector seating. Published factory data for this module puts the static voltage spread at 3mV or less, which gives a useful baseline for comparing a group of modules before they are wired together in a string. The module is the unit a Leaf repair plan is built around, but a working pack also needs series connections between modules, mounting hardware, and BMS integration. Twenty-four of these modules make up a 62kWh string, and the arithmetic is simple while the assembly discipline is not: consistent modules, correct series wiring, and a BMS that can read every sampling interface in the group. A workshop that regularly buys through EV battery suppliers, or that compares a Nissan Leaf battery module supplier on more than price, will find those details easier to judge than a specification sheet alone. Whether the modules arrive as CATL battery modules from a distributor or as part of a wider Nissan Leaf battery module order, the internal structure is what determines how the rebuild behaves afterward.
Module internals decide most of the practical questions in a Nissan Leaf battery repair: how much current the assembly can move, where voltage data comes from, and which parts need protection on the bench. Cells, busbars, sensor wiring, terminals, insulation, and housing each have a defined job, and knowing those jobs makes it easier to spot a damaged module and to understand what has to happen before the string runs again. The structure, in the end, explains the numbers.
A：Four CATL NMC cells in a 4S1P arrangement, laser welded busbars that tie the cells together and lead out to the terminals, pressure-welded sensor wires ending at a sampling interface, a PVC insulation wrap, and the housing that holds everything in place. Each part has a role: cells store energy, busbars carry current, sensor wiring feeds the battery management system, insulation prevents short circuits, and the housing takes mechanical load while setting the VDA 355 mounting envelope.
A：A 177Ah module can charge at 1C and discharge at 3C, which means hundreds of amps pass through the internal joints for short periods. Laser welding fuses the conductor to the cell terminal and produces a low-resistance joint with a flat, predictable current path. Lower joint resistance means less heat inside the module and steadier voltage under load, which is why weld appearance and busbar flatness are among the first things worth inspecting on a module.
A：Pressure-welded sensor wires run from each cell group to the sampling interface, where the pack harness links the module to the battery management system. The BMS reads group voltage, and often temperature, through those signals, then balances the groups and limits current when something drifts. Clean wire routing and a fully seated connector keep those readings accurate, so an unseated plug can look like a cell fault even when the cells are healthy.
