How is a lithium-nickel battery pack assembled? The process involves spot welding and insulation
A lithium battery pack is an assembly of dozens or even hundreds of identical cells, connected by metal strips that are spot-welded, electrically isolated from each other, monitored by a Battery Management System (BMS), and protected by a casing. Every step is crucial: an undersized connection can overheat, a forgotten insulation can cause a short circuit, and a poorly maintained cell will eventually fail due to vibrations. This guide describes how a battery pack is constructed, so you can learn to recognize a well-made battery. This is not an instruction manual: assembling lithium cells is a job for a trained professional with the proper equipment.
The essentials
- The cells within a battery pack must be identical, of the same reference, and matched.
- They are connected by nickel strips, sometimes reinforced with copper, and are spot-welded rather than soldered with a soldering iron.
- The insulation between cells, groups, and the enclosure is just as important as the connections.
- The Battery Management System (BMS) is connected to each cell group and to temperature sensors.
- Vibration resistance, a robust enclosure, and thorough quality control are what make the difference in the long run.
1. Select and pair the cells
It all starts with high-quality cells, of the same type and ideally from the same batch. Before assembly, we check their voltage and, according to the workshop's methods, their internal resistance or capacity, to create homogeneous groups. A cell that deviates from the others will work harder, age faster, and limit the performance of the entire battery. The selection of cells is detailed in our comparison: Molicel P45B, Samsung 50S, P42A, or 40T.
2. Layout and Mounting
The cells are placed in plastic holders that maintain a consistent distance between them. This spacing prevents the cell casings from rubbing against each other and leaves a small air gap between them. The arrangement follows a series-parallel configuration: for example, 20 groups in series of 10 cells in parallel to create a 20S10P battery. On an electric scooter, the shape of the deck often dictates a custom layout, in one or multiple layers.
3. Connections: Nickel and copper
The cells are connected using metal strips. Pure nickel is the material of choice: it welds well and is resistant to corrosion. Nickel-plated steel, which is cheaper, conducts electricity much less effectively and generates more heat: it is not suitable for a high-power battery.
The strip width is calculated based on the current. For very high currents, we double the nickel on a copper strip, as copper is a much better conductor. The nickel serves as a soldering interface with the cell. A connection that is too thin will overheat with each acceleration, which causes the neighboring cells to age prematurely. The sizing principles are similar to those in our guide on power cables and connectors.
4. Spot welding
The strips are attached to the cells by spot welding, also known as resistance welding. Two electrodes press against the strip and deliver a very short and intense current pulse. The metal locally melts at the point of contact, creating a strong bond in a fraction of a second.
Why not a soldering iron? Because a traditional soldering process heats the cell for several seconds: the heat can damage the joint, the internal separator, or the cell's chemistry. Spot welding delivers a significant amount of energy, but it is highly localized and brief. Its quality depends on the machine's settings, the electrode pressure, and the condition of the surfaces: a weld that is too weak will come loose under vibrations, while a weld that is too strong can pierce the bottom of the cell.
5. Insulation
On a cylindrical cell, the positive terminal is a small button in the center, and the rest of the metal casing is connected to the negative terminal. A strip of nickel that slips or a vibration that wears down the casing can therefore create a direct short circuit. We place insulating washers on the positive terminals, insulating paper between groups of different voltages, and on exposed edges, and we check that the plastic casing of each cell is intact.
The insulation between the battery pack and the enclosure, which is often metallic on an electric scooter, is just as important. It's one of the first things you'll notice when opening a low-quality battery.
6. The Battery Management System (BMS) and Wiring
The BMS is connected to each cell group by a measurement wire, called a balancing wire. These wires must be soldered in the correct location, in the correct order, and protected from tearing. Temperature sensors are placed between the cells. The power output cables are soldered to the end tabs with an appropriate gauge, and then protected by a sleeve. The role of the BMS is explained in our guide on the BMS and balancing.
7. Maintenance, protection, and enclosure
A scooter battery is subject to constant vibrations and shocks. Therefore, the battery pack is held in place by foam or shims, protected by a heat-shrinkable sleeve or an enclosure, and then installed in the deck or in a dedicated housing. Depending on the machine, we use an aluminum housing, as in our Performance 80V battery for Nami Burn-E, a 3D-printed enclosure, or a custom-designed casing made for the specific machine.
Final checks
- The voltage of each group of cells must be uniform
- BMS verification: reading all groups, checking for active protections
- Charge and discharge testing, and monitoring of connection temperature
- Insulation check between the battery pack and the enclosure.
| Sign of a well-made battery | Warning sign |
|---|---|
| Identifiable brand cells | Unmarked or renamed cells |
| Pure nickel, copper for high current applications | Thin strips of nickel-plated steel |
| Consistent welds, with multiple points of contact | Soldering the cells using a soldering iron |
| Washers and insulating paper everywhere | Perfectly compatible for direct contact with tires |
| Balance wires secured, BMS mounted | Loose wires, floating BMS |
Workshop tip
Do not open a battery to "see" inside or to attempt repairs yourself: even a tool that slips between two cell groups can cause a high-current short circuit. All of our scooter batteries are assembled in our workshop in France, using only branded cells. The risk of a faulty assembly is detailed in our guide on lithium battery fires.
Frequently Asked Questions
Can a battery that has been spot-welded be repaired?
Yes, a professional can replace a group of cells or a connection by removing the old solder joints. However, working on a charged battery pack requires a careful approach and specialized equipment.
Why not use salvaged cells?
Their history is unknown: capacity, internal resistance, and actual condition vary from cell to cell. A pack of mismatched cells becomes unbalanced and ages quickly.
Is nickel sufficient for a very powerful battery?
For very high currents, nickel alone would overheat or require extremely large cross-sections. It is therefore combined with copper, which carries the majority of the current.
Should you ventilate the battery of an electric scooter?
A battery housed in a closed deck is primarily cooled through conduction to the chassis. The most effective approach is to size the battery so that it generates minimal heat: this involves using cells suitable for the current draw and using generous wiring.