Spot Welding Basics: Building Strong Interconnects
Why spot welding is superior to soldering for battery packs.
The gold standard for connecting lithium cells is spot welding. While many beginners are tempted to reach for a soldering iron, soldering directly onto the terminals of a lithium cell is dangerous. The heat from a soldering iron can damage the internal seals and plastic separators of the cell. Spot welding, by contrast, uses a massive burst of energy in milliseconds to create a weld without heating the bulk of the cell.
The Spot Welding Process
A spot welder works by passing a very high current (hundreds of Amps) through two copper electrodes into the nickel strip and the cell terminal. The resistance at the junction generates enough heat to melt the metals together instantly. Because the pulse is so fast (usually 5ms to 20ms), the heat doesn't have time to travel into the volatile chemistry inside the battery.
Choosing the Right Welder
For hobbyists, there are three main types: Capacitor-discharge welders (the most consistent), Transformer-based (heavy but reliable), and Battery-powered (portable but inconsistent). Avoid the ultra-cheap $20 USB-powered welders; they lack the power to create deep, reliable welds, leading to 'cold joints' that can snap off during vibration.
Pure Nickel vs. Nickel-Plated Steel
This is a critical choice. Pure nickel has much lower resistance and won't rust. Nickel-plated steel is cheaper but generates more heat and can corrode over time, potentially leading to high-resistance joints and fire. You can test your strips by grinding the surface; if it sparks, it's steel. Pure nickel does not spark. Our ampacity calculator assumes pure nickel for safety.
Safety and 'Blowouts'
Always wear eye protection! A 'blowout' occurs when the electrode makes poor contact, causing a spark that can burn a hole through the cell casing. This releases internal gases and is extremely dangerous. Keep your electrodes clean, sharp, and apply firm, even pressure before triggering the pulse. Never weld on the center of the negative terminal, as that is the thinnest part of the cell.
FAQ
Can I ever solder to a battery?
Only if the cell has a pre-welded nickel tab. You can safely solder to the tab, but never directly to the flat steel casing of the cell.
What is the best thickness for nickel strips?
0.15mm is the industry standard for most ebike and power tool packs. It provides a good balance between current capacity and ease of welding. 0.2mm requires a much more powerful welder.
How many spots per cell?
Typically 4 to 6 spots (2-3 pairs) per terminal. This provides redundancy and ensures that the electrical load is spread across multiple points, reducing the risk of a single weld point overheating.