Battery cells get spot welded, not soldered, because a soldering iron has to hold heat against the terminal long enough to keep solder molten, and that heat travels straight into the cell where it can damage the separator and push a lithium cell toward thermal runaway. A resistance spot welder fires a current pulse lasting a few thousandths of a second, fusing the nickel strip to the terminal before the cell has time to heat up. That is the core reason every production pack and most serious DIY pack rebuilds use a tab welder.
This page covers why soldering is discouraged, the two welder types you will actually shop for (capacitive-discharge and transformer), how nickel strip thickness drives your pulse settings, and how to peel test a weld so you know it held. Building lithium packs carries real fire and explosion risk, and the work below is general process information, not a substitute for the cell and equipment manufacturers’ instructions. You build a pack at your own risk.
Why You Don’t Solder 18650 and 21700 Cells
The cells in e-bike packs, power-tool packs, and DIY power banks are almost always 18650 (18 mm by 65 mm) or 21700 (21 mm by 70 mm) lithium-ion cylindrical cells. They store a lot of energy in a small can, and that is exactly why heat is the enemy.
A soldering iron at the terminal has to overcome two things working against it. First, the steel cell can spreads heat sideways into the whole cell body. Second, to get solder to wet and flow you have to hold the iron there, which means dwell time measured in seconds. During those seconds the heat climbs into the internal jelly roll. The polymer separator that keeps the positive and negative layers apart is the most heat-sensitive part of the cell. Cook it, and you create an internal short. An internal short in a charged lithium cell is the classic ignition point for thermal runaway, where one cell vents, catches, and dumps its heat into its neighbors in a chain reaction.
Soldering also fights the terminal itself. The negative end of most cells is nickel-plated steel, which sheds heat fast and resists tinning. People who solder cells anyway tend to crank the iron hotter and lean on it longer to compensate, which is the opposite of what you want.
Resistance welding sidesteps all of it. The weld happens at the interface between the strip and the terminal, the current pulse is over in milliseconds, and the bulk of the cell never gets the chance to warm up. You feel a welded terminal right after the weld and it is cool to the touch.
How a Battery Tab Spot Welder Works
A tab welder is a small resistance welder. The general principle is the same one covered in the resistance and spot welding overview: two electrodes press against the work and a high-current pulse passes through, and resistance at the joint generates the heat that forms the weld. The difference is scale and timing. Battery work uses two closely spaced electrodes on the same side of the strip (an opposed-tip or “dual probe” arrangement), low voltage, and a pulse measured in milliseconds.
Because both probes sit on the same face, the current has to travel down through the strip, across the interface, and back up. The two welds form under the two probe tips. Probe spacing matters: too close and the current shortcuts across the surface between the tips instead of diving into the weld interface, giving you weak welds and a lot of sparking.
You will run into two machine families.
Capacitive-Discharge (CD) Welders
A CD welder charges a bank of capacitors and dumps the stored energy into the weld in a single fast pulse. The discharge is short and sharp, which suits thin nickel strip and is forgiving of a weak wall outlet because the capacitors do the heavy lifting, not the mains. Small kit-style CD welders and the popular all-in-one units (the kSger, Malectrics-style, and Sunkko-style boards people build into projects) fall here. They are the common entry point for 0.1 mm to 0.15 mm pure nickel.
The limit of a small CD unit is total energy. Once you move to thicker strip or heavier nickel-plated bus work, a single capacitor dump may not deliver enough energy, and you either get cold welds or have to make multiple passes.
Transformer (AC) Welders
Transformer welders step mains voltage down to a few volts at very high current and time the pulse with the welder’s controller. They deliver more sustained energy than a small CD unit, which is what lets them weld thicker strip. The familiar Sunkko 709A-class machines are transformer welders. Many offer a double-pulse mode, where a short pre-pulse cleans and seats the contact and a second main pulse forms the nugget. Double pulse tends to give more consistent welds on plated and slightly oxidized strip.
The trade-off is that transformer welders pull hard from the wall during the pulse and are heavier and bulkier. For pack building beyond thin strip, that is usually a fair trade.
| Capacitive-Discharge (CD) | Transformer (AC) | |
|---|---|---|
| Energy source | Capacitor bank dumped in one pulse | Stepped-down mains, controller-timed pulse |
| Comfortable strip range | 0.1-0.15 mm pure nickel (unit dependent) | 0.1-0.2 mm pure nickel, often double-pulse |
| Demand on wall outlet | Low (capacitors buffer the draw) | High during each pulse |
| Typical form | Compact boards, kit and portable units | Benchtop boxes (Sunkko 709-class) |
| Best for | Small packs, thin strip, portable use | Larger packs, thicker strip, repeat work |
Prices on these units move around and the import market churns models constantly, so treat any figure as a snapshot at time of writing rather than a fixed number. Buy on whether the machine can weld your strip thickness, not on a spec-sheet amp rating.
Nickel Strip Thickness Versus Pulse Settings
Strip thickness is the variable that drives everything else. You pick strip for the current your pack section will carry, then you set the welder to weld that strip without blowing through it.
A rough working frame: thin pure nickel (0.1 mm) welds at low energy and low pulse duration, and as you go thicker you need more energy or more pulses. Pure nickel and nickel-plated steel behave differently. Plated steel has higher electrical resistance, so it heats more readily at the joint and can feel “easier” to weld at a given setting, but pure nickel carries more current for the same cross section and is the better conductor for the pack. Builders often run pure nickel for current-carrying paths and reserve plated steel for low-current spots, or stack pure nickel for high-current packs.
| Pure nickel strip | Common pack use | General welding behavior |
|---|---|---|
| 0.1 mm | Low-current packs, light tool packs | Welds at low energy, easy to overdo and burn through |
| 0.12-0.15 mm | General e-bike and power-tool packs | The sweet spot for most hobby welders, CD or transformer |
| 0.2 mm | High-current packs, parallel bus links | Needs a stronger transformer or double-pulse unit, or stacked thinner strip |
These are general starting points, not a recipe. Every machine, electrode condition, and strip lot welds a little differently, so dial settings in on scrap. Start low, fire a weld, peel test, and step the energy or pulse up until welds hold without expelling sparks or punching a hole. Two telltales of too much energy: a shower of sparks at the tips on every shot, and a hole or heavy dimple punched clean through the strip. Too little energy gives you welds that peel off whole.
A few habits that make weld quality consistent regardless of machine:
- Clean, sharp electrode tips. Pitted or mushroomed copper tips spread the current and give cold, sparky welds. Dress them flat and clean.
- Firm, even probe pressure straight down. Light or angled pressure causes arcing instead of welding.
- Two welds per terminal minimum, spaced apart. Redundancy matters when a single weld can fail.
- Weld the strip flat to the terminal with no gap. A gap turns the weld into an arc.
Peel Testing: How You Confirm the Weld Held
Do not trust a battery tab weld by look. Confirm it by destroying a sample. Take a scrap cell or a steel slug, weld a length of your actual strip to it at your chosen settings, then grab the strip with pliers and peel it back.
A sound weld tears metal out of the parent surface. You will see small chunks of nickel left welded to the terminal where the dimples were, or the strip itself will tear before the welds let go. That is fusion. A bad weld lets the strip lift off the terminal with the weld dimples intact and shiny, meaning the metal never fused, it just stuck. Re-peel test any time you change strip thickness, switch from pure nickel to plated, change cell brand, or touch the machine settings.
This is the same logic behind verifying any resistance weld, and it mirrors the peel-test approach used for tacks and small assemblies in micro welding and small-part work. The destructive test on a coupon is the only honest check you have without a lab.
The Fire and Explosion Hazards You Have to Respect
Lithium cells are not benign. A shorted or overheated lithium cell can vent, ignite, and propagate to neighboring cells, and the fire is hard to put out because the cell supplies its own oxidizer. Treat every charged cell as a stored energy hazard the entire time you work.
The most common way DIY builders cause trouble is shorting a strip across a cell’s own terminals or across adjacent cells. A loose strip or a dropped tool that bridges positive to negative dumps the cell’s full short-circuit current in an instant, which heats metal red-hot and can start a fire. Specific practices that reduce that risk:
- Insulate your tools and your bench. Use a non-conductive work surface and keep bare metal away from terminals.
- Never let a free end of nickel strip flop across a terminal. Weld one end and control the other.
- Keep cells at a moderate state of charge while building when you can, since a fully charged cell stores more energy to release if something goes wrong.
- Inspect cells before use. The OSHA Safety and Health Information Bulletin “Preventing Fire and/or Explosion Injury from Small and Wearable Lithium Battery Powered Devices” lists bulging, cracking, hissing, leaking, rising temperature, and smoking as signs to pull a cell or device from service immediately. A cell showing any of those goes in a safe location away from combustibles, not into your pack.
- Set up for hot work. You are making sparks near an energetic chemistry, so apply the same fire precautions you would for any spark-producing task: a clear, non-combustible work area and a suitable extinguisher within reach, in line with the hot-work practices in welding fire prevention and the general guidance of ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes. Have a plan for a venting cell, which means a way to move a burning pack outdoors away from people and structures.
No technique makes a pack inherently safe, and nothing here guarantees a sound or long-lived pack. Pack design, cell matching, the battery management system, and charging are their own subjects and are out of scope for this page. A BMS is not optional on a real lithium pack, but how to spec and wire one belongs in its own guide.
Common Battery Tab Welding Problems
Welds peel off clean. Not enough energy, dirty or oxidized strip, electrode tips too far apart, or a gap between strip and terminal. Step energy up, clean the strip and tips, and press the strip flat before firing.
Holes punched through the strip. Too much energy or too long a pulse for that strip thickness. Back the setting down and re-test. Thin 0.1 mm strip is easy to over-weld.
Heavy sparking on every shot. Probes too close together (current shortcutting across the surface), light or uneven pressure, or worn tips. Widen the spacing slightly, press firmly and straight down, and dress the electrodes.
Cell gets warm during welding. Pulses too long, too many retries on the same spot, or you are actually arcing instead of welding. A correctly welded terminal stays cool. If a terminal heats up, stop and fix the setup before continuing, since heat in the cell is the exact failure mode you are spot welding to avoid.
One weld of a pair holds, the other does not. Uneven probe pressure or one tip in worse condition than the other. Even up pressure and dress both tips to match.
For the broader process family this sits inside, the resistance and spot welding guide covers spot, seam, and projection welding on sheet metal, which share the same current-time-pressure fundamentals scaled up.