18650 Spot Welding Strip: The Complete DIY Guide

A hobbyist builds a 10S e-bike pack from salvaged 18650 cells, welds it with cheap strip at full power, and watches the first hill climb melt the shrink wrap. Nine times out of ten the failure traces back to one overlooked part: the nickel strip welded across the terminals. This guide explains how to choose and correctly weld that strip so your pack survives.

What Is an 18650 Spot Welding Strip and Why Does It Matter?

If you have opened a dead laptop battery pack, you have seen it: a thin ribbon of metal stitched across a dozen cylindrical cells, held by small dimples rather than solder. That ribbon is the 18650 spot welding strip, and in a DIY pack it does two jobs at once: it holds the cells together, and it is the only path your current flows through. Undersize it, contaminate it, or weld it badly, and everything downstream fails.

What “18650” Actually Means

The name is a size code, not a chemistry code: an 18650 cell is 18 mm in diameter and 65 mm long and may be lithium-ion (NMC, NCA, LFP), NiMH, or LiFePO4. It started in laptops and power tools and now turns up in e-bikes, drills, vacuums, solar storage, and EVs. Because the format is standardized, builders salvage cells from old packs or buy new ones and build almost any voltage or capacity they need.

A single cell tops out around 4.2 V and empties near 2.5–3.0 V, so cells are wired in parallel to gain capacity, then parallel groups are wired in series to gain voltage. A 10S3P e-bike pack is ten groups of three cells: 36 V nominal, each group tied by nickel strip and linked in series by more strip. Every connection in the pack is made that way.

What the Strip Physically Is

An 18650 spot welding strip is a thin ribbon of either commercially pure nickel or nickel-plated steel, usually 0.05 to 0.30 mm thick and 5 to 10 mm wide. It comes on spools, in flat precut lengths, or as H, C, and Z bends that step from a positive terminal down to the next cell’s negative can.

The strip joins the terminal by resistance spot welding: two electrodes squeeze it against the cap, a short high-current pulse passes through, and contact resistance fuses a tiny nugget without heating the whole cell. Nickel is chosen because it is weldable, conductive, corrosion resistant, and available at battery-grade purity.

The gold standard is NASA’s resistance spot welding process specification for battery assemblies, which requires commercially pure nickel 200 or 201 tab material and no fewer than two weld spots per connection. NASA wrote that spec because the strip and its welds decide pack reliability, and academic work on micro-resistance spot welding of lithium-ion cells for electric-vehicle battery packs has mapped the same parameters in depth.

Why the Strip Is the Weakest Link

Consider what the strip survives. A 10S e-bike pack drawing 30 A on a climb pushes full group current continuously through each group’s strips, plus acceleration spikes, while a laptop pack may see momentary 100 A+ pulses and the whole strip flexes on every ride.

Now the failure modes: nickel-plated steel sold as “pure nickel” has roughly five times the resistivity of pure nickel and runs hotter; strip that is too thin acts like a fuse element; a single cold weld spot adds contact resistance that compounds across the pack; and oily shear cuts leave contamination that blocks nuggets. In each case the chemistry is fine, but the strip is what melts, oxidizes, peels, or overheats and cooks the cell’s shrink wrap from the outside in.

That is the failure from the opening of this guide: the pack survived bench testing and died on its first hill climb because the strip could not carry the current.

Why Spot Welding, Not Soldering

The short answer: soldering holds a 300+ °C iron on a terminal long enough for solder to wet the nickel, and that heat soaks into the cell, shrinking the separator, stressing the safety vent, and leaving corrosive flux residue behind.

Spot welding makes the same metallurgical bond in milliseconds, localized a few millimeters across, so the terminal never gets hot enough to damage the cell. Studies show terminal temperatures rising only tens of degrees during a proper weld, versus hundreds for hand soldering. That speed and low heat input is why every commercial pack manufacturer and every credible DIY guide spot welds.

Answering the Principal Question Up Front

So what should you buy? For most builders: 0.15 or 0.20 mm commercially pure nickel strip, 6 to 8 mm wide, from a supplier that states its resistivity and grade. That covers light and moderate packs up to roughly 15–20 A per parallel group. High-current builds step up to 0.25–0.30 mm, add width, or move to copper-nickel stacks. Nickel-plated steel suits only low-current packs like small power banks, and only if you know it is plated steel.

Everything that follows builds on that starting point: materials, sizing, welders, settings, technique, and catching mistakes before they burn a pack down.

Why Spot Welding Beats Soldering for 18650 Cells

Ask a room full of battery builders why they refuse to solder 18650 cells and you will get a dozen versions of one answer: “it gets the cells too hot.” True, but not the whole story. The real difference comes down to how each process makes heat, and where that heat is allowed to go.

How Resistance Spot Welding Actually Makes Heat

Resistance spot welding is sometimes called cold welding, which is a misnomer, since the joint briefly reaches a white-hot glow. The trick is location. Two electrodes clamp the nickel strip against the terminal and push a large current through them for a few milliseconds. The strip and electrodes have very low resistance, so almost no heat forms inside them. But the boundary between strip and terminal, however clean, has real contact resistance, and heat generated there follows the classic Joule relationship:

Q = I² × R × t

Because that interface resistance is many times higher than the bulk metal, the energy concentrates there and nowhere else. The two surfaces briefly fuse into a tiny weld nugget while the surrounding terminal stays close to ambient temperature. Doubling the current quadruples the heat, which is why these machines deliver hundreds of amps for pulses measured in milliseconds rather than seconds.

Why Soldering Is a Different Problem

Soldering inverts the whole equation. A soldering iron has to heat the terminal, the solder alloy, and the strip together to roughly 300–350 °C and hold it there for several seconds so the solder wets the nickel properly. Because heat flows freely through metal, it conducts straight through the terminal wall into the cell can, and from there into the electrolyte, separator, and jellyroll.

The separator in a lithium-ion cell is a thin polymer film that shuts down if it deforms or melts. Even partial thermal damage can thin it locally, and a thinned separator is exactly where dendrites find a path. Heat also drives electrolyte decomposition and gas generation. The cell may test fine the day you build the pack, then fail months later under load. This is a slow failure mode, which makes it worse, not better.

Spot welding flips the ratio in the builder’s favor: a very large current, a very small resistance, and a very short time. The energy is dumped before it has a chance to conduct anywhere.

What Published Research Says About Cell Temperature Rise

The numbers back this up. A study on automatic battery pack spot welding found that pushing a single heating pulse beyond about 120 ms raised cell surface temperature by roughly 5 °C, and the researchers adopted a conservative first-pulse limit of 80 ms specifically to protect 18650 cells (Energy Journal, 2017). Even at the “too long” end, the rise was single-digit degrees, not hundreds.

Compare that with a terminal sitting at 300 °C or more for several seconds. The thermal budgets are not in the same league.

Researchers at the University of Warwick took the question further, running in-depth micro-resistance spot welding tests on 18650 cells intended for electric vehicle packs. Under optimized parameters, their nickel-tab welds showed no intermetallic compounds at the interface, confirming a clean metallurgical bond with no brittle reaction layer and no evidence of cell degradation (WRAP, University of Warwick, published in International Journal of Advanced Manufacturing Technology). Done correctly, spot welding does not merely hurt the cell less than soldering; it leaves the cell effectively untouched.

Broader safety analyses of battery assembly, including NASA’s published procurement requirements for lithium-ion battery work, treat welding parameters as controlled processes precisely because uncontrolled heat input is a known risk to cell integrity (NASA current requirements document).

The Practical Takeaway

Spot welding wins for three compounding reasons:

• Heat is generated at the joint, not in the cell. The interface does the work; the chemistry stays cool.
• The timescale is milliseconds. Heat has no time to conduct into the jellyroll before the pulse ends.
• The bond is metallurgical, not adhesive. A weld nugget carries current through shared metal, while a solder joint depends on a filler alloy with meaningful resistance of its own.

That last point matters more than people expect. Solder is a resistor in your current path, and every milliohm of it becomes heat under load. A spot-welded nickel joint pushes that resistance down toward the noise floor.

The trade-off is equipment. Soldering needs a $20 iron; spot welding needs a welder, strips, and a bit of practice. That investment is covered in the equipment guides later in this article, but the physics above is why essentially every commercial pack, and every serious DIY build, is welded rather than soldered.

Pure Nickel vs Nickel-Plated Steel Strip: Which Should You Use?

If you have shopped for 18650 nickel strip online, you have probably stared at two listings that look almost identical, priced about three times apart, and wondered which one you actually need. The strip is the busbar of your pack: every amp your motor draws flows through it, every volt of sag you feel starts at its resistance, and every gram of heat it makes lands right next to a cell vent. The two materials behave differently in all of those respects, so choosing between them is really choosing how much current, abuse, and service life you expect from the pack.

What Is Actually Inside Each Strip

Battery-grade pure nickel strip is Nickel 200 or 201, a wrought grade with roughly 99.6% nickel or better, commonly supplied as cold-rolled sheet to ASTM B162. It is one material from edge to edge, so a cut edge is still nickel and still resists corrosion.

Nickel-plated steel is a sandwich in disguise: a cold-rolled steel core with about 2 microns of nickel plated onto each face. The plating exists to make the steel weldable and to keep the surface bright, not to carry the current. Because the steel core has substantially higher resistivity than nickel, resistance losses scale with the path the current takes, which is why plated strip heats more under load. That is the fundamental reason the two materials are not interchangeable, as the engineering discussion of why nickel plating is used at all makes clear.

The Ampacity Rule of Thumb

A practical sizing guide used across the DIY community is about 10 A per square millimeter of cross-section for pure nickel, and about 7 A/mm² for nickel-plated steel, on a continuous basis with the strip running in open air. Cross-section is simply width in mm multiplied by thickness in mm.

Strip (thickness × width)	Cross-section	Pure nickel (~10 A/mm²)	Plated steel (~7 A/mm²)
0.15 × 7 mm	1.05 mm²	~10.5 A	~7.35 A
0.15 × 10 mm	1.50 mm²	~15 A	~10.5 A
0.20 × 8 mm	1.60 mm²	~16 A	~11.2 A

Those numbers are conservative starting points, not ratings. Spot weld nuggets, terminal contact resistance, weld count, ambient temperature, and enclosure airflow all shift the real limit, and a strip buried under heat shrink runs hotter than one in open air. When a connection sits near the limit, add width or step up thickness rather than trusting the optimistic end of the range.

Corrosion, Weldability, and Cost

Pure nickel welds cleanly, holds up in humid environments, and does not rust where you cut it. Plated steel is easier on the wallet, welds well while the plating is intact, but the exposed steel at every cut edge and scratch becomes a corrosion site over years of service. In sealed, dry packs that rarely matters. In a bike pack exposed to weather, it can.

The price gap explains the market: pure nickel typically runs about three times the cost of plated steel, which is why plated strip dominates cheap tool packs, budget powerwalls, and low-drain applications, while e-bike, drone, and power-tool builders pay up for pure nickel.

Three Tests That Reveal What You Bought

Sellers mislabel strip constantly, so verify a new roll before it goes into a pack:

• Salt-water test: polish or scratch a small patch, submerge it in salt water for 12 to 24 hours, then inspect. Plated steel rusts visibly at the scratch; pure nickel does not.
• Magnet test: steel is strongly magnetic, pure nickel only weakly so. A strip that snaps to a magnet like a paperclip is steel.
• Resistance comparison: measure a short, identical length of your strip against a known sample with a milliohm meter. Plated steel reads noticeably higher.

Which One Should You Buy?

• Pure nickel: e-bike and scooter packs, cordless tool rebuilds, any pack drawing sustained current, anything built to last outdoors, and every build where you want a trustworthy baseline for weld settings.
• Nickel-plated steel: LED lighting banks, backup and solar storage with modest discharge, robot and hobby packs with short bursts, and tight-budget builds where you compensate by using a wider strip.

A sensible middle path: buy a small roll of each. Use pure nickel for the main series and parallel bridges that carry real current, and plated steel for balance leads, sense wires, and low-current jumpers where the savings are free. Whatever you pick, size it with the ampacity table above and verify the material with a quick test before committing a whole pack to it.

Choosing Strip Thickness, Width, and Shape

After choosing between pure nickel and nickel-plated steel, the next decision is geometry: how thick, how wide, and what shape. These three choices set how much current your pack carries, how strong each weld is, and whether your spot welder can form a proper nugget at all. Get them wrong and even good cells underperform.

Standard Thicknesses and What They Are For

Commercial nickel strip for 18650 work comes in a predictable range of thicknesses. Each one has a realistic job:

Thickness	Typical use	Weldability notes
0.10 mm	Low-current packs, DIY welders with weak output	Welds easily but burns through fast; thin and fragile
0.15 mm	The default for most DIY 18650 packs	Good balance of strength, conductivity, and ease of welding
0.20 mm	E-bike packs, moderate to high current	Needs a welder with real current capability
0.25–0.30 mm	Powerwalls, high-drain builds	Difficult for cheap AC welders; often needs double pulses

0.15 mm pure nickel is the standard starting thickness for most DIY 18650 packs. It carries the current a typical parallel group demands, penetrates under a mid-range welder, and stays flat across terminals. For high-current e-bike and powerwall builds, step up to 0.2–0.3 mm, or weld two layers of 0.15 mm strip together for a similar cross-section without buying new material. Research on micro-resistance spot welding of lithium-ion cells confirms thickness shifts the weld window, since a thicker strip absorbs more heat before the nugget forms (Warwick in-depth evaluation of micro-resistance spot welding).

Width: Wider Is Usually Better, Up to a Point

Strip width is measured across the face that sits on the terminal. Common widths run from 5 mm to 10 mm, with 6 mm and 8 mm the most popular. Wider strip means more cross-sectional area, lower resistance, less heating under load, and more room to place multiple weld points. The limits are cell spacing and cost: on a tightly packed matrix, an 8 mm strip may overlap onto adjacent cell vents, which you never want. A good rule is to use the widest strip that leaves at least 1–2 mm of clearance from the neighboring cell’s positive terminal rim.

Solid Versus Slotted Strip

This is one of the most misunderstood choices in pack building. On a solid strip welded with two spots side by side, much of the welding current takes the shortcut straight across the metal between the electrodes instead of flowing down through the terminal, leaving a weaker nugget.

That is why slotted strip exists: the gap breaks the shunt path so nearly all the current goes where you want it. The trade-off is a small loss of cross-section and slightly higher resistance along the current path, which matters little in low-drain packs but should be weighed in high-current designs.

Pre-Cut Shapes: H, C, Z and Ladder Strips

Pre-cut shapes exist to save time and improve consistency:

• H-shape – two pads joined by a narrow bridge; the bridge can sit over the gap between cells.
• C-shape – an open-sided link, easy to trim and position on staggered layouts.
• Z-shape – steps up or across, useful for offset series connections.
• Ladder strip – one long piece with regular tabs, ideal for welding a whole parallel group in a single pass.

Ladder strip speeds up assembly more than anything else on this list, but it carries a hidden risk: the narrow rung between weld points is often the ampacity bottleneck of the whole pack. Current from every cell in the group funnels through that thin section, so a ladder strip sized for a laptop pack can quietly overheat in a power tool build. Always size ladder strip by its narrowest point, not its overall width.

Matching Strip Choice to Pack Current Demands

Start from your worst-case continuous current and work backward. A modest 10S e-bike pack pulling 20 A peak is comfortable with 0.15 mm pure nickel at 8 mm wide. A 4P powerwall discharging at 100 A is not; there you want 0.25–0.30 mm strip, doubled layers, or a copper-nickel stack (covered later in this guide). A 2025 experimental study on 18650 joints found a 0.15 mm nickel strip with six weld points reached a maximum shear force of about 2380 N, while 0.12 mm strip produced a more homogeneous weld microstructure with fewer micro-cracks (journal study on 18650 spot weld geometry) — a useful reminder that thickness changes both mechanical strength and metallurgical quality.

The honest takeaway: buy 0.15 mm pure nickel in two widths (6 mm and 8 mm), keep solid strip for practice and low-current work, and invest in slotted or pre-cut shapes only when your pack’s current demands justify them.

Welder Types and Equipment for Welding Nickel Strip to 18650 Cells

The strip only performs as well as the machine driving current through it. Fortunately, you do not need a factory setup to weld nickel to 18650 cells. The hobby market now spans everything from a $50 transformer box to five-figure CNC stations, and the differences between them matter more than the price tags suggest.

AC Transformer Welders

The simplest spot welders are modified AC transformers, usually salvaged from microwave ovens or built from a pair of wound cores. They push a low-voltage, high-current sine wave through the tips on every half-cycle. Because the current is AC and comparatively hard to modulate, timing control is coarse: you get a fixed number of AC cycles, not a precise millisecond pulse. That makes them serviceable for 0.10–0.15 mm nickel-plated steel, but frustrating on thicker pure nickel, where a few milliseconds too long burns through the strip and a few too few leaves a weld that peels. Their virtues are cheapness, simplicity, and repairability with parts you can find anywhere.

Capacitor and Inverter DC Welders

The dominant class for DIY pack builders is the capacitor or supercapacitor inverter welder. These units rectify mains power, store the energy in a capacitor bank, and dump it as a tightly controlled millisecond pulse through an inverter bridge. A typical hobby unit delivers up to 2500 A and 14.5 kW peak, which is plenty to weld 0.3 mm pure nickel onto 18650 terminals. You dial pulse length in single milliseconds, save preset programs, and switch polarity for different tip arrangements. For most readers building e-bike packs or powerwalls, this is the sweet spot of cost, control, and repeatability.

DIY Car-Battery Controllers

At the other end sits the home-built route: a car battery as the current source, a MOSFET board as the switch, and a microcontroller for timing. The well-known open-source Arduino spot welder design runs off a 12 V 40 Ah car battery and produces a configurable double pulse in which the first pulse is 12% of the main pulse duration, giving a pre-heat that improves consistency. Community builds of this type are documented on sites like the Pumping Station One battery spot welder wiki. They are genuinely capable machines, but they demand electronics skill, a safe enclosure for the FET bank, and respect for the fact that a car battery can deliver thousands of amps into a short circuit.

CNC and Production Machines

On the industrial end, servo-driven micro-resistance spot welding (micro-RSW) systems control force, displacement, and current in closed loop. Published research on welding 18650 tabs found that weld current is the most significant parameter affecting weld strength, followed by weld time, and that round electrode tips outperformed other geometries for electrode life and resistance to oxidation (International Journal of Advanced Manufacturing Technology, 2022). Those findings translate directly to your bench: control current first, time second, and keep the tips round and clean.

Electrode Material, Tip Geometry, and Pens vs. Fixed Heads

Electrodes are almost always copper alloy, with class 1–2 chromium-copper or beryllium-copper chosen for hardness at temperature. Tip geometry matters: round, slightly domed tips concentrate current predictably and last longest, while pointed tips dig in and leave craters. As a working range, tip spacing of roughly 5–8 mm on an 18650 terminal is typical, and it should sit inside the terminal’s diameter with margin from the vent and rim.

Two form factors exist:

• Welding pens — handheld, spring-loaded, flexible cables. Best for irregular packs and repair work, but tip pressure depends on your grip.
• Fixed heads / foot pedal — electrodes mounted in a stand with a foot switch or pedal. More consistent force, better for batch production.

Tip dressing is not optional. Contaminated or deformed tips are the single most common cause of inconsistent welds, so dress or replace them regularly and wipe them between sessions.

What Specs Actually Matter When Buying or Building

1. Peak current and pulse range — enough headroom for your thickest planned strip, not just 0.1 mm foil.
2. Timing resolution — 1 ms steps are practical; anything coarser forces guesswork.
3. Double-pulse capability — pre-heat improves weld consistency on oxidized terminals.
4. Force control — springs or gas assist beat hand pressure for repeatability.
5. Tip accessibility — can the heads reach the middle of your 7×10 parallel group without shorting neighbors?

Whatever machine you choose, treat it as an adjustable instrument rather than a fixed-power tool. The next section turns those knobs into concrete settings.

Weld Settings: Current, Time, Pressure, and the Double Pulse

There is no universal “correct” setting on a battery spot welder. There is only the setting that works on your strip, your cells, and your electrode tips today. Treat the numbers below as a starting point on a dial, not a destination. Every real session begins with test welds on scrap strip and ends with a peel test before a single spot touches a cell you care about.

A Starting Framework by Strip Thickness

The single biggest variable is how much nickel you are trying to melt through. Thicker strip sinks heat away from the joint, so it needs both more energy and a longer pulse. For pure nickel welded to a standard 18650 positive terminal, these are sensible first dials:

Strip thickness	Energy level	Weld time
0.10 mm	Low	2–4 ms
0.15 mm	Low to medium	3–6 ms
0.20 mm	Medium	5–8 ms
0.25 mm	Medium to high	6–10 ms
0.30 mm	High	8–14 ms

Width matters too. A 5 mm strip spreads the same current over a wider area than an 8 mm strip, so widening the tab usually means nudging the energy up as well. A guide to nickel strip specifications gives useful cross-checks on resistivity and current capacity when you are matching strip to pack demand.

What Published Research Says About the Window

Laboratory work gives us hard numbers to aim at. A Warwick-based study backed by TVS evaluated micro resistance spot welding of 0.2 mm nickel tabs onto 18650 terminal material and identified a preferred window of 1800–2000 A at 8–12 ms. Critically, the researchers reported that failure above roughly 1800 A and 8 ms shifted from nugget pull-out to a sound joint where the tab tore rather than detached — meaning the weld had become stronger than the strip itself. That is exactly what you want: a connection that fails in the nickel, never at the interface.

Broadly the same physics shows up across the literature on lithium-ion cell welding, including work indexed through International Journal of Advanced Manufacturing Technology and reviews such as the Engineering Journal study on resistance spot welding of battery tabs. The consistent finding is that heat input scales with the square of the current, so small changes in amperage dominate small changes in time. Dial current first, trim time second.

The Double Pulse: Why a Pre-Heat Helps

A double pulse fires a short, weaker burst before the main weld. The first pulse burns off light oxide and contamination, seats the strip firmly against the terminal, and gently raises local temperature; the second pulse then forms the nugget on a clean, settled interface. Nickel-plated steel benefits disproportionately from this because its plating and the underlying steel respond differently to heat — the pre-heat evens that out and reduces the violent expulsion you see with a single hard shot. On many hobby machines, a first pulse of 30–50% of the main energy with a 5–10 ms gap between pulses is a good place to begin.

Electrode Force: The Setting People Forget

Welding current only heats where resistance exists, and contact resistance depends entirely on how hard the tips press. Push too hard and you flatten the contact area, dispersing the current path and reducing heat exactly where you need it. Push too lightly and the interface resistance spikes unpredictably, causing expulsion, spatter, and pitted terminals. Both extremes produce weak welds, which is why consistent tip pressure matters more than most DIY guides admit. Practical machines like the Arduino Spot Welder or commercial units described in the US Solid JFBSW manual rely on firm, repeatable pen pressure just as much as on their pulse settings.

Always Calibrate With Test Welds

Cell coating thickness, terminal cleanliness, strip alloy, tip geometry, and ambient temperature all move the goalposts. Weld two scraps together, increase energy until the strip tears during a peel test, then back off one step. That calibrated setting, confirmed on the actual cells you are about to use, is your setting for the session — and the only number in this section that truly matters.

Step-by-Step: Welding Nickel Strip to 18650 Cells

Choosing the right strip and owning a capable welder are only half the battle. The difference between a pack that lasts five years and one that fails on its first hill climb is almost always the workflow. Below is the same sequence professional pack builders follow, scaled down to a kitchen-table setup, with the checkpoints that catch problems while they are still cheap to fix.

Step 1: Sort and Voltage-Match Every Cell

Before a single spot is welded, measure the resting voltage of every cell with a decent multimeter. All cells destined to sit in parallel must be within about ±0.01 V of each other, ideally sitting at a storage charge near 3.6–3.7 V. This matters more than most beginners realize: two cells at 4.15 V and 3.90 V wired in parallel will rush current between themselves at rates that can exceed 10 A for several minutes. That internal heating happens before you ever pick up the welder, and it degrades the chemistry permanently.

Group cells by measured capacity and internal resistance where possible, then charge or discharge any outliers slowly until the whole batch converges. Label groups with a paint pen so you never lose track of which parallel set is which.

Step 2: Build a Simple Holder and Check Polarity Twice

A laser-cut acrylic cell holder or even a pair of wooden blocks with a 1 mm spacer keeps cells aligned, spaced at the correct 18.5–19 mm pitch, and pressed flat against the table. Loose cells roll mid-weld and produce skewed spots.

With cells seated, trace the polarity of every single cell with a marker on the exposed terminal. Standard 18650s are positive-up, but rewrap cells from laptop pulls and some industrial formats break that rule. Then trace your series and parallel layout on paper and match it to the physical arrangement before cutting any nickel. A reversed cell welded into a finished series string is, realistically, a total loss.

Step 3: Clean and Cut Your Strip

Nickel strip arrives with a light oil film from the rolling mill. Wipe it down with isopropyl alcohol and let it flash dry. Cut lengths with sharp scissors rather than tin snips, and deburr the edges. For parallel groups, cut strip long enough to reach every cell with a small overhang at each end, which you can fold down or trim later. Pre-cut H-shaped and Z-shaped pieces save enormous time on larger packs because they span both polarities without extra layers.

Step 4: Dial In Settings With Test Welds

Never begin on the pack itself. Weld two scrap strips to a dead cell or a spare strip laid flat on a copper busbar, then run the peel test. Grip the strip with pliers and peel it back: a good weld tears the nickel itself or leaves a crater in the terminal, while a bad weld lifts cleanly and leaves the strip shiny. Adjust energy, pulse count, or electrode force until you get consistent torn-nickel results.

Step 5: Weld the Pack in a Deliberate Sequence

Work group by group, parallel cells first, series links second. Lay the strip down, hold it flat with a fingernail or a small plastic shim at the far end, and set your pen or electrode firmly before triggering the weld. Keep the electrode tips perpendicular to the strip and wipe any black oxide off them every twenty spots or so, because buildup steals current and produces progressively weaker welds.

Minimum spot count per cell terminal: two spots for anything below roughly 5 A continuous service, and three to four for e-bike or powerwall duty. NASA’s published welding procedure requires at least two weld spots per connection with a specified maximum joint resistance, a useful reminder that redundancy is not optional even in high-budget builds. Published micro-resistance welding studies on lithium-ion cells, such as the in-depth evaluation from the University of Warwick group (available here), consistently show that weld quality and heat-affected zone size scale with electrode pressure and dwell, so hold pressure steady and do not rush the pulse.

Step 6: Mind Shunting on Solid Strip

When welding solid (non-slotted) strip across a parallel group, current from the second spot can bypass the terminal and travel through the nickel to the first spot, weakening both. The practical fix is simple: weld adjacent cells in an alternating, outside-in pattern, or switch to slotted strip so each terminal is forced to carry its own current, which forces current flow through the terminal as discussed earlier in this guide.

Step 7: Final Verification and Rest Period

Weld at a moderate state of charge, roughly 30–50%, never a full charge. When the last spot is done, set the pack aside for twenty to thirty minutes, then check every parallel group with a multimeter. Any group that has dropped more than a few millivolts relative to its neighbors, or any cell that is noticeably warm, has an internal fault or a bad weld shunting current. Catch it now while the pack can still be disassembled.

Finish by insulating every exposed terminal with fish paper rings and Kapton tape over the strip runs, then re-check total pack voltage and polarity at the main leads. Only then should the pack go into its BMS and enclosure.

Common 18650 Strip Welding Mistakes and How to Fix Them

Every failed DIY pack leaves clues at the weld: a strip that lifts with a fingernail, a black halo around the tip, a busbar that warms while the cells stay cool. Each symptom has only a handful of causes, and very few are the welder’s fault. Here are the recurring failures, the real cause, and the fix that works.

Cold Welds That Peel Off

Symptom: the strip lifts with a light twist, leaving two clean dimples behind. Cause: not enough energy reaching the interface: low current, a pulse that is too short, worn or dirty tips, or too little electrode force to break surface oxide. Fix: dress the tips, seat the pens flat and firm, then step energy up one notch until a peel test tears the strip instead of the joint. Run test welds on scrap strip first, exactly as the US Solid JFBSW battery spot welder manual walks through, before touching a cell.

Burn-Through, Spatter and Black Rings

Symptom: sparks, a hole blown through the strip, or a scorched ring around the weld. Cause: excessive heat input, and published parametric research notes these burn marks can later develop into rust under humid service conditions (parametric study of resistance welding parameters). Fix: shorten the pulse, drop the power, then inspect your tips: a worn, dished face concentrates current into a far smaller area than a fresh flat one. A black ring means you are close to piercing the thin positive cap, so back off rather than accepting “it holds.”

Dirty or Oxidized Terminals

Symptom: inconsistent results, tips sticking, light scorching. Cause: oil, fingerprints, adhesive residue from salvaged packs, or oxide from damp storage; skin oils alone change contact resistance. Fix: wipe terminals and strip with isopropyl alcohol, dry fully, and store cells in low humidity. A light scuff helps heavily oxidized salvage cells, but never abrade the terminal itself.

Current Shunting Through a Solid Strip

Symptom: the first spot welds well and every spot after it is weaker. Cause: once the first spot makes contact, the strip becomes a parallel path, so later pulses split between the intended joint and metal already joined. Solid strip spanning a series connection gives welding current a shortcut through neighbouring cells. Fix: weld in a planned sequence and prefer slotted or H/C/Z shapes that force current through the terminal rather than around it. That is the entire reason shaped strips exist.

Re-Welding the Same Spot

Symptom: a dimple that was “nearly there,” hit two or three more times. Cause: heat stacks. Excessive single-pulse heating time measurably raises cell surface temperature, and repeated triggers pile that input onto one small area (in-depth evaluation of micro-resistance spot welding of Li-ion cells). Fix: one short, decisive pulse. If you want a warm-up, use the double-pulse function instead of re-triggering; open-source controllers like Arduino Spot Welder V3 expose pre-heat and main-pulse timings separately. A failed spot should be re-made 1–2 mm away on fresh metal.

Mismatched Strip Material Claims

Symptom: spattery, steel-like welds, or a strip that runs hot in service, on a spool sold as pure nickel. Cause: counterfeit material. Pure nickel and nickel-plated steel are nearly indistinguishable by sight, and selling plated steel as pure nickel is a well-documented problem in the DIY community, as long-running build threads such as the Endless Sphere spot welding repository repeatedly warn. The plating exists to make steel weldable and corrosion-resistant, not to make it a good conductor. Fix: verify before you build.

Undersized Series Links and Busbars

Symptom: a pack that tests fine but gets warm under load, with heat localised at the bridges. Cause: the strip is the weakest link in most DIY packs. A 6 mm bridge in a ladder strip on a high-current pack will run hot long before the cells do, and the heat then migrates into the terminal and the shrink wrap. Fix: widen series links to roughly full terminal width and step up strip thickness on the sections carrying pack current. Check with a thermal camera or a finger test on a sustained load run before the pack goes into service.

None of these failures need expensive gear. Peel-test a scrap row, check voltage sag under load, and look at your tips and your strip. Fix the symptom, re-test on scrap, then commit another cell.

Safety, Quality Control, and Testing Your Welds

A finished-looking pack is not a finished, safe pack. Once the last weld is done, the real work begins: verifying that every joint will carry current for years, insulating anything that could short, and confirming the pack behaves as designed. This section covers the practices you should never skip.

Welding Around Lithium Cells: Real Safety Practice

Spot welding nickel strip to an 18650 is far safer than soldering, but it is not risk-free. Three hazards deserve your full attention:

• Molten spatter. A bad weld or too much power can eject tiny globules of molten nickel. Always wear safety glasses; a single spark landing on an exposed terminal can short a cell instantly.
• Short circuits from stray metal. Nickel offcuts, trimmed strip ends, and even a loose screwdriver blade can bridge two terminals. Keep the work surface non-flammable (bare wood is poor; a steel or ceramic tile is better), clear of loose metal, and never set a partially welded pack down on a pile of trimmings.
• Thermal runaway. A single 18650 that is hard-shorted can vent and ignite, and one flaming cell readily propagates to its neighbors. Have a fire plan before you strike the first weld: a Class D or lithium-rated extinguisher is ideal, and a bucket of dry sand next to the bench is a cheap, effective fallback. Never keep a welding station beside your bed or inside an unventilated closet.

The same discipline applies to wiring: no rings, no watches, no dangling necklaces near exposed terminals. A wedding ring bridging two busbars is a classic way to lose a finger.

Destructive Peel Testing: The Gold Standard for DIY

How do you know a weld is actually strong? Published research on automatic battery pack welding used destructive breakage testing as the chosen method to evaluate weld strength against welding time parameters, and for good reason: it is the only test that definitively reveals what is happening inside the joint. That study and related work on micro-resistance spot welding of lithium-ion cells for electric vehicles both lean on mechanical testing to validate weld quality.

For a DIY builder, the practical version is simple:

1. Weld a scrap strip to a scrap cell using the exact settings you intend for the pack.
2. Peel the strip off with pliers, twisting slowly.
3. Inspect the failure mode. Ideal: the nickel strip tears, leaving metal still fused to the terminal. Acceptable: the weld nugget pulls out cleanly as a ring. Bad: the weld pops off leaving two clean, shiny circles, which is a cold weld that will fail in service.

Run this test whenever you change strip thickness, replace electrode tips, or notice tip discoloration. Five minutes of scrap testing can save an entire pack build.

Electrical Verification: Contact Resistance as a Quality Screen

Post-weld joint quality can also be screened electrically. NASA’s battery welding specification, PRC-0009, caps joint resistivity for unspecified welds at 1.0 Ω, and production lines routinely use low-resistance checks for 100% inspection of every weld. You do not need aerospace equipment, but you do need a milliohm meter or a four-wire Kelvin setup to make meaningful measurements.

On a DIY bench, a workable proxy is comparing cell voltages under load: weld a small parallel group, apply a known load, and check that all cells in the group hold within a few millivolts of each other. A noticeably laggard cell often points to a high-resistance weld on its terminal rather than a bad cell.

Insulation: Fish Paper, Kapton, and Shrink Wrap

Exposed nickel edges and fresh weld areas are the pack’s most vulnerable spots. Before shrink wrapping, cover every terminal face with a fish paper ring (also called barley paper) to insulate the positive terminal and any exposed nickel, and secure loose strip runs with Kapton (polyimide) tape, which tolerates heat and will not degrade like cheap electrical tape.

Pay special attention to the positive end of each cell, where the terminal ring sits only fractions of a millimeter from the steel can. Weld spatter or a lifted strip edge touching that rim creates a direct can-to-positive short that no protection circuit will stop.

A Practical Pre-Service Checklist

Before a pack powers anything you depend on:

• Visually inspect every weld for discoloration, spatter craters, or lifted strip edges.
• Perform peel tests on your representative scrap welds, and shear-test a duplicate of any critical joint.
• Verify total pack voltage, per-group balance, and absence of self-discharge over 24–48 hours.
• Confirm insulation coverage, then confirm again after shrink wrapping.

Safety gear and testing take an extra hour or two. Compared to replacing a burned cell, a ruined controller, or worse, it is the cheapest insurance in the entire build.

Advanced Topics: High-Current Packs, Copper-Nickel Stacks, and What’s Next

Everything covered so far is enough to build a normal pack. Some projects are not normal. A 10S e-bike climbing long hills, a 48V powerwall discharging at 100 A, an RC pack pulling 200 C for ten seconds, these push a single layer of nickel past what it can carry without heating up. In that territory the strip stops being a convenience part and becomes a design decision. This section covers the techniques experienced builders use for high-current work, plus where the industry is heading.

Copper-Nickel Stacks: Putting Nickel Where the Welder Needs It

Copper is the obvious answer to high current. Its resistivity is roughly 1.7 × 10⁻⁸ Ω·m, against about 7 × 10⁻⁸ Ω·m for pure nickel, so a copper bar of the same size carries far more current with less heating. The catch is that the same low resistance makes copper almost impossible to spot weld. Resistance welding works by concentrating heat at the contact point; copper conducts that heat and current away so fast that the joint never reaches melting temperature. You get scorch marks and a loose strip instead of a nugget.

The workaround is a sandwich: lay a copper strip on the cell terminal, place a nickel strip on top, and weld through the nickel. The nickel gives the welder a resistive, weldable layer so the nugget forms where it should, while the copper underneath handles most of the current. Builders use pure nickel or nickel-plated steel as the top layer, cut slightly narrower than the copper so the electrodes land on nickel rather than copper.

Doubled Nickel and Proper Busbars

The simplest capacity upgrade is stacking two layers of 0.15 mm pure nickel, giving about 0.30 mm of total cross-section. This is a well-proven DIY route when 0.30 mm single strip is too thick for your welder, because most hobby machines cannot push enough current through it in a single pulse. Weld the bottom layer to the cells first, then weld the second layer to the first, and add a soldered busbar across the top for long runs.

For stationary storage and e-bike packs, serious builders eventually bolt or solder a copper busbar over the nickel interconnects and treat the strip as a fuse link rather than the main conductor. The strip handles assembly and short fault currents; the busbar handles the continuous load.

Steel-Can Versus Nickel-Can Cells

Not every 18650 has a nickel-plated steel can. Some have plain steel tops, some are fully nickel-plated, and a few rare cells use stainless. Plain steel terminals weld more forgivingly but corrode if the plating is scratched, while heavily plated cans behave like pure nickel and need softer settings. Test on the cell’s negative rim first, since that is usually the most representative surface, and never weld the positive cap of an unprotected cell without checking the vent structure underneath it.

What Production Lines Do: Resistance Welding and Laser Welding

Automotive pack manufacturing has not abandoned spot welding. Research on cylindrical-cell modules found that micro-resistance spot welding remains the common joining technique for low- to medium-volume applications, while high-volume lines increasingly move to laser welding for busbars because it is faster, more controllable and far less mechanically intrusive. That same work evaluated joint performance at 10 A, 20 A and 30 A load currents, measuring contact resistance and temperature rise, the two metrics that actually determine pack efficiency and cell ageing (Warwick review, published in International Journal of Advanced Manufacturing Technology, doi.org/10.1007/s00170-022-09775-z). That framing is worth stealing for DIY: judge a weld by how much its resistance warms under load, not by how it looks.

Hobby tools keep improving too. Open-source controllers such as the Arduino Spot Welder V3 give pulse control once reserved for industrial machines, and community-maintained guides like the Pumping Station One spot welder wiki document builds and settings in the open.

A Practical Decision Framework

Pack type	Typical demand	Strip choice
Backup pack, low drain	Under 10 A	0.15 mm pure nickel, 8 mm wide, 2 spots per joint
Powerwall / solar storage	10–40 A	0.20–0.30 mm pure nickel plus copper busbar
E-bike, mid-drive	20–60 A	0.30 mm pure nickel or doubled 0.15 mm, slotted
RC / high burst	Over 100 A	Copper-under-nickel stack, soldered busbar

Ask three questions in order: how much continuous current will this pack see, can my welder deliver a good nugget at that strip thickness, and is the strip the conductor or just the attachment? Match the answer to the table, run peel tests on scrap cells before touching the real pack, and measure temperature rise after the first heavy discharge rather than trusting a calculation.

Conclusion: advanced does not mean complicated. Copper-nickel stacks, doubled nickel and added busbars are the same idea applied harder, put weldable material where the electrode touches, and conductive material where the current flows. Build deliberately, test every weld, and your pack will still be running long after the shrink wrap stops sagging.

Frequently Asked Questions

What thickness of nickel strip is best for 18650 spot welding?

0.15 mm pure nickel strip is the best all-round starting point for DIY 18650 packs — it welds easily with most hobby welders and carries moderate current. Use 0.2–0.3 mm strip, or two stacked 0.15 mm layers, for high-current e-bike and powerwall builds, and drop to 0.10 mm only for very low-drain projects.

Can I solder nickel strip to 18650 cells instead of spot welding?

Soldering is discouraged because the sustained 300 °C-plus heat conducts into the cell and can damage the internal separator and chemistry. Spot welding deposits heat in a few milliseconds at the surface, which is why it is the standard method for cylindrical cell packs from hobby benches to aerospace assemblies.

How do I tell pure nickel strip from nickel-plated steel?

Pure nickel and plated steel look identical, so use a physical test: scratch a small area, soak it in salt water for 12–24 hours, and plated steel will rust at the scratch while pure nickel stays clean. A magnet test and a simple resistance comparison also help, and pure nickel is noticeably softer and more expensive.

Why do my welds keep peeling off the cell terminal?

Peeling welds are cold welds — usually the energy is too low, the pulse too short, or the surfaces are dirty or oxidized. Clean terminals with isopropyl alcohol, increase power or pulse time gradually, and re-run the peel test: a sound weld tears the nickel strip rather than lifting cleanly off the terminal.

How many weld spots should each strip connection have?

Use a minimum of two weld spots per connection — this is the floor in NASA’s battery welding process specification. Many builders add more: published testing on 18650 joints showed six weld points consistently improve load distribution and peel strength compared with fewer spots.

What welder settings should I start with for 0.15 mm nickel strip?

As a starting point, use a low-to-medium energy setting with a 3–6 ms pulse on 0.15 mm pure nickel, with a pre-heat (double) pulse enabled. Always dial in on scrap strip and a spare cell first — tip condition, terminal coating and cleanliness all shift the ideal setting, so the peel test decides the final value.

Is nickel-plated steel strip safe to use in a battery pack?

Yes, for low-to-moderate current applications — it is standard in commercial tool packs and is easy to spot weld. Its roughly 7 A/mm² capacity versus 10 A/mm² for pure nickel, and its lower corrosion resistance in humid environments, make it a poor choice for high-drain or outdoor packs.

What state of charge should 18650 cells be at when spot welding?

Weld at a moderate state of charge, roughly 30–50%, and never fully charged. A cell at full charge stores maximum energy, so any accidental short during welding releases it far more violently; after welding, let cells rest and check for abnormal heat or voltage drop before continuing the build.