Editing Battery Refurbishment (section)

== Step 2: Tools and materials ==

=== The spot welder ===

[[File:Battery guide - spot weld nugget cross section.png|thumb|420px|Cross-sections through resistance spot welds. Current is forced through the contact point between the two sheets, which has the highest resistance in the circuit; the metal fuses there and solidifies as a '''nugget''' (diameter ''d''), leaving a small indentation from the electrodes. This is the joint a battery spot welder makes.]]

A spot welder passes a very large current — of the order of 800–1200 A — through the joint for only '''1–10 milliseconds'''. Because the pulse is far shorter than the time heat needs to conduct through the terminal into the cell, the interface melts and re-solidifies while the cell body barely warms: a correctly executed weld raises the cell surface temperature by a few degrees.<ref name="mc-vs" /> That is the whole reason spot welding is the industry-standard method and soldering is not.

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Choosing a welder'''
! Class !! Capability !! Suitability
|-
| '''Rechargeable handheld / "mini" welders''' || 0.1–0.15 mm nickel, low-drain packs || Adequate for a one-off vintage pack rebuild, which is what this guide is about. Energy consistency is the weak point.
|-
| '''Mid-range bench welders''' (e.g. Sunkko 737G+ class) || 0.1–0.3 mm nickel, adjustable 2–7 mm needle spacing || The practical choice if you expect to rebuild more than a couple of packs.
|-
| '''Capacitive-discharge dual-pulse systems''' || Copper as well as nickel, joule-level energy control || Overkill for retro work, but the dual-pulse conditioning genuinely does cure inconsistent welds on oxidised strip.
|}

'''Dual pulse''' machines fire a low-energy conditioning pulse first to break through the surface oxide, then the main fusion pulse. Because oxide thickness varies from spot to spot, single-pulse machines deliver varying energy to the joint even at a fixed setting; the conditioning pulse removes that variable. If your welds are inconsistent despite good electrodes and steady pressure, this is the reason.<ref name="mc-weld">The Maker's Chest, [https://themakerschest.com/blogs/spot-welding-hub/how-to-spot-weld-battery-tabs-settings-technique-and-common-mistakes "How to Spot Weld Battery Tabs: Settings, Technique and Common Mistakes"]. Source for the electrode types and maintenance intervals, the power calibration procedure, electrode spacing and pressure guidance, the strip-thickness settings, the pull test and visual criteria, weld contact resistance figures, and the fault table reproduced in this guide.</ref>

'''Electrodes.''' Copper alloy tips conduct and extract heat well but wear quickly; tungsten tips are harder and last longer but conduct less. For 18650 and 21700 work, slightly rounded points of '''1–1.5 mm radius''' are the usual choice. '''Inspect the tips every 30–50 welds''' (every 20–30 for copper alloy) and dress them with a diamond file or fine abrasive. A mushroomed tip spreads the current over a larger area, quietly dropping weld quality without any obvious symptom.<ref name="mc-weld" />

=== Nickel strip ===

[[File:Battery guide - NiCd pack with recycling markings.jpg|thumb|300px|A shrink-wrapped four-cell NiCd pack with pre-welded solder tabs. Note also the Ni-Cd recycling symbol and the crossed-out wheeled bin mark.]]

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Nickel strip: thickness against current'''<ref name="strip">Current ratings collated from nickel strip supplier data and battery-building references; see also the strip-thickness welding guidance in [https://themakerschest.com/blogs/spot-welding-hub/how-to-spot-weld-battery-tabs-settings-technique-and-common-mistakes The Maker's Chest, "How to Spot Weld Battery Tabs"]. Published ratings differ substantially between suppliers because they assume different permitted temperature rises and duty cycles, which is why a range is given here rather than a single figure.</ref>
! Thickness !! Indicative continuous current !! Welding notes
|-
| '''0.10 mm''' || A few amps || Welds very easily and burns through just as easily. Fine for low-drain vintage packs.
|-
| '''0.15 mm''' || Commonly quoted at '''5–10 A''' (8 mm wide pure nickel is usually given as 5–7 A continuous) || '''The default gauge.''' Every welder class handles it. Start here.
|-
| '''0.20 mm''' || Commonly quoted at '''10–15 A''' || Needs a stronger transformer or a dual-pulse machine. Two welded layers of 0.15 mm are an alternative.
|-
| '''0.25–0.30 mm''' || Higher still || Needs a capable bench or professional welder; beyond most handhelds.
|}

Published current ratings for nickel strip vary widely between suppliers — figures as high as 17 A for 0.15 mm and 25 A for 0.2 mm appear in some catalogues — because they assume different temperature rises and duty cycles. Treat them as a range and size generously; a vintage laptop rarely draws more than a few amps anyway.

'''Pure nickel versus nickel-plated steel.''' Pure nickel has lower resistance and carries more current for a given cross-section. Nickel-plated steel is cheaper and, because its resistance is higher, actually welds more easily on a weak machine — but it carries less current and adds resistance to the pack. That is a trade-off, not an upgrade. '''Use pure nickel where you can.'''

=== The soldering iron, and where it is allowed ===

A soldering iron is still needed — just not on a lithium cell.

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Where the iron may and may not go'''
! Joint !! Allowed? !! Notes
|-
| Wire to nickel strip (after the strip is welded to the cell) || '''Yes''' || The strip acts as a heat buffer; the cell is not in the heat path.<ref name="mc-vs" />
|-
| BMS balance and sense leads to strip or PCB pads || '''Yes''' || Ordinary electronics soldering.
|-
| Output connector to the pack leads || '''Yes''' ||
|-
| Splicing one nickel strip to another || '''Yes''' ||
|-
| Pre-welded '''solder tab''' on a NiCd or NiMH cell || '''Yes, briefly''' || The tab, never the can. Keep the joint under about 3 seconds.
|-
| Tab of a '''tabbed coin cell''' || '''Yes, briefly''' || Never the coin cell body.
|-
| '''Bare NiCd or NiMH can''' || '''Strongly discouraged''' || Buy pre-tabbed cells instead.
|-
| '''Lithium cell terminal or can''' || '''No''' || This is the rule the rest of the guide is built around.
|-
| '''LiPo pouch tab''' || '''No, in practice''' || Buy the pouch cell with its PCM already fitted.
|}

'''If you have no spot welder and the pack is a low-drain one-off''', the least-bad soldering technique is: a high-wattage iron (60–80 W) with a large tip for thermal mass, generous flux, terminal abraded and pre-tinned in a fraction of a second, joint completed in '''under one second''' of contact, 10–15 seconds of cooling before touching an adjacent connection, and never a second application to the same terminal.<ref name="mc-vs" /> This reduces the heat input; it does not remove the risk. For anything that will be cycled regularly, borrow or buy a welder.

'''On nickel cells the technique is:''' scuff the tab with emery cloth, clean it with isopropyl alcohol, tin the iron generously so heat transfers fast, and stay on the joint '''no more than about three seconds'''.

=== The rest of the kit ===

* '''Digital multimeter''' — for cell voltages, pack voltage, thermistor resistance and continuity.
* '''Smart charger / analyser with a capacity test''' (a four-bay Li-ion/NiMH analyser is the usual hobbyist choice) — this is how cells get graded and matched.
* '''Internal-resistance meter or a charger that measures IR.'''
* '''Milliohm meter''' (optional) — the only quantitative way to check weld quality.
* '''Digital calipers''' — for measuring original cells and pouch dimensions.
* '''Kapton (polyimide) tape''' — holds strip in place while welding and insulates afterwards; it tolerates the heat.
* '''Fish-paper or pre-cut insulating rings''' for the positive end of cylindrical cells.
* '''Heatshrink sleeving''', including large-diameter sleeving to re-wrap the finished block.
* '''Nickel strip''' in the width the pack needs, plus scrap for calibration.
* '''Scrap or dead cells''' to calibrate the welder on. Never calibrate on your good cells.
* '''Insulated side cutters, thin pliers, plastic spudgers, a thin scraper''' for opening ultrasonically welded casings.
* '''A hot-air gun''' — used sparingly and at a distance, only to soften adhesive.
* '''Isopropyl alcohol, cotton buds, a small wire brush.'''

See [[Recommended Tools]] for the general workshop toolkit.