Battery Refurbishment

The inside of a typical laptop pack: six Panasonic CGR18650DA cells in a 3-series, 2-parallel arrangement, joined by nickel strip, with the protection and gas-gauge PCB below. A match is included for scale.

The other outcome. The lithium-polymer pouch cells in this MacBook Pro battery have swollen enough to force the pack casing apart. A pack in this condition is never charged, cut, crushed or carried loose.

This is a generic guide to rebuilding rechargeable battery packs for vintage computers and handheld devices whose original packs are no longer manufactured — laptops, portables, PDAs, organisers, calculators, test equipment and games handhelds. It covers identifying what is inside the pack, choosing replacement cells, joining them safely, dealing with the pack's own electronics, and testing the result.

Machine-specific guides cover the quirks of individual packs. This page covers the parts of the job that are the same on every pack.

The single most important rule on this page: cells are joined by spot welding, not soldering. Everything else follows from that.

⚠️ Safety Warning

A lithium cell contains a flammable liquid electrolyte and enough stored energy to ignite it. A cell that is shorted, crushed, punctured, overheated, overcharged or reverse-charged can enter thermal runaway: an internal chemical reaction that cannot be stopped once it starts.

A lithium-ion cell struck with a hammer during a deliberate destructive test. Crushing a charged cell creates an internal short, which drives it into thermal runaway. This is what a "small" cell does when it is abused.

A cell in thermal runaway vents roughly one to two litres of hot, flammable gas for every amp-hour of its capacity — so a 2.5 Ah 18650 produces several litres in a few seconds — and the heat it releases is usually enough to set off the cells next to it in turn. The gas is mostly carbon dioxide, hydrogen and carbon monoxide, and the decomposing electrolyte also produces hydrogen fluoride, which is corrosive and acutely toxic.

The rules that are not negotiable

• Never apply a soldering iron to a lithium cell's terminal or can. Cell manufacturers specify a maximum cell-case temperature during assembly of 60–80 °C. A soldering iron running at 280–380 °C reaches that in one to three seconds and keeps conducting heat inwards after it is lifted. The separator inside the cell — the microporous film that keeps anode and cathode apart — is a polyethylene or polypropylene membrane whose pores are deliberately designed to close by melting if the cell overheats. Those polymers melt in the region of 130–160 °C. Lead-free solder does not even become liquid until about 217 °C.
• Never charge a cell or pack that is swollen, dented, punctured, leaking or smells sweet. The sweet smell is vented electrolyte. Retire it.
• Never charge a lithium cell that has sat below about 1.5 V per cell for a week or more. Copper dendrites grow inside a cell left dwelling below 2.50 V/cell for more than about a week, and they can bridge the electrodes as a partial or total internal short. Such a cell "might become unstable, causing excessive heat or show other anomalies" when recharged.
• Never work on a pack that is still installed in a machine you value.
• Never leave a rebuilt pack charging unattended, and never charge one without a working protection circuit.
• Never mix chemistries, capacities, brands or ages within one pack. A mismatched cell is the cell that fails.

Fire

A lithium-ion fire is burning electrolyte — a flammable organic liquid, so a Class B fuel — but with a crucial difference from an ordinary Class B fire: the cell generates its own heat and its own oxidiser internally, so it cannot be smothered. Cutting off the air does not stop it. Only removing heat does.

That has practical consequences for a workshop:

• A CO₂ or dry-powder extinguisher knocks the flame down but does not cool the cells, so the pack frequently reignites a minute or two later, and again after that.
• Water works, but by cooling, and it takes a lot of it. Fire services attack lithium battery fires with sustained large volumes of water for exactly this reason. A hand extinguisher does not carry enough to cool a pack below the runaway threshold.
• The realistic plan for a hobbyist is not to fight it. Get the pack outside and away from anything combustible if you can do so safely, get everyone out, and call the fire service. Tell them it is a lithium battery.
• Treat the smoke as toxic and do not breathe it. Hydrogen fluoride, carbon monoxide and hydrogen chloride are among the products.
• Expect delayed reignition. A pack that appears to have gone out can restart hours later. Leave it outside on a non-combustible surface and keep watching it.

None of this applies to lithium-metal primary cells — non-rechargeable lithium coin cells and the lithium thionyl chloride cells used in some industrial and test equipment. Those contain metallic lithium, which reacts with water. They are a Class D fire and are dealt with using dry sand or a Class D extinguisher, never water.

Before you touch anything

Minimum precautions

Precaution	Why
Safety glasses	Cells vent upwards and sideways, and nickel strip flicks when it tears.
A non-combustible work surface	A ceramic tile, a steel tray or a paving slab. Not a wooden bench top and not a carpet.
A way to get the pack outside	A metal bucket or tin with a lid within arm's reach. A venting cell is dealt with by getting it out of the building, not by fighting it indoors.
A bucket of dry sand and a water source	Sand smothers and contains; water cools. Know which you would use for which chemistry before you need to decide.
Insulated tools	Tape all but the last few millimetres of your side cutters and pliers. Dropping an un-taped tool across a pack is the classic way to start a fire.
Remove rings and metal watch straps	A wedding ring across a pack terminal will glow before you can get it off.
Good ventilation	Vented electrolyte is an irritant, and hydrogen fluoride is among the decomposition products.
No smoking, no naked flame, no hot-air gun near a cell	Adhesive softening with heat is done briefly and at a distance.

Work one connection at a time. The overwhelming majority of accidents in pack rebuilding are momentary short circuits caused by a loose strip, a dropped tool or a cell rolling into a live terminal — not by exotic chemistry.

Is refurbishment the right answer?

Rebuilding is worth doing when the original pack is unobtainable and the machine will not run without it. It is not always the best answer.

Deciding what to do with an obsolete pack

Situation	Recommended approach
A modern equivalent pack is still sold	Buy it. Rebuilding is for the packs nobody makes.
The pack is a simple series stack of cells with no electronics	Ideal rebuild candidate. Most NiCd and NiMH packs from the 1980s and early 1990s are this.
The pack has a simple protection PCB (PCM) and no data bus	Good rebuild candidate. The PCM can usually be reused.
The pack is a "smart" SMBus pack with a fuel gauge	Possible, but the electronics are the hard part. See Smart packs below.
The pack is only needed to hold CMOS/RTC settings	Rebuilding a tabbed coin cell is easy and cheap — see Coin cells below.
The machine is on mains permanently and the pack is only dead weight	Consider a dummy pack: the original casing, gutted, with the terminals left open or a link fitted if the machine requires one. Many retro portables run happily this way and it removes the fire risk entirely.
The pack is leaking, corroded, or has damaged the machine	Deal with the machine first. See Battery Explosion, Capacitor or Corrosion Damage.
The cells are lithium and have been flat for years	Assume they are scrap. Do not attempt to revive them.

Step 1: Identify what you have

Nothing else can be decided until the chemistry, the cell format and the pack configuration are known.

Chemistry

The four chemistries found in vintage computer and handheld packs

Chemistry	Nominal	Fully charged	Discharged	Typical era and use	Notes for the rebuilder
NiCd (nickel-cadmium)	1.2 V/cell	~1.45 V	1.0 V	Late 1970s–early 1990s laptops, portables, calculators, RTC backup	Tolerant, but cadmium is toxic and NiCd packs leak corrosive electrolyte as they age. Pre-tabbed cells are still made.
NiMH (nickel-metal hydride)	1.2 V/cell	~1.45 V	1.0 V	Early–mid 1990s onwards; the usual NiCd replacement	Higher capacity than NiCd in the same can. Same nominal voltage, so it is often a drop-in.
Li-ion (cylindrical)	3.6–3.7 V/cell	4.20 V	2.50–3.00 V	Mid 1990s onwards; laptops, camcorders	Never soldered. Requires protection electronics.
LiPo (lithium-polymer pouch)	3.7 V/cell	4.20 V	3.00 V	Late 1990s onwards; PDAs, organisers, thin handhelds	Soft foil pouch. Extremely easy to puncture. Never soldered at the cell; the tabs are the only place an iron may go, and even then reluctantly.

Chemistry is normally printed on the pack label, on the cell wrapper, or moulded into the casing. If the label is gone, cell count against pack voltage is decisive: a 1.2 V nominal cell only ever produces multiples of 1.2 V (2.4, 3.6, 4.8, 6.0, 7.2, 9.6, 12 V), while lithium packs come out at multiples of 3.6 or 3.7 V (3.6, 7.2, 10.8, 11.1, 14.4, 14.8 V).

Reading the pack label

Two laptop packs and a bare 18650 cell. The label voltages give the series count away: 10.8 V is 3 cells in series (3 × 3.6 V), 14.8 V is 4 in series (4 × 3.7 V). The 4400 mAh rating on a pack built from ~2200 mAh cells means two parallel strings.

The pack label tells you the configuration before you open it.

• Voltage ÷ cell nominal voltage = the series count (S). 10.8 V ÷ 3.6 V = 3S. 14.4 V ÷ 3.6 V = 4S. 7.2 V ÷ 1.2 V = 6 NiCd/NiMH cells in series.
• Pack capacity ÷ single-cell capacity = the parallel count (P). A 4400 mAh pack built from 2200 mAh cells is 2P. A 4S2P pack therefore contains eight cells.
• Watt-hours = pack voltage × pack amp-hours. A 10.8 V 3.8 Ah pack is about 41 Wh — which is exactly what the "Transport Only 41 Wh" marking on some packs refers to.

Write the configuration down before you cut anything. It is the specification for the rebuild.

Cylindrical lithium cells: the number is the size, not the chemistry

An 18650 cell (left) and a 21700 (right). The numbers are dimensions in millimetres, nothing more.

An 18650 against AA and AAA cells with a coin for scale. An 18650 is visibly fatter and longer than an AA and will not fit an AA holder.

The familiar five-digit numbers are purely dimensional and follow the scheme given in IEC 60086-1 for cylindrical cells: the first two digits are the nominal diameter in millimetres, the next two the height in millimetres, and the fifth digit indicates the cylindrical shape. (Some manufacturers instead read the last three digits as the height in tenths of a millimetre, which gives the same answer for the common sizes, and some use their own designations entirely.)

So 18650 means "18 mm diameter, 65.0 mm long, cylindrical" and tells you nothing whatsoever about what is inside it. The same cell is sometimes written as a 1865 with the trailing shape digit dropped; it is the same thing. Cells with lithium cobalt oxide, NMC, NCA and lithium iron phosphate chemistries have all been built in this size, and by the 2020s so had sodium-ion and even potassium-ion cells — with different nominal voltages and different charging requirements. Never assume chemistry from the size code.

Cylindrical lithium cell sizes you will meet in vintage equipment

Designation	Diameter	Length	Where it turns up
18650	18 mm	65.0 mm	The overwhelmingly common laptop and camcorder cell. Introduced by Sony in 1991.
18500	18 mm	50.0 mm	Slimmer packs, some camcorders
17670	17 mm	67.0 mm	Some early camcorder and portable packs
16340 (RCR123)	16 mm	34.0 mm	Cameras, small devices
14500	14 mm	50.0 mm	AA-sized lithium. Not an AA — it is 3.7 V, not 1.2 V or 1.5 V, and putting one in an AA device destroys the device.
21700	21 mm	70.0 mm	Modern; useful only if the pack has room
26650	26 mm	65.0 mm	Modern high-capacity; rarely fits a vintage pack

Cell length is quoted for the bare cell. Protected cells carry a small protection PCB and a strip conductor under the wrapper: a protected 18650 typically measures around 68 mm rather than 65 mm, and some designs increase the diameter instead. Either way the cell may no longer fit a compartment designed for an unprotected one, so measure before ordering.

Decoding the chemistry prefix

Most cylindrical cells carry a manufacturer part number whose leading letters describe the cathode chemistry.

Chemistry prefixes on cylindrical lithium cells

Prefix	Cathode	Characteristics	Typically found in
ICR	Lithium cobalt oxide, LiCoO2	High capacity, modest discharge current, least thermally tolerant	Laptops, cameras — the classic 1990s/2000s laptop cell
IMR	Lithium manganese oxide, LiMn2O4	High discharge current, thermally stable, lower capacity	Power tools
INR	Nickel manganese cobalt, LiNiMnCoO2 (NMC)	Balanced capacity and current; the modern default	General purpose — usually the best modern substitute
NCR	Nickel cobalt aluminium, LiNiCoAlO2 (NCA)	Very high capacity, moderate current	Panasonic/Sanyo high-capacity cells
IFR	Lithium iron phosphate, LiFePO4	Safest, longest cycle life, but 3.2 V nominal and 3.6 V charged	Not a drop-in substitute — the voltage is wrong for a Li-ion pack

A part number such as ICR18650-26F therefore reads: lithium cobalt oxide chemistry, 18 mm × 65 mm cylindrical, 2600 mAh, revision F.

IFR/LiFePO4 cells are not a substitute for Li-ion cells in an existing pack. At 3.2 V nominal they will not reach the voltage the machine expects, and a Li-ion charger will overcharge them.

NiCd and NiMH cell sizes

Sub-C NiCd cells, 1.2 V 1200 mAh, in the paper-wrapped form found inside many legacy packs. Sub-C is 23 mm × 43 mm.

Loose NiMH cells. NiMH is the usual modern replacement for an obsolete NiCd pack: same 1.2 V nominal, more capacity in the same can.

Nickel cells use a completely different naming scheme, based on the old ANSI size names with fractions denoting shortened versions of a standard size. A "2/3 A" cell has the diameter of an A cell and roughly two-thirds of its length. These fractional sizes are everywhere in vintage laptop and handheld packs because manufacturers picked whatever fitted the moulding.

Common NiCd/NiMH cell sizes and dimensions

Size	Diameter	Length	Notes
1/3 AAA	10.5 mm	16 mm
2/3 AAA	10.5 mm	30 mm
AAA	10.5 mm	44.5 mm
1/3 AA	14.2 mm	17.5 mm	Common as a memory-backup cell
1/2 AA	14.2 mm	30 mm
2/3 AA	14.2 mm	28.7 mm	Very common in handheld and organiser packs
4/5 AA	14.2 mm	43 mm
AA	14.2 mm	50 mm	48 mm for a flat-top cell
4/3 AA	14.2 mm	65.2 mm
1/2 A	17 mm	25 mm
2/3 A	17 mm	28.5 mm	Extremely common in early laptop packs
4/5 A	17 mm	43 mm
A	17 mm	50 mm
4/3 A	17 mm	67 mm
Fat A	18 mm	50 mm
1/2 SC	23 mm	26 mm
2/3 SC	23 mm	28 mm
4/5 SC	23 mm	34 mm
SC (Sub-C)	23 mm	43 mm	The classic high-drain legacy cell
4/3 SC	23 mm	50 mm
1/2 C	26 mm	24 mm
2/3 C	26 mm	31 mm
C	26 mm	46 mm
1/2 D	33 mm	37 mm
D	33 mm	58 mm
F	33 mm	91.2 mm
F3 prismatic	5.6 × 16.5 × 22 mm		The flat "chewing gum" cells in 9 V packs and camcorders
F6 prismatic	5.6 × 16.5 × 48 mm

Diameter can vary by as much as 1 mm between manufacturers, and length increases if the cell has a protruding end cap. Measure the original cell with calipers and compare against the datasheet of the cell you intend to buy, not against the nominal figure in a table.

A 9 V NiMH battery opened up. The prismatic cells are stacked in series and joined by welded metal tabs — the same construction used inside handheld device packs.

Pouch (LiPo) cells

Pouch cells are identified by a six-digit code giving thickness × width × length in tenths of a millimetre: a 503759 cell is 5.0 mm × 37 mm × 59 mm. The measurement excludes the sealed edge flanges and the tabs, so always measure the original cell body and check the replacement's datasheet drawing.

Pouch cells in handheld devices almost always arrive with a small protection circuit module (PCM) already welded to the tabs and heat-shrunk in place, providing over-charge, over-discharge, over-current and short-circuit cut-out. Buy the replacement with its PCM fitted wherever possible; that keeps the iron away from the cell tabs entirely.

Coin cells, CMOS and RTC batteries

Many vintage machines keep their configuration in CMOS RAM backed by a coin cell soldered or "pigtailed" to the board. When the OEM part is long gone, the fix is to rebuild the assembly around a tabbed coin cell.

• Buy pre-tabbed cells. A tabbed cell lets you solder to the tab, well away from the cell body. Soldering to a bare coin cell can make it vent or explode.
• Coin cell numbers are dimensional too. In a four-digit code the first two digits are the diameter in millimetres and the last two are the height in tenths of a millimetre, so CR2016, CR2025 and CR2032 are all 20 mm across and differ only in thickness — 1.6, 2.5 and 3.2 mm. The CR prefix denotes lithium manganese dioxide chemistry at 3 V nominal.
• A CR2032 replaces a CR2016 or CR2025 if there is physical room, and gives the longest life; a CR2025 substitutes for a CR2032 where space is tight, at the cost of runtime.
• Use vertical tabs where possible, and never bend a tab to fit — a folded tab can short across the cell body.
• Re-sleeve the finished cell. Heatshrink of 19.1–19.2 mm inner diameter fits a CR20xx cell. All bare metal must end up covered.
• A dead CMOS cell reads a few millivolts or 0.00 V on a meter. On many machines a working main battery masks a dead CMOS cell, so remove the main pack and hold the power button for 30 seconds before testing.

Spotting fake and reclaimed cells

The market for cylindrical lithium cells is full of counterfeits, and a rebuilt pack is only as good as the cells in it.

Warning signs when buying cells

Sign	What it means
Claimed capacity above about 3500 mAh in an 18650	Fake. Genuine 18650 cells run roughly 2000–3500 mAh. "6000 mAh" and "9900 mAh" 18650s do not exist.
Cell weighs 30–35 g	Fake or grossly under-filled. A genuine high-capacity 18650 weighs about 42–50 g. A kitchen scale is the cheapest counterfeit detector there is.
Misspelled brand names, misaligned logos, blurred printing	Counterfeit wrapper.
Scratches, dents, rust, burn marks or weld scars on the terminals	A reclaimed cell pulled from a scrap pack and re-wrapped, sold as new.
No manufacturer datasheet exists for the part number	Treat the ratings as fiction.
Price far below the going rate for a known brand	Cells from Samsung, LG, Panasonic/Sanyo, Molicel and Murata cost what they cost.

The only proof is measurement: charge the cell, run a capacity test, and compare against the claim.

Step 2: Tools and materials

The spot welder

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. That is the whole reason spot welding is the industry-standard method and soldering is not.

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.

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.

Nickel strip

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.

Nickel strip: thickness against current

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.

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.
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. 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.

Step 3: Assess and document the pack

1. Photograph everything, from every angle, before and during disassembly. Cell orientation, strip routing, wire colours, thermistor position, foam and insulator placement. You will need this in an hour.
2. Measure the pack terminal voltage. Compare it with the label voltage. A pack reading zero may simply have a protection circuit latched off; a pack reading a plausible voltage still has energy in it and must be treated as live.
3. Record the label data: chemistry, voltage, capacity, watt-hours, part number.
4. Work out the expected configuration (nS × nP) from the label as described above.
5. Inspect for swelling, corrosion, leakage and a sweet smell. Corrosion around a NiCd pack means electrolyte has escaped — check the machine's PCB as well.

Step 4: Open the pack

Vintage packs are ultrasonically welded, glued, screwed, or all three.

1. Look for screws first, including under labels and rubber feet.
2. For a welded seam, work a thin, blunt blade or a plastic spudger into the seam and lever gradually all the way round rather than forcing one point. Expect to break some internal clips; they can be glued on reassembly.
3. Never cut into a pack blind. You do not know where the cells sit relative to the casing wall. Cutting into a cell is exactly the failure mode the safety section describes.
4. Do not use a hot-air gun on the casing over the cells. Warm the seam locally if you must, briefly.
5. Once open, immediately tape over any exposed terminal you are not working on.

Step 5: Map the pack before you cut anything

A sealed NiMH laptop pack. Nothing about the outside tells you how the cells are wired inside — which is why the pack gets mapped before anything is disconnected.

Draw a diagram. Record:

• Cell count, orientation and physical arrangement — which way each cell faces, and which cells are in each parallel group.
• The series/parallel wiring — trace each nickel strip and mark on your diagram what it joins.
• The thermistor — a two-wire component pressed against a cell. Note which cell and where on that cell. The most common type is a 10 kΩ NTC, reading 10 kΩ at 20 °C, with resistance falling as it warms. You can identify it with an ohmmeter: warming the pack with your hand is enough to see the value change.
• Balance / sense leads — some fuel-gauge chips run a separate wire to each series node. These must be reconnected in the correct order, starting from cell one and working up.
• Fuses, PTC devices and thermal cut-outs in the strip work — these are easy to mistake for plain strip.
• The pack connector pinout. A typical smart pack has five or more contacts: positive and negative at the outer edges, with thermistor, clock and data on the inner contacts. There is no standard for the arrangement of the inner contacts, so find positive and negative with a voltmeter and work outwards from there.

Step 6: Keep the pack electronics alive

This step is skipped at your peril on smart packs.

Some fuel-gauge and protection circuits lose critical calibration data — including the digitised shunt resistor value that the coulomb counter depends on — if their supply is interrupted for even a fraction of a second.

The technique is to supply the board from a bench supply at the same voltage through a 100 Ω resistor, connected before the old cells are disconnected, and removed only after the new cells are in place and supplying the board. The resistor is low enough to keep a digital circuit powered and high enough to protect against an accidental short.

The same 100 Ω resistor is the safe way to probe an apparently dead pack. Some packs use a solid-state switch that leaves the terminals at zero volts until it is enabled: with the voltmeter on the outer terminals, tie one end of the resistor to ground and touch each remaining contact in turn, then repeat with the resistor tied to a positive rail, and watch for the pack output waking up.

If nothing responds, the pack may be locked by a manufacturer activation code. Those codes are proprietary and are not published — even to service staff — so a code-locked pack cannot be revived. Some manufacturers also fitted an end-of-life switch that disables the pack at a set age or cycle count.

Step 7: Remove the old cells

1. Discharge is not possible on a pack that will not power up — so treat every cell as charged.
2. Cut the strip, not the cell. Snip through the nickel between cells rather than trying to prise strip off a cell you intend to keep.
3. Where a cell is to be salvaged, grip the strip by an edge and roll it off the terminal slowly. Rolling needs the least force. Take care not to let the freed strip fall across the cell and short it.
4. Preserve the original strip layout as a template if the geometry is unusual — lay it on your bench in position.
5. Keep the protection/fuel-gauge PCB, the thermistor, the connector and any moulded insulators. On a vintage pack these are the unobtainable parts, not the cells.
6. Tape the terminals of every removed cell immediately and put them in a non-conductive container away from the work area.

Step 8: Choose the replacement cells

Selection rules

Rule	Reason
Same chemistry as the original	The machine's charger is designed around one chemistry's charge termination. NiCd → NiMH is the one common exception (see below).
Same physical size, or smaller	Measure with calipers. Remember protected cells are 2–5 mm longer.
Same series count	The pack voltage must match what the machine expects.
All cells identical — same manufacturer, same part number, same production batch where possible	Mixed cells are the commonest cause of a rebuilt pack that dies early.
Do not chase maximum capacity	A modest, genuine, well-known cell outperforms an optimistic no-name one. The original charger was designed for the original current levels.
Buy pre-tabbed nickel cells if you have no welder	Solder to the tab, never the can.
Buy pouch cells with the PCM already fitted

Salvaged cells. Harvesting 18650s from scrap laptop packs is standard practice, but only cells that pass the full grading procedure below belong in a pack you will leave charging.

Step 9: Test and grade every cell

Four 18650 cells in a four-bay analyser. Chargers of this type set the charge voltage by chemistry (Li-ion 4.2 V, LiFePO4 3.6 V, NiMH 1.2 V) and measure each cell's real capacity — which is how cells are graded and matched.

Every cell that goes into the pack — new or salvaged — passes through the same five checks.

1. Visual inspection

Reject any cell that is swollen, dented, cracked, corroded, shows dried or wet residue, has burn marks, or smells sweet. A sweet smell means the electrolyte has leaked. Minor scratches in the wrapper are cosmetic; a compromised wrapper should be re-sleeved before use because the whole can of a cylindrical lithium cell is the negative terminal.

2. Resting voltage

A usable lithium cell rests somewhere between about 2.6 V and 4.2 V.

• Below 2.5 V — deeply discharged. Copper dendrites grow in a cell left below this for more than about a week.
• Below 1.5 V for a week or more — do not attempt to charge it. Copper shunts may already have formed.
• Above 4.2 V — dangerous. Retire it.

"Sleeping" packs and boosting

A pack reading zero volts is not necessarily a pack full of dead cells. Li-ion protection circuits cut off somewhere between 2.2 and 2.9 V per cell depending on the manufacturer, and a pack left in storage can self-discharge past that point and switch itself off. Many chargers and analysers have a boost or wake-up function that applies a small current to bring the protection circuit back to life, after which a normal charge proceeds.

• If the voltage does not rise to a normal level within about a minute of boosting, discard the pack.
• Check polarity with great care before boosting. A sleeping pack does not reveal its voltage, and applying a voltage in reverse causes permanent damage.
• The 1.5 V / one-week rule overrides all of this. Do not boost a cell that has dwelled that low that long, however tempting.

Boosting is worth trying: in a Cadex study of 294 mobile-phone batteries returned under warranty, 30 % were merely inactive and needed a boost, and 91 % were restored to 80 % capacity or better.

3. Charge, watching for heat

Charge each cell individually and keep track of its temperature. A cell that becomes noticeably warm at a low charge current has a high internal resistance or an internal short. Discard it. The palm of your hand is a surprisingly good instrument here; an infrared thermometer is better.

4. Self-discharge

Leave the charged cells resting for at least a week. Any cell that has lost more than about 0.1 V is discarded. A healthy cell loses roughly 0.5–2 % of its charge per month. Self-discharge means the electrodes are beginning to touch internally, and it only gets worse.

This step takes a week of calendar time and no effort, and it is the single most effective filter for cells that would otherwise fail inside your rebuilt pack.

5. Capacity and internal resistance

• Capacity test: charge fully to 4.20 V, then discharge at a modest constant current (around 0.2 C) to the cell's specified cut-off, typically 2.8–3.0 V. A healthy cell measures within about 10 % of its rated capacity. Below roughly 80 % of rating, the cell is worn; below 60 %, it is scrap.
• Internal resistance: a good 18650 typically measures around 30–50 mΩ. Significantly above 50 mΩ means the cell will run hot and drag the pack down. Well over 100 mΩ is a dead cell.
• Load test (quick field check): across a 2 Ω load a healthy cell's voltage sags only about 0.1–0.3 V depending on state of charge. A cell that sags much more than its neighbours, or sags inconsistently between tests, is bad.

6. Match the cells

Cells in the same parallel group must be matched for capacity. A protection circuit shuts the whole pack down when any one group hits its over-charge or over-discharge threshold, so the weakest group sets the capacity of the entire pack. Group your graded cells so that each parallel group totals roughly the same capacity as every other.

Then bring every cell to the same voltage before assembly — typically around 3.8 V. Charging cells individually to the same voltage before they are welded together avoids a large balancing current flowing the instant the parallel groups are joined.

Step 10: Lay out and insulate

1. Arrange the cells exactly as the original diagram shows, paying attention to orientation. Getting one cell backwards in a series string is a short circuit through the pack.
2. Check the polarity of every cell with a meter after laying them out, before any welding.
3. Fit an insulating ring to the positive end of each cylindrical cell. The positive terminal is only the small raised button; the flat ring around it is connected to the negative can. Without the insulator, a strip that shifts slightly shorts the cell out. Pre-cut fish-paper rings are sold for exactly this.
4. Hold the block together with a cell holder, Kapton tape, or the pack's original moulded carrier.
5. Cut the nickel strip to length with a few millimetres of overhang each side.
6. Make sure the strip lies flat. A strip that is bowed from the roll leaves a gap at the interface, and a spot welder cannot bridge a gap. Straighten strip by drawing it over a flat edge.
7. Tape the strip down with Kapton so it cannot move between welds.

Step 11: Spot weld

Calibrate on scrap first

Never start on your real cells.

1. Set the machine to roughly 50 % of its capacity.
2. Weld a piece of your actual strip to a scrap or dead cell.
3. Pull test it. If the strip peels off cleanly, raise the power by 10–15 % and repeat.
4. Continue until the pull test passes — the strip tears rather than the weld letting go.
5. Push slightly higher until you see burn-through or discolouration of the terminal; the setting below that is your maximum.
6. Your working range is: passes the pull test, without burning through. Write the setting down against the strip gauge, strip material and cell type. Recalibrate when any of those changes.

Calibration takes fifteen or twenty minutes and it is the difference between a pack that lasts and a pack that comes apart.

Technique

• Electrode spacing: 2–5 mm. Too close (under 2 mm) and the current takes the short path through the strip surface instead of down through the interface, leaving shallow welds. Too far (over 7 mm) and the energy is dissipated heating the strip instead of the joint. Use the closer end of the range for 0.1 mm strip and the wider end for 0.2 mm.
• Pressure: firm and steady. Enough that the electrodes cannot be slid sideways without lifting them; not so much that the strip deforms before the pulse fires. Too little pressure causes surface arcing and sparks with poor penetration below; too much deforms the strip and risks slipping.
• Hold the electrodes vertical, press, and trigger without hesitating — hesitation lets the pressure waver.
• Maintain pressure for about half a second to a second after the pulse while the nugget solidifies, then lift cleanly. Do not lift immediately, and do not drag the electrodes sideways.
• Two weld spots minimum per strip end per terminal, four for anything carrying real current. Keep the spot positions consistent from cell to cell.
• Work methodically along the strip from one end to the other. Finish one cell before moving on.
• On the positive end, the strip contacts the raised button; the slight offset is normal and the weld still forms at the button interface.

Check the welds

Visual. A good weld leaves two small, clean, round indentations of consistent size where the electrode tips sat. Slight darkening is normal. Elongated, smeared or asymmetric marks mean the pressure or position wavered; sparking marks away from the intended spots, visible holes, or discolouration of the cell terminal all mean too much energy.

Pull test. The definitive check: grip the strip with pliers and pull it straight away from the terminal. A good weld tears the nickel and leaves a fused remnant on the cell. A bad weld peels off cleanly leaving the terminal essentially unmarked. Do this destructively on your scrap during calibration, not on every production weld.

Resistance. A well-made nickel tab weld measures roughly 0.05–0.3 mΩ. Higher means a cold weld, oxide contamination or misalignment. This needs a milliohm meter, which is optional for a one-off job but the only quantitative measure available.

Weld fault table

Spot welding faults and cures

Symptom	Likely causes	Fix
Strip peels off cleanly on the pull test, minimal mark on the terminal	Power too low; dirty or worn electrode tips; not enough pressure; electrodes too close together; strip not flat against the terminal	Check and dress the electrodes first, confirm the strip is flat, then raise power in 10 % steps
Holes burnt through the strip; terminal discoloured	Power too high; electrode tips too sharp; electrodes too close together	Reduce power; use a slightly blunter tip radius; widen the spacing a little
Weld spots vary in size and depth at a fixed setting	Worn electrodes; inconsistent hand pressure; strip not consistently flat; varying surface oxide	Dress the tips, practise consistent pressure on scrap, consider a dual-pulse machine
Electrodes stick to the strip	Power too high; nickel contamination on the tip; tip too pointed	Reduce power; clean and reshape the tip; increase the tip radius slightly
Lots of sparking at the tips	Insufficient pressure; oxidised strip surface	Press harder; clean the strip; use dual pulse if available

Step 12: Wiring, protection and the thermistor

Once every cell-to-strip joint is welded, the soldering iron comes out.

1. Solder the main leads to the end strips, not to a cell.
2. Reconnect the balance and sense leads in the correct order, starting at cell one.
3. Refit the thermistor against a cell, in the position it originally occupied. On a great many packs the machine's charger uses temperature to detect end of charge; a thermistor left dangling in free air will cause overcharging. This matters most on NiCd and NiMH packs.
4. Fit or refit the protection circuit. Every lithium cell group must be monitored individually. A pack without protection must never be charged or discharged unattended.
5. Insulate everything. Kapton over exposed strip, heatshrink over the block, insulators back where the original ones were.
6. Check for shorts with a meter before applying any charge, and confirm the pack terminal voltage matches the expected series count × cell voltage.

Smart packs, fuel gauges and the permanent-failure flag

A "smart" pack is two things: the chemical battery (the cells) and the digital battery (a microcontroller with a coulomb counter, usually talking SMBus). Replacing the cells fixes only the first.

Expect the following after a rebuild:

• The fuel gauge will be wrong, because the coulomb counter's learned full-charge capacity still describes the old cells. Calibrate the pack: charge fully, run the machine down until it reports a low battery, then charge fully again. The full discharge sets the discharge flag and the full charge sets the charge flag, and the gauge interpolates between them.
• Calibration is not permanent. Where a pack is in regular use, recalibrate every three months or after about 40 partial cycles.
• Max Error is the gauge's own estimate of how far it has drifted. Some manufacturers suggest calibrating at 8 %; above 12 % may raise an alarm and 16 % may make the pack refuse to work. There is no unified standard — every manufacturer picks its own thresholds.
• Impedance-tracking gauges self-learn and may need several cycles rather than one to settle.
• Some controllers latch a permanent-failure flag when they detect an over-voltage, under-voltage or over-temperature event — which is exactly what a pack does as its original cells die. Once latched, a new set of cells will not clear it, and the flag can only be reset by writing to the controller's EEPROM with the right tooling. On many vintage packs this is the point at which the rebuild stops being practical.
• SMBus is not rigidly standardised, so even a successfully rebuilt pack should be checked for compatibility with the machine's charger rather than assumed to work.

Step 13: First charge and verification

1. Charge slowly and attended, on a non-combustible surface. A slow first charge brings all the cells to parity.
2. Feel for heat repeatedly during the first charge. A pack that becomes warm at low current has a problem — stop and investigate.
3. Check individual cell-group voltages at the balance leads once charged. They should be within a few tens of millivolts of each other.
4. Discharge in the machine and confirm the runtime is plausible for the capacity fitted.
5. Leave the charged pack for a week and re-measure. Intrinsic defects show up as self-discharge, and a repaired pack should always be checked for it.
6. Recharge and recalibrate the gauge as described above.
7. Only then reassemble the casing.

Chemistry-specific notes

Replacing NiCd with NiMH

The rechargeable pack from an HP-41CX calculator — the kind of small, obsolete assembly that has to be rebuilt rather than replaced.

The NiCd pack from an Atari ST Book. Early-1990s packs like this are simple series stacks of tabbed cells and are the easiest kind to rebuild.

NiCd cells in the older fractional sizes are becoming difficult to buy, and NiMH is the usual substitute. It generally works, because both are 1.2 V nominal and the cell sizes are shared, and NiMH gives more capacity in the same can — but it is a substitution, not a drop-in, and there are three things to check.

• Charge termination. Nickel chargers detect full charge by a small voltage drop after the peak (−ΔV) or by a rise in temperature (dT/dt). NiMH produces a much smaller −ΔV than NiCd, so an old NiCd charger designed around a large voltage drop can miss the end of charge and cook the pack. This is why the thermistor matters so much on nickel packs, and why a rebuilt NiMH pack should be watched carefully through its first few charges.
• Trickle charge rate. NiCd tolerates a continuous trickle charge that NiMH does not. A machine that keeps its pack on a permanent trickle is a poor candidate for a NiMH conversion.
• Physical fit and thermal space. Match the form factor, the connector layout and the space around the cells.

Where a machine keeps its pack on permanent float and cannot be modified, staying with NiCd — or fitting a dummy pack and running from mains — is the safer answer.

Nickel packs generally

• Use pre-tabbed cells. Soldering to a bare nickel cell can is possible but poor practice; the tab is there so you do not have to.
• Solder to the tab, never the can, and keep the joint under about three seconds.
• Nickel cells self-discharge substantially — a rebuilt nickel pack that reads low after a few weeks on the shelf is behaving normally, unlike a lithium pack.
• Old NiCd packs leak. Check the machine's PCB and battery compartment for the white crystalline residue of leaked electrolyte and neutralise it before fitting a new pack. See Battery Explosion, Capacitor or Corrosion Damage.

Lithium-polymer pouch packs in handhelds

• Never puncture, fold, crease or trap a pouch cell. The foil is the containment.
• Buy the cell with its PCM fitted.
• Match thickness first — pouch cells swell slightly with age and a cell that just fits when new will press against the case later.
• Retain the original connector and, wherever possible, transplant it rather than rewiring the device.
• A swollen pouch cell is not repairable and is not a "still works" cell. Retire it.

Storage

Lithium cells and packs are stored part-charged, never full and never empty.

• Store at about 3.7–3.85 V per cell, roughly 40–60 % state of charge. Manufacturers recommend 40–50 % for storage; the trade-off is that too low a charge risks the cell drifting down into the sleep-mode and copper-dendrite region while it sits. If in doubt, keep it at the higher end of the range and keep it cool.
• Store cool. Room temperature or below; never in a hot loft, a car, or beside a radiator.
• Check stored packs every few months and top them up if they have drifted down.
• Store the pack outside the machine where the machine allows it, in a non-conductive container, with the terminals taped or covered.

Temperature and state of charge both matter, and they compound:

Li-ion self-discharge per month

State of charge	0 °C	25 °C	60 °C
Full charge	6 %	20 %	35 %
40–60 % charge	2 %	4 %	15 %

A fully charged cell in a warm room loses five times as much per month as a half-charged one, and it ages faster while doing it. This is the whole argument for storing part-charged.

Nickel packs behave differently: their self-discharge is much higher, so a NiCd or NiMH pack that reads flat after a few months on the shelf is normal rather than faulty. Charge them before storage and top them up periodically. NiMH is typically good for 300–400 cycles and standard NiCd for over 1000, after which rising self-discharge starts to dominate.

Disposal

The Ni-Cd recycling symbol and the crossed-out wheeled bin. Batteries carrying these marks must not go in household waste.

Old cells never go in household or kerbside waste. Crushed in a bin lorry or a waste transfer station, a lithium cell starts a fire — this is now one of the commonest causes of waste-industry fires.

• Tape both terminals of every cell with non-conductive tape before it leaves the bench. This is the single most important step.
• Store waste cells in a non-conductive container — never loose in a tin or a drawer with other metal.
• Damaged, swollen or vented cells go into a separate container, ideally with dry sand, and are taken for disposal promptly rather than stored.
• In the UK, the Waste Batteries and Accumulators Regulations 2009 make collection and recycling compulsory and prohibit batteries from being landfilled or incinerated. Any distributor or retailer selling more than 32 kg of portable batteries a year must provide a free collection point on its premises — which is why supermarkets and larger shops have a battery box near the entrance. Household waste recycling centres also accept them, and many councils run a small-electricals and battery kerbside collection.
• Cadmium (NiCd) and lithium both require specific recycling routes, which is what the crossed-out wheeled bin and the chemical symbol under the recycling triangle are telling you.

Troubleshooting a rebuilt pack

Common problems after a rebuild

Symptom	Likely cause	Action
Machine does not see the pack at all	Solid-state switch off; missing thermistor connection; pack locked by an activation code; sense leads in the wrong order	Probe with the 100 Ω resistor technique; verify the thermistor reads ~10 kΩ at room temperature; re-check the sense-lead order against your diagram
Pack charges but reports the wrong capacity	Fuel gauge still holds the old cells' learned capacity	Run a full charge / full discharge / full charge calibration cycle; repeat several times on an impedance-tracking gauge
Machine shuts down with the pack apparently still part-charged	One weak or mismatched cell group hitting the low-voltage cut-off first	Measure each group at the balance leads; the odd one out is the problem. This is what cell matching prevents
Runtime far short of the capacity fitted	Counterfeit or reclaimed cells; cells not capacity-tested; high-resistance welds	Capacity test the cells individually; check weld quality
Pack or a specific cell runs hot	Cold weld or high-resistance joint; damaged or high-IR cell	Locate the hot spot; re-weld the joint or replace the cell
Pack dies within weeks	Self-discharging cell that was never rested and re-measured	Do the one-week self-discharge test on every cell next time
Pack was fine, then permanently refused to work	Latched permanent-failure flag in the controller	Needs EEPROM-level access to reset; often the end of the road for that pack
Strip comes loose in service	Welds that passed a light pull but were never properly calibrated	Recalibrate on scrap and re-weld; a good weld tears the strip
NiMH pack overheats on charge	Charger expecting NiCd's larger −ΔV; thermistor not touching a cell	Refit the thermistor correctly against a cell; consider staying with NiCd

Related pages

• Battery Explosion, Capacitor or Corrosion Damage — cleaning up after a pack that leaked into the machine
• CRT Discharge Procedure — the other job on this wiki that will hurt you if you rush it
• Recommended Tools
• Capacitor Failure Symptoms