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 lithium-polymer pouch cells in this MacBook Pro battery have swollen enough to force the pack casing apart. A pack in this condition is not charged, cut, crushed or carried loose.

Battery refurbishment is the rebuilding of rechargeable battery packs for vintage computers and handheld devices whose original packs are no longer made: laptops, portables, PDAs, organisers, calculators, test equipment and games handhelds. Rebuilding a pack means identifying what is inside it, choosing replacement cells, joining them, dealing with the pack's own electronics and testing the result, and the method is the same whoever made the pack. Machine-specific guides cover the quirks of individual packs, and Manufacturer battery data at the end lists the batteries each manufacturer fitted, with links to the platform pages.

Lithium cells are joined by spot welding. A soldering iron is used on the wiring and the strip, and kept off the cells themselves; most of the procedure follows from that rule.

Safety warning

A lithium cell holds a flammable electrolyte and enough stored energy to ignite it. A cell that is short-circuited, crushed, punctured, overheated or overcharged can go into thermal runaway, and the heat it releases can set off the cells next to it.

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.

A cell in thermal runaway vents roughly 1 to 2 litres of gas for every amp-hour of capacity, so a 2.5 Ah 18650 releases several litres. The gas is mostly hydrogen, carbon monoxide and carbon dioxide, with hydrocarbons, in proportions that depend on the chemistry and the state of charge. Burning lithium-ion cells also release hydrogen fluoride: tests on seven types of commercial cell measured 20 to 200 mg per watt-hour of rated capacity.

Basic rules

• Keep the soldering iron off a lithium cell's terminals and can. Cell manufacturers specify a maximum can temperature of 60–80 °C during assembly. An iron runs at 280–380 °C, and one to three seconds on a terminal is enough to pass that limit, with heat still conducting into the cell after the iron is lifted. The separator that keeps the electrodes apart is a microporous polyethylene or polypropylene film, and its pores are designed to close by melting when the cell overheats.
• Do not charge a pack that has started to swell. Stop charging and switch the device off. Retire any cell that is swollen, dented, punctured or leaking.
• Do not charge a lithium cell that has sat below 1.5 V for a week or more. Copper shunts can form inside a cell left that low, and on recharge it "might become unstable, causing excessive heat or show other anomalies".
• Take the pack out of the machine before working on it.
• Do not leave a rebuilt pack charging unattended, and do not charge or discharge it without a working protection circuit.
• Build a pack from matched cells of one chemistry, make and capacity.

Fire

If a battery starts to smoke or catches fire, the London Fire Brigade's advice is to get out, raise the alarm, call 999 and not to tackle the fire yourself. Lithium-ion fires spread quickly and give off toxic smoke. Tell the fire service that a lithium battery is involved.

Lithium-thionyl chloride memory-backup cells (the 3.6 V half-AA type) are a different case. Electrochem's safety data sheet says water spray may be ineffective on a lithium fire, but that copious water may be used to cool a battery fire and put out burning material around it. The electrolyte releases toxic sulfur dioxide.

Before you touch anything

Minimum precautions

Precaution	Why
Safety glasses	Cells vent upwards and sideways, and nickel strip flicks when it tears.
Safety gloves	If you are spot welding, use electrical safety gloves. If you accidentally short a battery, it has a high chance of causing severe burns. Nickel strips flick when they burn, which has a high chance of burning skin.
A non-combustible work surface	A ceramic tile, a steel tray or a paving slab. Not a wooden bench top or a carpet.
A way to get the pack outside	A metal bucket or tin with a lid, or a bucket of dry sand, within arm's reach, so that a hot or venting cell can be carried out of the building.
Insulated tools	Tape all but the last few millimetres of side cutters and pliers. A bare tool dropped across a pack is a short circuit.
Rings and metal watch straps removed	A ring across a pack terminal heats up faster than it can be taken off.
Ventilation	Vented electrolyte is an irritant, and hydrogen fluoride is among the combustion products.
No naked flame and no hot-air gun near a cell	Adhesive is softened briefly and from a distance.

Work on one connection at a time. A loose strip, a dropped tool or a cell rolling into a live terminal all make a short circuit.

Is refurbishment the right answer?

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

Deciding what to do with an obsolete pack

Situation	Approach
A modern equivalent pack is still sold	Buy it. Rebuilding is for the packs nobody makes.
The pack is a series stack of cells with no electronics	A straightforward rebuild.
The pack has a simple protection PCB (PCM) and no data bus	A 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 only holds CMOS or RTC settings	Rebuilding around a tabbed coin cell is cheap. See Coin cells below.
The machine always runs from mains and the pack is dead weight	Consider a dummy pack: the original casing, gutted, with the terminals left open or a link fitted if the machine needs one. It removes the fire risk.
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	Treat them as scrap. See the 1.5 V rule above.

Step 1: Identify what you have

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

Chemistry

Chemistries found in vintage computer and handheld packs

Chemistry	Nominal per cell	Notes for the rebuilder
NiCd (nickel-cadmium)	1.2 V	The electrolyte is potassium hydroxide and the cells contain cadmium; see Battery Explosion, Capacitor or Corrosion Damage for leakage. Pre-tabbed cells are still made.
NiMH (nickel-metal hydride)	1.2 V	The usual replacement for NiCd, with higher specific energy. It needs a charger that can detect its full charge; see Replacing NiCd with NiMH.
Li-ion (cylindrical)	3.6–3.7 V	Charged to 4.20 V (NMC and NCA types). Needs protection electronics. Kept away from the iron.
LiPo (lithium-polymer pouch)	As Li-ion	A soft foil pouch, easily punctured. Buy replacement cells with their protection board fitted.

Chemistry is normally printed on the pack label, on the cell wrapper or moulded into the casing. If the label is gone, the cell count against the pack voltage settles it: nickel packs come in multiples of 1.2 V (2.4, 3.6, 4.8, 6.0, 7.2, 9.6, 12 V), lithium packs in multiples of 3.6 or 3.7 V (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 three cells in series (3 × 3.6 V), 14.8 V is four (4 × 3.7 V). A 4400 mAh rating on a pack built from 2200 mAh cells means two parallel strings.

The label gives the configuration before the pack is opened. Pack voltage divided by the cell's nominal voltage is the series count (S): 10.8 V ÷ 3.6 V is 3S, 14.4 V ÷ 3.6 V is 4S, and 7.2 V ÷ 1.2 V is six nickel cells in series. Pack capacity divided by one cell's capacity is the parallel count (P): a 4400 mAh pack built from 2200 mAh cells is 2P, so a 4S2P pack holds eight cells. Watt-hours are pack voltage times amp-hours; a 10.8 V 3.8 Ah pack is about 41 Wh.

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

Cylindrical lithium cell sizes

An 18650 cell (left) and a 21700 (right). The numbers are dimensions in millimetres.

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

The five-digit numbers are dimensions. For taller cylindrical cells the first two digits are the diameter in millimetres and the rest give the height, so an 18650 is 18 mm across and 65.0 mm long. The same cell is sometimes called an 1865. The number says nothing about the chemistry: lithium cobalt oxide, NMC, NCA and lithium iron phosphate cells have all been made in the 18650 size, and by the 2020s so had sodium-ion and potassium-ion cells, with different voltages and charging requirements. Check the chemistry separately.

Cylindrical lithium cell sizes

Designation	Diameter	Length	Notes
18650	18 mm	65.0 mm	The usual laptop cell. Sony developed it in 1991; Panasonic claims 1994.
18500	18 mm	50.0 mm
17670	17 mm	67.0 mm
16340 (RCR123)	16 mm	34.0 mm
14500	14 mm	50.0 mm	The size of an AA cell, but 3.7 V nominal. It will destroy a device built for 1.5 V AA cells.
21700	21 mm	70.0 mm	Useful only where the pack has room.
26650	26 mm	65.0 mm

These lengths are for the bare cell. A protected cell carries a small protection circuit under the wrapper, and a protected 18650 is about 68 mm long; some protection circuits add to the diameter instead. Measure the compartment before ordering.

Decoding the chemistry prefix

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

Chemistry prefixes on cylindrical lithium cells

Prefix	Cathode	Traits	Typical use
ICR	Lithium cobalt oxide, LiCoO2	High capacity, low discharge current	Laptops, cameras
IMR	Lithium manganese oxide, LiMn2O4	High discharge current, stable	Power tools
INR	Nickel manganese cobalt, LiNiMnCoO2 (NMC)	Balanced capacity and discharge	General purpose
NCR	Nickel cobalt aluminium, LiNiCoAlO2 (NCA)	Very high capacity	Panasonic high-capacity cells
IFR	Lithium iron phosphate, LiFePO4	Long life, most thermally stable	Not a Li-ion substitute; see below

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

LiFePO₄ (IFR) cells are 3.2 V nominal and are charged to 3.65 V, against 4.2 V for an NMC or NCA cell. They will not reach the voltage a Li-ion pack is expected to give, 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 old packs. Sub-C is 23 mm × 43 mm.

Loose NiMH cells, the usual replacement for an obsolete NiCd pack.

Nickel cells use the old ANSI size names, with fractions for shortened versions of a standard size: a 2/3 A cell has the diameter of an A cell and about two-thirds of its length. Fractional sizes are common in laptop and handheld packs.

NiCd and NiMH cell sizes

Size	Diameter	Length
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
1/2 AA	14.2 mm	30 mm
2/3 AA	14.2 mm	28.7 mm
4/5 AA	14.2 mm	43 mm
AA	14.2 mm	50 mm (48 mm flat top)
4/3 AA	14.2 mm	65.2 mm
1/2 A	17 mm	25 mm
2/3 A	17 mm	28.5 mm
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
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
F6 prismatic	5.6 × 16.5 × 48 mm

Diameter can vary by up to 1 mm between manufacturers, and a protruding end cap adds length. Measure the original cell with calipers and compare it with the datasheet of the cell you intend to buy.

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 packs.

Pouch (LiPo) cells

Pouch cells usually carry a six-digit size code. The first two digits are commonly the thickness in tenths of a millimetre and the next two pairs the width and length in whole millimetres, so a 503759 cell is 5.0 × 37 × 59 mm. The scheme is not a universal standard, so check the supplier's drawing.

Buy a replacement pouch cell with its protection circuit module (PCM) already fitted where one is offered. That keeps the iron away from the cell tabs.

Coin cells, CMOS and RTC batteries

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

• Buy pre-tabbed cells and solder to the tab. A tabbed cell can still explode if the iron is held on the tab too long.
• Coin cell numbers are dimensions too. The last two digits are the height in tenths of a millimetre and the digits before them the diameter in millimetres, so CR2016, CR2025 and CR2032 are all 20 mm across and 1.6, 2.5 and 3.2 mm thick.
• A cell that takes a CR2016 can take a CR2025 or CR2032 if there is room.
• Use vertical tabs where possible, and do not bend a tab to fit; a bent tab can short across the cell. A cell with horizontal tabs needs 19.2–19.3 mm heatshrink to cover the edges. All bare metal must end up covered.
• A dead coin cell reads a few millivolts or 0.00 V. On a laptop, a working main battery can mask a dead CMOS cell: remove the main pack and hold the power button for 30 seconds before testing.

Where the board charges its backup cell (a NiCd or NiMH barrel, or a rechargeable lithium coin cell), a primary lithium cell must not be fitted in its place without a series diode to block the charging current. The manufacturer sections below say which machines charge their cells.

Spotting fake and reclaimed cells

Counterfeit cylindrical cells are common, and a rebuilt pack is only as good as its cells.

Warning signs when buying 18650 cells

Sign	What it means
A claimed capacity of 5000, 6800 or 9900 mAh	Genuine branded 18650s sit in about the 2.6–3.5 Ah class. Claims like these are fake.
A cell well below its model's published weight	Genuine cells of the common models weigh roughly 46–49 g. A kitchen scale will catch a hollow fake.
Cells in one "new" lot that differ by several grams, or arrive at very different voltages	A mixed or rewrapped lot.
Missing or odd can codes, top rings, vent disks or wrapper printing	A counterfeit or rewrapped cell.
Internal resistance far above the model's published figure	A worn, reclaimed or fake cell.

The proof is measurement: charge the cell, run a capacity test and compare the result with 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.

A spot welder passes a large current through the joint for 1–10 milliseconds. The interface between strip and terminal melts and re-solidifies before much heat can conduct into the cell, and a correctly made weld raises the cell's surface temperature by only a few degrees. Soldering keeps the iron on the terminal for seconds.

Cheap welders are the limit on strip thickness. Cell Saviors notes that most low-cost welders struggle with 0.15 mm nickel and most cannot weld 0.20 mm at all, and handheld rechargeable welders usually cannot manage 0.25–0.3 mm. A small vintage pack built with 0.1 or 0.15 mm strip is within reach of a modest machine.

Dual-pulse machines fire a small conditioning pulse before the main pulse. If welds stay inconsistent with good electrodes and steady pressure, the usual cause is surface oxide on the strip, which is what the conditioning pulse deals with.

Copper-alloy electrodes conduct and extract heat well but wear faster; tungsten tips are harder. For 18650 and 21700 work, slightly rounded tips of about 1–1.5 mm radius are usual. Inspect tungsten tips every 30–50 welds and copper-alloy tips every 20–30, and dress them when the spots grow larger and shallower at the same setting.

Nickel strip

A shrink-wrapped four-cell NiCd pack with pre-welded solder tabs, the Ni-Cd recycling symbol and the crossed-out wheeled bin mark.

Pure nickel strip: welding starting points

Thickness	Notes
0.1 mm	Low end of the welder's range. Welds easily and burns through easily.
0.15 mm	Mid range. The most common gauge and the best place to start calibrating a new machine.
0.2 mm	Mid to upper range. Two welded layers of 0.15 mm are an alternative.
0.25–0.3 mm	Needs a capable bench or professional welder.

Use pure nickel. It is about twice as conductive as nickel-plated steel, and plated steel is sometimes sold as pure nickel. Suppliers' current ratings for strip vary widely; size the strip for the machine's current with a margin.

The soldering iron, and where it is allowed

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 sits between the iron and the cell.
Protection board sense leads to strip or pads	Yes	Ordinary electronics soldering.
Output connector to the pack leads	Yes
Nickel strip to nickel strip	Yes
Pre-welded solder tab on a NiCd or NiMH cell	Yes, briefly	The tab, not the can.
Tab of a tabbed coin cell	Yes, briefly	Not the cell body.
Bare NiCd or NiMH can	Avoid	Buy pre-tabbed cells.
Lithium cell terminal or can	No	See Basic rules.
LiPo pouch tab	No, in practice	Buy the pouch cell with its PCM fitted.

With no spot welder and a low-drain one-off pack, the least-bad soldering technique is a 60–80 W iron with a large tip, flux applied generously and the terminal pre-tinned quickly, the joint finished in under one second of contact, 10–15 seconds of cooling before the next connection, and never a second application to the same terminal in a session. This reduces the heat input without removing the risk. For a pack that will be cycled regularly, borrow or buy a welder.

On nickel cell tabs, scuff the tab with emery cloth, clean it with isopropyl alcohol, tin the iron well so the heat transfers quickly, and keep the contact short.

The rest of the kit

• A digital multimeter, for cell and pack voltages, thermistor resistance and continuity.
• A charger or analyser with a capacity test, for grading and matching cells.
• An internal-resistance meter, or a charger that measures internal resistance.
• A milliohm meter (optional), the only quantitative check on weld quality.
• Digital calipers, for measuring original cells and pouches.
• Kapton (polyimide) tape, to hold strip in place while welding and to insulate afterwards.
• Fish-paper or pre-cut insulating rings for the positive end of cylindrical cells.
• Heatshrink sleeving, including large-diameter sleeving to 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. Do not calibrate on the cells you intend to use.
• Insulated side cutters, thin pliers, plastic spudgers and a thin scraper for opening welded casings.
• A hot-air gun, used briefly and from a distance to soften adhesive.
• Isopropyl alcohol, cotton buds and 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 insulators.
2. Measure the pack terminal voltage and compare it with the label. A pack reading zero may have a protection circuit latched off; a pack reading a plausible voltage has energy in it and is live.
3. Record the label data: chemistry, voltage, capacity, watt-hours and part number.
4. Work out the expected configuration (nS × nP) from the label as described above.
5. Inspect for swelling, corrosion and leakage. Corrosion around a NiCd pack means electrolyte has escaped, so check the machine's board 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. Expect to break some internal clips; they can be glued on reassembly.
3. Do not cut into a pack blind. The cells may sit right against the casing wall.
4. Do not use a hot-air gun on the casing over the cells. If the seam must be warmed, warm it locally and briefly.
5. Once the pack is open, tape over every exposed terminal you are not working on.

Step 5: Map the pack before you cut anything

A sealed NiMH laptop pack. Nothing on the outside shows how the cells are wired inside, which is why the pack is mapped before anything is disconnected.

Draw a diagram. Record:

• the cell count, orientation and physical arrangement, and which cells form each parallel group;
• the series and parallel wiring, tracing each nickel strip;
• the thermistor, a two-wire part pressed against a cell. Note which cell and where on it. The most common type is a 10 kΩ NTC, which reads 10 kΩ at 20 °C and falls as it warms, so it can be found with an ohmmeter;
• the sense leads. Some fuel-gauge chips run a wire to each cell, and these must be reconnected in sequence starting from cell one;
• fuses, PTC devices and thermal cut-outs in the strip work, which are easily mistaken for plain strip;
• the pack connector pinout. A typical smart pack has five or more contacts, with positive and negative usually at the outer edges and the thermistor, clock and data on the inner contacts, which are often unmarked. Find positive and negative with a voltmeter first.

Step 6: Keep the pack electronics alive

On a smart pack, interrupting the controller's supply for even a fraction of a second can erase its memory, including the value of the digitised shunt resistor that its coulomb counter depends on.

Supply the controller from a source of the same voltage through a 100 Ω resistor before the old cells are disconnected, and remove it only once the new cells are in place.

The same resistor is used to probe a pack that shows no voltage. Some packs have a solid-state switch that holds the terminals at zero until it is enabled. With the voltmeter on the outer terminals, tie one end of the resistor to ground and touch the other end to each remaining contact in turn, then repeat with the resistor tied to the positive side, and watch for the output to appear.

If nothing responds, the pack may need an activation code. Battery makers keep these codes secret, even from service staff.

Step 7: Remove the old cells

1. Treat every cell as charged; a pack that will not power up cannot be discharged first.
2. Cut the strip between cells with the cutters. Do not lever strip off a cell you intend to keep.
3. To salvage a cell, grip the strip by an edge and roll it off the terminal slowly, and keep the freed strip from falling across the cell.
4. Keep the original strip layout as a template if the geometry is unusual.
5. Keep the protection or fuel-gauge board, the thermistor, the connector and any moulded insulators. On a vintage pack these are the parts nobody sells.
6. Tape the terminals of every removed cell straight away and put the cells in a non-conductive container away from the work.

Step 8: Choose the replacement cells

Selection rules

Rule	Reason
Same chemistry as the original	The machine's charger is built around one chemistry's charge termination. NiCd to NiMH needs care; see Replacing NiCd with NiMH.
Same physical size, or smaller	Measure with calipers. A protected 18650 is about 3 mm longer than a bare one.
Same series count	The pack voltage must match what the machine expects.
All cells identical: same maker, part number and batch where possible	Cells in a parallel group should match within 50 mAh and 20 mΩ.
Modest, genuine cells	A known-brand cell of modest capacity beats an optimistic unknown one, and the original charger was designed for the original current levels.
Pre-tabbed nickel cells if there is no welder	Solder to the tab, not the can.
Pouch cells with the PCM already fitted

Salvaging 18650s from scrap laptop packs is common. Only cells that pass the grading steps below belong in a pack that will be left on charge.

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 and measure each cell's real capacity.

Every cell that goes into the pack, new or salvaged, goes through the same checks.

1. Visual inspection

Reject any cell that is swollen, dented, cracked, corroded, leaking or scorched. A scratched wrapper is cosmetic, but a split one should be replaced before use: the whole can of a cylindrical lithium cell is the negative terminal.

2. Resting voltage

• A lithium-ion cell should not be discharged below 2.50 V. Copper dendrites grow in a cell left at low voltage for more than about a week.
• Do not attempt to charge a cell that has sat below 1.5 V for a week or more.
• An NMC or NCA cell is full at 4.2 V.

"Sleeping" packs and boosting

A pack that reads zero is not necessarily full of dead cells. Depending on the maker, a Li-ion protection circuit cuts off somewhere between 2.2 and 2.9 V per cell, and a pack left in storage can self-discharge past that point and switch itself off. Some chargers and analysers have a "boost" function that applies a small current to wake the protection circuit, after which a normal charge can follow.

• Discard the pack if the voltage does not rise to a normal level within about a minute of boosting.
• Check polarity with great care. A sleeping pack does not show its voltage, and a voltage applied in reverse causes permanent damage.
• The 1.5 V, one-week rule overrides all of this.

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 on its own and watch its temperature. A cell that warms noticeably at a low charge current has high internal resistance or an internal fault. Discard it.

4. Self-discharge

Leave the charged cells to rest for a week and measure them again. A cell whose voltage has fallen further than its batch-mates' is self-discharging and is discarded. Intrinsic defects often show up as high self-discharge.

5. Capacity and internal resistance

• Capacity: charge fully (4.2 V for NMC and NCA, 3.65 V for LiFePO₄), then discharge at a fixed current, such as 0.5 A, to a cut-off of about 2.8 V for an NMC 18650. Use the same rate for the whole batch. Cell Saviors treats 80 % of rated capacity as acceptable for reuse, below 70 % as marginal and below 60 % as time to retire the cell.
• Internal resistance: on a DC test, a good 18650 from a reputable maker typically reads 50–150 mΩ. Above 300 mΩ the cell is degraded, and above 500 mΩ it should not go into a pack.
• A cell that gets warm during a 0.5 A discharge, or shows a flat spot in its discharge curve, is retired.

6. Match the cells

Cells in the same parallel group are matched for capacity and resistance, within 50 mAh and 20 mΩ of each other, and each parallel group should total about the same capacity as the others. The weakest group reaches the protection circuit's limits first and sets the capacity of the whole pack.

Bring every cell to the same voltage before assembly, so that no large balancing current flows when the parallel groups are joined.

Step 10: Lay out and insulate

1. Arrange the cells exactly as the original diagram shows. One cell reversed 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 and before welding.
3. Fit an insulating ring to the positive end of each cylindrical cell. The positive terminal is only the raised button; the flat ring around it is part of the negative can, and a strip that shifts slightly will short the cell without the insulator.
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 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

Do not start on the real cells.

1. Set the machine to about half 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 strip tears before the weld lets go.
5. Go slightly higher until the strip burns through or the terminal discolours; the setting below that is the maximum.
6. The working range passes the pull test without burning through. Write the setting down against the strip gauge, strip material and cell type, and recalibrate when any of them changes.

Calibration takes 15–20 minutes.

Technique

• Space the electrodes 2–5 mm apart. Closer than that, the current takes the short path through the strip and the weld does not bond underneath; much further apart (over about 7 mm), the energy goes into heating the strip. Use the closer end for 0.1 mm strip and the wider end for 0.2 mm.
• Press firmly and steadily, enough that the electrodes cannot slide.
• Hold the pressure for about half a second to a second after the pulse while the nugget solidifies, then lift cleanly. Do not drag the electrodes between positions.
• Make at least two weld spots per strip end per terminal, and four where the pack carries a high current. Keep the spot positions consistent from cell to cell.
• Work along the strip from one end to the other and finish each cell before moving to the next.

Check the welds

A good weld leaves two small, clean, round indentations of consistent size; slight darkening is normal. Elongated or smeared marks, spark marks outside the spot, burn-through or a discoloured terminal are faults.

The pull test is the definitive check: grip the strip with pliers and pull it away from the terminal. A good weld tears the nickel and leaves some of it fused to the cell; a bad one peels off and leaves the terminal almost untouched. Do this on scrap during calibration, not on every production weld.

A well-made nickel tab weld measures around 0.05–0.3 mΩ on a milliohm meter. Higher readings point to a cold weld, oxide or misaligned electrodes.

Weld faults

Spot welding faults and cures

Symptom	Likely causes	Fix
Strip peels off cleanly on the pull test, little mark on the terminal	Power too low; dirty or worn tips; too little pressure; electrodes too close; strip not flat	Dress the electrodes, flatten the strip, then raise power in small steps
Holes burnt through the strip; terminal discoloured	Power too high; tips too sharp; electrodes too close	Reduce power; use a blunter tip; widen the spacing a little
Weld spots vary in size at a fixed setting	Worn electrodes; uneven pressure; strip not flat; surface oxide	Dress the tips, practise pressure on scrap, consider dual pulse
Electrodes stick to the strip	Power too high; nickel on the tip; tip too pointed	Reduce power; clean and reshape the tip; increase the tip radius slightly
Heavy sparking at the tips	Too little pressure; oxidised strip	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 sense leads in sequence, starting at cell one.
3. Refit the thermistor against a cell, where it was originally. Nickel chargers use temperature rise as one of their end-of-charge signals, so a thermistor left in free air can lead to overcharging.
4. Fit or refit the protection circuit. Each lithium cell must be monitored individually, and a pack without protection is never charged or discharged unattended.
5. Insulate everything: Kapton over exposed strip, heatshrink over the block, insulators back where the originals were.
6. Check for shorts with a meter before applying any charge, and confirm the pack voltage matches the series count times the 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 on SMBus). New cells fix only the first.

• The fuel gauge will be wrong after a rebuild, because it still holds the old cells' capacity. Calibrate it with a full charge, a full discharge in the machine and a full charge. In regular use, Battery University suggests calibrating every three months or after 40 partial cycles.
• The gauge's "Max Error" is its own estimate of drift. Some manufacturers recommend calibrating at 8 %; above 12 % may raise an alarm and 16 % may make the pack unserviceable. Every maker sets its own thresholds.
• Impedance-tracking gauges learn by themselves but may need several cycles.
• Some gauge chips permanently disable a pack when they detect a serious fault. Texas Instruments' bq20z-series gauges, for example, set a permanent-failure flag for a large voltage difference between series cells, which is what one dying cell produces, and for a safety over-voltage. Replacing the cells does not clear the flag. On the bq20z40 it is cleared by sending a two-word key through the gauge's ManufacturerAccess command, and on many vintage packs that is where the rebuild stops.
• SMBus allows variations between makers, so check a rebuilt pack against the machine's charger.

Step 13: First charge and verification

1. Charge slowly, attended and on a non-combustible surface. A slow first charge brings the cells to parity.
2. Feel for heat repeatedly during the first charge. A pack that warms at low current has a problem; stop and investigate.
3. Measure each cell group at the sense leads once charged. They should agree closely.
4. Discharge in the machine and check that the running time is plausible for the capacity fitted.
5. Leave the charged pack for a week and measure it again for self-discharge.
6. Recharge and calibrate the gauge as described above.
7. Only then close the casing.

Chemistry-specific notes

Replacing NiCd with NiMH

The rechargeable pack from an HP-41CX calculator, a small obsolete assembly that has to be rebuilt.

The NiCd pack from an Atari ST Book, a simple series stack of tabbed cells.

NiMH has the same 1.2 V nominal voltage as NiCd and more energy in the same size of cell, but the machine's charger decides whether the swap is safe.

• A charger detects full charge by a small voltage drop after the peak (negative delta V) or by a rise in temperature. The voltage drop of NiMH is faint, and a NiMH charger has to respond to a drop of 5 mV per cell or less. A charger built for NiCd can miss it.
• NiMH tolerates less overcharge. Battery University gives a trickle rate of about 0.05C for NiMH against 0.1C in the original NiCd chargers.
• Battery University's conclusion is that an original NiCd charger is unsuitable for NiMH, and that a NiMH cell in a NiCd charger will overheat.

Where the machine charges its own pack from a NiCd charging circuit, stay with NiCd, or fit a dummy pack and run from mains. Keep the thermistor against a cell whichever chemistry is fitted.

Nickel packs generally

• Use pre-tabbed cells and solder to the tab, briefly.
• Nickel cells self-discharge much faster than lithium ones, so a rebuilt nickel pack that reads low after a few weeks on the shelf is not necessarily faulty. NiMH is good for 300–400 cycles and standard NiCd for over 1,000 before rising self-discharge interferes.
• Old NiCd packs leak. Check the machine's board and battery compartment for the white crystalline residue of leaked electrolyte and deal with it before fitting a new pack. See Battery Explosion, Capacitor or Corrosion Damage.

Lithium-polymer pouch packs in handhelds

• Do not puncture, fold, crease or trap a pouch cell. The foil is the only containment.
• Buy the cell with its PCM fitted.
• Match the thickness first, and leave the cell some room in the case.
• Keep the original connector and transplant it where possible.
• A swollen pouch cell is retired, not reused.

Storage

Store lithium cells and packs part-charged. Manufacturers recommend 40–50 % state of charge; a lower charge risks the cell drifting down into the protection cut-off while it sits, so if in doubt keep it at the higher end and keep it cool.

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 at 25 °C loses five times as much per month as a part-charged one. Keep stored packs out of lofts, cars and airing cupboards, check them every few months, and store them outside the machine where it allows, in a non-conductive container with the terminals covered.

Nickel packs self-discharge much faster, so one that reads flat after a few months on the shelf is normal. Charge them before storage and top them up from time to time.

Disposal

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

Old cells do not go in household or kerbside waste. A crushed or punctured lithium cell is a fire risk.

• Tape both terminals of every cell with non-conductive tape before it leaves the bench.
• Keep waste cells in a non-conductive container, not loose in a tin or drawer with other metal.
• Put damaged, swollen or vented cells in a separate container, with dry sand, and take them for disposal promptly.
• In the UK, the Waste Batteries and Accumulators Regulations 2009 make collection and recycling compulsory and stop batteries being incinerated or sent to landfill. A distributor or retailer that sells more than 32 kg of portable batteries a year must offer a take-back service, which is why larger shops have a battery box. Household waste recycling centres also take them.

Troubleshooting a rebuilt pack

Common problems after a rebuild

Symptom	Likely cause	Action
Machine does not see the pack	Solid-state switch off; thermistor disconnected; activation code; sense leads out of order	Probe with the 100 Ω resistor method; check the thermistor reads about 10 kΩ at 20 °C; recheck the sense-lead order against your diagram
Pack charges but reports the wrong capacity	Fuel gauge still holds the old cells' capacity	Run a full charge, full discharge, full charge; repeat on an impedance-tracking gauge
Machine shuts down with the pack apparently part-charged	One weak or mismatched cell group reaching the low-voltage cut-off first	Measure each group at the sense leads; the odd one out is the problem
Running time far short of the capacity fitted	Counterfeit or reclaimed cells; untested cells; high-resistance welds	Capacity-test the cells; check the welds
Pack or one cell runs hot	Cold weld or high-resistance joint; damaged or high-resistance cell	Find the hot spot; re-weld the joint or replace the cell
Pack dies within weeks	A self-discharging cell that was never rested and re-measured	Do the one-week self-discharge test on every cell
Pack worked, then refused to work at all	Permanent-failure flag set in the gauge	Needs the gauge's key and tools to clear; often the end for that pack
Strip comes loose in use	Welds that passed a light tug but were never calibrated	Recalibrate on scrap and re-weld; a good weld tears the strip
NiMH pack overheats on charge	NiCd charger missing the NiMH end of charge; thermistor not touching a cell	Refit the thermistor against a cell; go back to NiCd

Manufacturer battery data

Most vintage machines carry a small battery to keep a clock and configuration memory alive, and portables add a main pack. The chemistry decides both how a cell fails and how its leakage is neutralised: alkaline and NiCd leakage is caustic and takes a mild acid such as vinegar, while lithium-thionyl chloride residue is acidic and takes bicarbonate. The full cleaning procedure is on Battery Explosion, Capacitor or Corrosion Damage. Where a board charges its backup cell, a primary lithium cell fitted in its place needs a series diode.

Apple

Apple's Service Source volumes print a battery verification procedure with a replace-below figure for the PRAM battery, and the figure differs between families. Measure against the machine's own figure: a 3.1 V cell is due for replacement in a Quadra 840AV and fine in a Macintosh IIsi.

Apple PRAM battery replace-below figures

Replace below	Machines, by the volume that gives the figure	Notes
2.8 V	Macintosh SE, SE/30, Macintosh IIcx, IIci and Quadra 700	The SE and SE/30 procedures have the cover off, the CRT discharged and the logic board out before the cell is measured; see CRT Discharge Procedure.
3.0 V	Macintosh IIsi, LC, LC II, LC III, LC 475 and Quadra 605, LC 520, 550 and 575, LC 580 and Performa 580CD, Macintosh TV, Performa 400 series, Performa 500 series, Performa 6200/6300 series, Power Macintosh/Performa 5200 and 5300, 5260 and 5280, 5400 and 5500, 6400 and 6500 series, Power Macintosh 4400	The all-in-one machines are measured with the I/O door and the logic board removed.
3.2 V	Macintosh II, IIx and IIfx, Quadra 610, Centris 610 and WS 60, Quadra 800 and WS 80, Quadra 840AV, Quadra 900, 950 and AWS 95, Power Macintosh 6100 and WS 6150, 7100 series, 8100 and WS 8150	The Quadra 800 and Power Macintosh 8100 procedures have the logic board out first.
No figure printed	Performa 630 series, Power Macintosh 8200/8500 and WS 8550, 9500 series, 7300/7500/7600 and WS 7350, Twentieth Anniversary Macintosh	These volumes give no replace-below figure.

Most of the 3.0 V and 3.2 V machines take 3.6 V lithium cells. Apple's technical specification pages give a 4.5 V alkaline battery for the LC 575 and LC 580, the Performa 580CD and 588CD, and the Power Macintosh and Performa 4400, 5200, 5260, 5300, 5400, 5500, 6200, 6300, 6400 and 6500, although some of their Service Sources describe a lithium cell. Check the cell fitted before cleaning up a leak. Four other groups of Apple machines have batteries of their own:

Other Apple batteries

Machine	Battery	Notes
Macintosh 128K and 512K, Macintosh Plus	4.5 V, user-replaceable, in a compartment at the rear	Alkaline. Larry Pina's parts list for the analogue board gives B1 as an Eveready 523, 4.5 V. Leakage is neutralised with vinegar.
LC 630, Performa 630 and Performa 630CD DOS Compatible	4.5 V alkaline	Apple's 630-family Service Sources give no replace-below figure. Alkaline leakage takes vinegar.
Macintosh Portable	Main battery: sealed lead-acid, 6.5 V, up to 10 hours. Backup: 9 V transistor battery. Power adapter output 7.0–7.6 V (7.5 V nominal).	Apple's take-apart notes that the main battery contains toxic materials. Apple's battery verification procedure is to measure the main battery, recharge it if it reads below 5.7 V, and replace it if it will not recharge. The Macintosh Bible (fourth edition) says that if the Portable's batteries "fall below 5.4 volts, they can't be recharged".
eMate 300	Built-in pack of four AA NiMH cells, up to 24 hours between charges, one-hour fast charge, 500-cycle life. The charge LED is off with no adapter, amber while charging and green when charged.	A rebuild candidate: four AA-size NiMH cells.

IBM

IBM used four kinds of clock battery across the PC and PS/2 range: an external 6 V lithium battery on a lead, a two-cell 6 V lithium pack in a holder, a Dallas module with the cell sealed inside the chip, and an ordinary CR2032. Where IBM gives a check, the battery is taken out of circuit before it is measured.

IBM clock and main batteries

Machine	Battery	IBM's check or part	Notes
PC AT (5170)	6 V lithium battery on a lead to J21, IBM part 8286121	Disconnect it, meter on the 12 V DC range across pins 1 and 4: 6.0 V DC minimum. A 161 error follows any battery change; run SETUP.	No charging circuit, so a rechargeable battery is pointless. Owners fit a CR-P2 in a holder or four AA cells.
PC Convertible (5140)	Main pack of eight rechargeable NiCd cells in one unit, plugged onto the power supply card, about eight hours per charge		A NiCd pack; check the power supply card for leakage.
PS/2 Model 25 (8086)	None; no clock
PS/2 Model 30 (8086)	3 V lithium BR-2/3A (2/3A size) soldered to the riser card at B1	No FRU; IBM replaced the riser	Pull the riser, not the planar. A BR-2/3A or a two-AA holder replaces it.
Model 25-286, 30-286, 35, 40, 55 SX	Dallas DS1287 RTC module, cell sealed inside	FRU 8509237	Replace the whole module with a DS12887, DS12887+ or bq3287MT. Do not use the DS12C887, which handles the century byte differently.
Model 55 LS, 65 SX	DS1287 module plus a Dallas DS1220AD 2 KB NVRAM module, each with its own cell	FRU 8509237 and 64F0722	The DS1220AD+ is still made.
Model 50, 50 Z, 60, 70, P70, 80	6 V two-cell lithium pack in the battery/speaker assembly (FRU 33F5950); equivalent to a CR-P2 or DL223A	FRU 72X8498. Remove it from the assembly and measure on the 12 V DC range; replace below 5.5 V DC (retain tip H024809)	Low voltage gives intermittent 161, 162 and 163 errors.
Model 25 SX, 56, 57, 76, 77, 90, 95, PS/2 E	CR2032 coin cell	FRU 33F8354. IBM gives 2.5–3.7 V as correct and replace below 2.5 V (95xx products)
Model P75	Two CR2477 coin cells on a small board with a lead	FRU 64F9987	No direct equivalent is sold.
ThinkPad 340 (NiCd) and 340CSE (NiMH)	Main pack; separate backup battery	Main pack: recharge, and replace it if it still reads under 10.0 V DC. Backup battery: 2.9–3.3 V DC	A pack above 8.0 V should read 4–30 kΩ between terminals 3 and 4, or the pack is faulty.
ThinkPad T30	Li-ion main pack, 0 to 12.6 V at terminals 1 and 5; separate backup cell	Main pack: recharge for at least 3 hours and replace it if it still reads under 11.0 V DC. Backup cell: 2.5–3.2 V DC	A pack above 11.0 V should read 4–30 kΩ between terminals 4 and 5; if it does, the fault is on the system board.

When the sealed cell in a Dallas module is flat, the whole module is replaced. Ardent Tool also describes reworking a DS1287 to take an external cell.

Commodore

The Amiga 500 Plus, the A501 memory expansion for the Amiga 500, and the A2000, A3000 and A4000 back their clocks with a rechargeable barrel cell, and those cells leak. Commodore's A4000 bill of materials lists BT176 as "Battery, NICAD, Rechargeable, 3.6V". The A3000 has the same kind of cell on the left side of the main board, and the leaks reach the A2000, A501 and A4000 too.

Commodore Amiga clock batteries

Machine	Battery	Location	Notes
Amiga 500 with A501 expansion	Varta rechargeable barrel cell, 3.6 V	On the A501 trapdoor RAM card	Leaks onto the card; the clock chip is usually an OKI M6242.
Amiga 500 Plus	Varta rechargeable barrel cell, 3.6 V	Main board
Amiga 2000	Rechargeable barrel cell	Main board
Amiga 3000	NiCd barrel cell, 3.6 V	Left side of the main board
Amiga 4000	NiCd, rechargeable, 3.6 V (BT176)	Main board

Remove the original cell from any Amiga that still has one, and clean and check the board around it. The Amiga charges its clock cell, so the replacement is either another 3.6 V NiCd (the A3000 guide gives 60 mAh) or a CR2032 behind a series diode and a resistor of at least 200 Ω. A lithium cell fitted directly would be charged. NiCd electrolyte is alkaline.

Atari

Atari batteries

Machine	Battery	Location	Notes
Mega ST	Two AA cells, 3 V, backing the Ricoh RP5C15 clock	Battery housing on top of the case, behind the left fan vent; the lead plugs in under the left rear corner of the top cover	Alkaline cells; take them out for storage.
Mega STE	3.6 V lithium cell on a lead with a plug (Tadiran TL-5242/W)	Held to the case with velcro	Non-rechargeable. Replace with a 3.6 V lithium cell on a lead, reusing the connector.
TT030	3.6 V lithium, 400 mAh, Atari part C301020-001	Main board	Backs the clock and 50 bytes of RAM.
Falcon030	Dallas DS1287 clock module (Atari C398170-001) with an integrated 3.6 V lithium cell and crystal, keeping the time, date and 50 bytes of configuration RAM	U64	Paweł Góralski replaces the module with a DS12887+ in a socket, or cuts the old module open and wires in an external cell, and resets the NVRAM afterwards.
Stacy	Clock: lithium 3 V 560 mAh, C103655-001. Main power: twelve C cells or an external DC supply	Clock cell at LB on the main board	A primary lithium cell; do not fit a NiCd or NiMH in its place. Leakage from the C-cell bay is alkaline.
ST Book	Seven AA cells or a rechargeable pack, at J901; cut-off comparators annotated 6.25 V	Battery tray	The NiCd pack pictured above is an ST Book pack. Remove AA cells for storage.
Portfolio	Three AA cells; each RAM card has its own lithium backup cell, which Atari says keeps the card's data for "a year or more"	Battery compartment; card cells in the cards	A card that forgets its contents out of the machine has a flat card cell.

Acorn

The BBC Master keeps its clock battery in a holder on a lead. The Archimedes and Risc PC range uses two different arrangements: alkaline AA cells in a holder on the 300 and 400 series, and a single rechargeable 1.2 V nickel cell soldered to the board on the later machines. All of them leak onto the board.

Acorn clock and CMOS batteries

Machine	Battery	Location	Acorn's check or note
BBC Master series	Acorn's manual describes a lithium manganese dioxide cell, with an optional keyboard-mounted rechargeable battery that the board charges	Holder next to the speaker, on PL8	At least 2.6 V at the clock chip with the mains off. RetroClinic, which sells replacement packs, reports that the packs found in Masters are alkaline AA cells fitted with a diode and resistor, and that alkali from them creeps up the cable to the board connector.
A305, A310, A410, A420, A440	Two LR06 (AA) 1.5 V manganese alkaline cells	Holder assembly 0176,009, wired to PL11	Acorn specified replacement once a year. On the A440, IC16 pin 8 should read about 2.8 V with the power off; if it does not, check PL11, D3 and the cells, each of which should read above 1.4 V.
A3000	NiCd, 1.2 V 280 mAh, B1, Acorn part 0817,013	Soldered to the main board	With the power off, IC6 pin 8 should read about 1.1 V. If it is under 1 V, change B1 and check D2 and C15.
A540	NiCd, 1.2 V 280 mAh, BT1	Soldered to the main board	With the power off, IC22 pin 8 should read about 2.8 V; if it does not, check the charge state of BT1.
A5000	1.2 V 280 mAh rechargeable cell, BT1	Main board	Charged from +5 V through D15 and R272 while the machine is on; it backs the PCF8583 clock, IC58. Acorn's A5000 service manual gives no battery type or part number.
A3010, A3020, A4000	Nickel, 1.2 V 280 mAh, BT2, Acorn part 0817,014 ("BAT NI 1V2 280MAH VT PCB")	Soldered to the main board	The same cell and part number on all three boards.
Risc PC 600 and 700	NiMH, 1.2 V 280 mAh, BT1, Acorn part 0817,016	Main board	Trickle-charged from +5 V while the machine is on. On Acorn's circuit diagram the charge path runs from +5 V through diode D2 and R130 (180 Ω), with R212 (180 Ω) in the negative lead; the PCF8583 clock (IC20) is fed through R133 and decoupled by C70. A board that reports "CMOS unreadable" with a good cell can have a fault in that path: on one 700 board the stardot user philpem found D2 failed, an open via under C70 and a leaking decoupling capacitor next to R133. See Acorn Risc PC 700 Maintenance Guide.

The 1.2 V cells on the later boards are charged by the board. A common repair is a CR2032 in a holder behind a series diode, so the board cannot charge it. The alkaline cells in the 300 and 400 series leak potassium hydroxide, as do the NiCd cells.

Amstrad

Amstrad batteries

Machine	Battery	Notes
PC1512	Four non-rechargeable AA cells backing the HD146818 clock and configuration RAM	A flat set produces "Please fit new batteries" at start-up, and the machine reloads its default settings. The cells sit in a compartment in the system unit; see the cabinet parts list in the Amstrad PC1512 Service Manual.
PPC512 and PPC640	Compartment for ten alkaline C cells, up to eight hours' use	Alkaline leakage takes vinegar.
NC100 Notepad	Four AA cells; CR2032 3 V lithium backup cell	Remove the AA cells for storage.

Psion

Psion's handhelds split into machines that run on primary cells with a lithium coin cell holding memory while they are changed, and machines with a built-in rechargeable pack. On the first group, never remove the main cells and the backup cell together, or the internal memory is lost. The battery service pages for the rechargeable machines are Psion Revo Battery Replacement, Psion Series 7 Battery Service, Psion netBook Battery Service and Psion netBook Pro Battery Service.

Psion batteries

Machine	Main power	Backup	Notes
Organiser (1984)	9 V PP3	None	Records are on the Datapak; a flat battery loses the time.
Organiser II	9 V PP3, alkaline	None	Works down to 5.5 V; a new cell reads up to 10.5 V off load. Fit the new battery within 90 seconds or internal memory is lost.
Series 3, 3a, 3c	2 × AA	CR1620 lithium
Series 3mx	2 × AA	CR2025 lithium
Siena	2 × AAA	CR1620 lithium	No adapter socket, so the backup cell alone holds memory during a battery change.
Series 5, 5mx	2 × AA alkaline	CR2032 lithium	Rechargeable cells run for a short time and go flat with little warning.
Revo, Revo Plus	Two NiMH AAA cells, 650 mAh, soldered to a connector and sealed in the case	None	A thermistor in the pack is used by the charge controller; keep it when replacing the cells.
Series 7, netBook	Li-ion pack, three 18650 cells in series with a controller board, 10.8 V 1500 mAh	CR2032 lithium	Do not wire cells straight to the pack contacts; the controller board balances the three cells.
netBook Pro	Li-ion pack, 12.6 V 2200 mAh	Two alkaline AAA cells	The AAA cells can leak; remove them for storage.

Sega

Sega backup batteries

Machine	Battery	Location	Notes
Mega CD / Sega CD (Model 1)	Rechargeable lithium coin cell: part 401-0036 (AL2032-HC1) or 401-0037 (ML2016-HS1), charged by the MB3790 at IC6	Main board	A plain CR2032 is not a drop-in replacement on a charging circuit; fit a rechargeable cell, or a primary cell behind a diode.
Saturn	CR2032, listed as "Lithium Battery (CR2032)", "Positive Side B"	Holder behind a battery lid (Sega part 253-6915-03)	User-replaceable. Lost saves and a reset clock mean a flat cell.
Dreamcast	BT1, a rechargeable lithium coin cell. Sega's parts list gives three alternatives: 401-0066 ML2032T26 (Hitachi), 401-0067 ML2020/G1B (Panasonic) and 401-0068 ML2430-VS1 (Sanyo)	Soldered to sub board 2, the controller port board	The board charges the cell through R1 (13 Ω, 1 W), and Sega says to replace it with the same or an equivalent type, so a primary CR2032 does not belong here. Sega's instruction manual says the cell charges while the console is on, takes about two hours, and then holds the date and time for about 20 days. An ML2032 in a vertical holder is the usual replacement; LIR2032 cells are 3.6–3.7 V and are not recommended.
Dreamcast VMU	Two CR2032 cells	Under a screw-secured lid at the rear of the VMU	A VMU with flat cells still works as a memory card, but beeps when the console is switched on.

Nintendo

Nintendo's cartridge consoles keep saves in the cartridges. Nintendo's own Game Pak test for the Super NES expects a cartridge save battery to read 2.7–3.2 V DC. Save cartridges carry a tabbed lithium coin cell soldered to the board to keep the save RAM alive, so the save goes when the cell is removed; the Game Boy cartridge in iFixit's guide takes a CR2025. Fit a tabbed cell of the same size. See Super Nintendo General Maintenance.

The GameCube keeps its saves on memory cards. Its real-time clock is backed by a tabbed CR2032 cell soldered to the controller port board; when the cell is flat, the clock and system settings reset each time the console is switched off. Fit a tabbed cell of the same size; see the GameCube maintenance guide.

The Game Boy Advance runs from two AA cells. Its power LED turns red below 2.35 V and the console shuts down below 1.7 V. Nintendo warns against carbon-zinc cells, whose higher internal resistance can shut the console down suddenly and lose data from games that save to flash memory.

Related pages

• Battery Explosion, Capacitor or Corrosion Damage: cleaning up after a cell or pack has leaked into a machine
• Lithium Thionyl Chloride Cell Safety Data Sheet
• CRT Discharge Procedure
• Recommended Tools
• Capacitor Failure Symptoms