Editing Battery Refurbishment (section)

== ⚠️ 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.'''

[[File:Battery guide - lithium ion cell explosion damage.jpg|thumb|420px|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.'''<ref name="vent">Measured vent-gas studies of 18650 cells, including Sandia National Laboratories, "Modeling cell venting and gas-phase reactions in 18650 lithium ion batteries during thermal runaway", ''Journal of Power Sources'' (2021), and the summary at [https://www.batterydesign.net/safety/thermal-runaway/ Battery Design, "Thermal Runaway"]. Source for the 1–2 litres of vent gas per amp-hour figure and for the composition of the vent gas.</ref> The gas is mostly carbon dioxide, hydrogen and carbon monoxide, and the decomposing electrolyte also produces '''hydrogen fluoride''', which is corrosive and acutely toxic.<ref name="vent" />

=== 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.<ref name="mc-vs">The Maker's Chest, [https://themakerschest.com/blogs/spot-welding-hub/spot-welding-vs-soldering-battery-packs-which-is-better-and-when "Spot Welding vs Soldering Battery Packs: Which Is Better and When?"]. Source for the 60–80 °C cell assembly limit, the 280–380 °C iron tip range, the millisecond-versus-seconds heat-conduction argument, weld contact resistance figures, and the division of labour between welder and iron.</ref> 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.<ref name="sep">Wikipedia, [https://en.wikipedia.org/wiki/Separator_(electricity) "Separator (electricity)"]. Source for separator materials (polyethylene, polypropylene) and for the shutdown mechanism in which the micropores close by melting when the cell overheats. Polyethylene melts in the region of 130 °C and polypropylene around 160 °C; ordinary lead-free solder melts at about 217–220 °C, and a soldering iron runs far hotter still.</ref> 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.<ref name="bu-808a">Battery University, [https://batteryuniversity.com/article/bu-808a-how-to-awaken-a-sleeping-li-ion BU-808a: ''How to Awaken a Sleeping Li-ion'']. Source for the 1.5 V/cell one-week rule, the sleep-mode and "boost" behaviour, the 2.2–2.9 V/cell protection cut-off range, the reverse-polarity warning, and the Cadex recovery statistics.</ref><ref name="bu-802b">Battery University, [https://batteryuniversity.com/article/bu-802b-what-does-elevated-self-discharge-do BU-802b: ''What does Elevated Self-discharge Do?'']. Source for the 2.50 V/cell floor, the copper dendrite mechanism, the Li-ion self-discharge rates, and the self-discharge-versus-temperature-and-state-of-charge table.</ref>
* '''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.<ref name="bu-911">Battery University, [https://www.batteryuniversity.com/article/bu-911-how-to-repair-a-laptop-battery/ BU-911: ''How to Repair a Laptop Battery'']. Source for SMBus pack terminal layout, the 100 Ω probing and keep-alive technique, thermistor identification, the solid-state switch and activation code problem, fuel-gauge behaviour after repair, and the pack-repair guidelines quoted in this guide.</ref>
* '''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 CO<sub>2</sub> 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.<ref name="vent" />
* '''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 ===

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''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.