Energy storageBench electronicsSpecialist reuse

Cylindrical lithium-ion cell

Also called 18650, 21700, tool battery.

The energy-storage prize in modern trash, and the fire. Tool packs, e-bike and scooter packs, good laptop packs, some vacuums. A pack is discarded because one series group is weak. The rest might be fine, which you prove by testing, not by hoping.

If you are new to this category

Batteries store energy. The useful ones in modern scrap are lithium cells, and they are also the ones that burn if they are crushed, overcharged, or wired together carelessly. A pack is usually thrown out because one cell in the chain is weak, not because every cell is dead. Swollen means stop.

Tape over bare metal ends as soon as a cell is loose. Charge only with a charger made for that chemistry and that number of cells in series.

A loose lithium cell

The flat end is negative, the button end is positive. Nominal voltage is about 3.6 to 3.7 volts. Full, for an ordinary cell, is 4.20 volts. Below about 2.5 volts for a long time, or any swelling, means it is for recycling, not for a project. Tape the ends so a bunch of them cannot short in a drawer.

Lithium-ion cells. A punctured, crushed, overcharged, or reverse-charged cell vents and burns hot enough to ignite the bench. Swollen pouches are already failed.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Sizes	18650 (18×65 mm) and 21700 (21×70). Anything labeled 5000 mAh in an 18650 wrapper is lying; real high-capacity 18650s are around 3500 mAh.
Voltage	3.6–3.7 V nominal, 4.20 V full for standard cells. Some LiHV tool cells are 4.35 V. If the pack does not say, stop at 4.20.
Packs	12 V tools are often 3S. 18/20 V tools are 5S. Scooters and hoverboards are often 10S (36 V nominal). The BMS may be in the pack or only a thermistor.
Protection	Loose harvested cells are usually unprotected. Your charger and your wiring have to be.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

A tested pack for a tool
Lights
A bench battery with a real BMS and a fuse

Leave this one when

Swollen, dented, rusted, below about 2.5 V for a long time, or hot during a modest charge. Recycle those.

Buying a new one is the better idea when

New cells from a supplier that is not a marketplace wrapper. Salvage is for packs you already have open.

How to tell if this one still works

Resting voltage, then capacity and sag under load. Match cells that will share current.

Step-by-step on the bench: Sort salvaged lithium cells

Tools you will want: Charger with a display, Meter, Tape for the ends.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

BMS: Battery management board. The small circuit that cuts a lithium pack off if it is too full, too empty, or drawing too much current. It has to match the number of cells.
nominal: The nameplate voltage, not the exact voltage at every moment. A '12 V' lead-acid battery sits around 12.6 V when full and sags under load.
series: Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.

Where people actually find it

6 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Laptop	Prize	a pack	sometimes	Bottom cover, or internal.
Cordless vacuum	Prize	a pack	usually	The cartridge.
Cordless drill	Prize	3–10	usually	The pack.
E-scooter or hoverboard	Prize	a large pack	usually	The tray.
Powered or Bluetooth speaker	Worth it	a pack	sometimes	Portable speakers.
Flashlight	Worth it	1	sometimes	The body.

Prize — take this firstLaptopa pack · sometimes
In this particular machine
18650s in older packs; many newer packs are pouches instead. See the pouch entry.
Where to look
Bottom cover, or internal.
How to get it out
Only if the pack is not swollen and you are set up to test cells. Otherwise recycle the pack whole.
The condition to expect
One weak series group is the usual failure. Others may test fine.
Prize — take this firstCordless vacuuma pack · usually
In this particular machine
18650 or 21700, series-parallel. One group is weak.
Where to look
The cartridge.
How to get it out
Test each group. Tape the ends as you separate them.
The condition to expect
Mixed. Do not keep the weak group.
Prize — take this firstCordless drill3–10 · usually
In this particular machine
3S on 12 V class tools, 5S on 18/20 V. Often 2P or more.
Where to look
The pack.
How to get it out
Measure each series group. Match before reuse.
The condition to expect
One group bad is typical.
Prize — take this firstE-scooter or hoverboarda large pack · usually
In this particular machine
Often 10S (36 V nominal) on hoverboards.
Where to look
The tray.
How to get it out
Test groups. Recycle the pack if any cell is crushed.
The condition to expect
One bad group, or worse.
Worth pullingPowered or Bluetooth speakera pack · sometimes
In this particular machine
18650s.
Where to look
Portable speakers.
How to get it out
Test them. The pack is often the fault.
The condition to expect
Mixed.
Worth pullingFlashlight1 · sometimes
In this particular machine
18650, sometimes unprotected.
Where to look
The body.
How to get it out
Test it. A light left on flattens it.
The condition to expect
Mixed.