I recently discovered there's a "thing" going around where people are charging lithium cells by attaching a stripped USB lead directly to them.
Normally lithium cells are very safe if undamaged and correct charging procedures are used.  But overcharging them like this can result in cell damage and potentially fire.

Many of the "found" lithium cells salvaged from disposable devices have no extra protection circuitry because it's not needed in their application.  But when recharging them it is very important to control the charge current and stop charging at around 4.2V

Direct charging with a USB lead has very little current limiting and the charger will often smash more than its rated current into them, potentially damaging the charger too.  It will also keep charging them beyond 4.2V and that poses a genuine risk of internal chemistry damage, potentially resulting in avalanche failure where an internal short circuit occurs.  If that happens the full energy capacity of the cell will be released extremely quickly resulting in the electrolyte venting as a flammable vapour, and if sparks blow out too it can ignite resulting in a flamethrower effect.

I want to stress that lithium cells are not dangerous because of the lithium.  There's very little lithium metal in a rechargeable cell, and it's diffused through the electrode materials.  The real hazard is their ability to store and release very high amounts of energy.  (Like an overloaded Star Trek phaser.)  If used correctly and protected from physical damage, lithium cells are very safe.

You can easily and cheaply get TP4056 modules on eBay that let you charge a cell correctly from a standard USB lead.  The keywords to find them are 5pcs USB TP4056.  It's often cheaper to buy them in packs of 5 or 10.  There are two types - with or without the extra DW01 protection chips.  It doesn't add much to the cost, which is less than 1 £$€ per module.

If you are OK with soldering surface mount resistors you can swap the existing 1.2K (122) resistor for a 2.2K (222) one to change the charging current from the standard 1A to 500mA.  Alternatively a conventional quarter watt 2.2K resistor can have its leads shaped for soldering to the surface mount pads.  The ideal charging current for a lithium cell is equal to its capacity in mAh.  So a 500mAh cell is best charged at 500mA, although 1A can be used for faster charging at the expense of capacity and charge cycles.

So far all the cells from modern "disposable" vapour devices that I've tested have taken a good charge.  When salvaging cells for use in projects use common sense and caution with recharging.  At this point in time the cells do seem to be standard rechargeable types, but that could change in the future if they discover a cheaper option.

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