Editing Battery Refurbishment (section)

=== 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.<ref name="bu-802b" />
* '''Below 1.5 V for a week or more''' — '''do not attempt to charge it.''' Copper shunts may already have formed.<ref name="bu-808a" />
* '''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.<ref name="bu-808a" />

* '''If the voltage does not rise to a normal level within about a minute of boosting, discard the pack.'''<ref name="bu-808a" />
* '''Check polarity with great care before boosting.''' A sleeping pack does not reveal its voltage, and applying a voltage in reverse causes permanent damage.<ref name="bu-808a" />
* '''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.<ref name="bu-808a" />

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.<ref name="bu-808a" />