[fleitz]

It's inherent to any lithium ion battery design which is why every laptop, iphone, etc that uses a lithium ion battery has circuitry that cuts power when the batter reaches 1% to 5% it can occur in your laptop if you discharge the battery entirely and then leave your battery uncharged for months (or less if stored cold).

Some kind of chemical change occurs in Li batteries when completely discharged.

[sounds]

Good answer. In case anyone wants more detail, wikipedia is a little vague, but has the basic idea right:

"Overdischarge supersaturates lithium cobalt oxide, leading to the production of lithium oxide...deep discharge may short-circuit the cell, in which case recharging would be unsafe."

and

"Overcharge up to 5.2 Volts leads to the synthesis of cobalt(IV) oxide...if overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. In extreme cases this can lead to combustion."

http://en.wikipedia.org/wiki/Lithium-ion_battery

The possibilitiy of thermal runaway or fire from overcharging is actually fairly likely with Li+ ions, since they will react with any non-inert substance.

The short circuit caused by undercharging can affect other cells in the pack, leading to overheating and combustion.

[throwaway64]

storing hot is what is detremental to lithium batteries, storing them cold increases lifetime

[jambalaya]

Should I store batteries I don't use in the fridge or freezer? I have a backup laptop battery for instance.

[throwaway64]

maximum lifetime is obtained when you store them at around 66% charge at 4C, which is the temperature of your fridge, take them out ever few months and charge them, you can get less than 1% capacity degradation a year.