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[_] correct treatment of laptop batteries

Steve Barker steve at darda.co.uk
Sun Dec 20 12:54:04 GMT 2009

> the genius at apple store suggested leaving on power was bad for a battery
and that it should be fully charged and then run off the battery till its
flat and then recharged. This sounds a bit odd seeing as the battery is
lithium. any expert views i can follow?

The "genius" in the apple store is definitely wrong. This used to be true
for NiMH batteries, where discharging fully was thought to help extend both
voltage depression effects (where cell voltage would start to drop more
quickly when in use) and general capacity (the "memory effect").

Fully discharging cells cures voltage depression in NiMH cells, but if you
fully discharge a whole NiMH battery pack then the stronger cells in the
battery will "reverse" the weaker ones. The weaker ones go flat first, and
then get charged backwards by the others. This is bad for the weaker cells,
and will kill a NiMH battery pack more quickly. Model enthusiasts will use a
proper electronic discharger that discharges the cells without damaging
them.

There is no need at all to purposefully discharge a lithium polymer battery.
The chemistry is totally different. Each cell in a lipo pack has a working
voltage between 4.2v (fully charged) and 3v (fully discharged). If you run
any lipo battery based RC models then you'll know how carefull you have to
be.. fly your lipo helicopter for a few minutes after the power has dropped
each time and then youll have to chuck the battery away within a few charges
as youve completely messed it up! A laptop will switch off when any of the
cells in the pack reach 3v to protect the battery. Then when it is charging
it will charge each cell individually and balance them all out. There is no
memory effect though.

Model enthusiasts preserve your lipo batteries when not in use by keeping
them in the fridge. Even better if they are discharged half way
(apparently). 

Totally impractical for a laptop battery, but a cold laptop might be nice on
a warm day.