From: parkyn@fraser.sfu.ca (Dale Parkyn)
Subject: PowerBook battery wisdom
Summary: Don't drain powerbook batteries
Keywords: PowerBook PB battery memory life charge charging
Organization: Simon Fraser University, Burnaby, B.C., Canada
Date: Fri, 28 Aug 1992 14:45:26 GMT

I would like to pass along the following information about battery charging.
A friend tried draining PB battery through 6V light system, surprising amount
of energy left below the PowerBook's shut down point.  But...

   The battery would no longer begin to charge though!

After reading the following maybe self-styled experts, read as myth passers,
I think we understand what happened.

Here it is:

>As an owner of two powerbook batteries (for the 170 if it matters) which
>are currently getting virtually random cycling and use, I've been
>wondering about these full-cycle battery chargers you see
>advertised various places.  The one I see most often is, I gather,
>made by Lind Electronic Design and fully discharges as well as
>charges batteries so as to prevent the dreaded "Memory Effect"
>common in ni-cads.

The "memory effect" is almost never observed in NiCd batteries used in
consumer applications.  It occurs only when a NiCd battery is repeatedly
discharged to _exactly_ the same level and then recharged, many times in
a row.  This can happen in some very specialied applications, but won't
occur in a typical consumer application such as in a PowerBook.  The
"virtually random cycling and use" that you are giving your batteries is
exactly the sort of use which ensures that the memory effect won't
occur.

There is an effect in NiCd batteries known as "voltage depression",
which occurs if you overcharge the battery (if you leave it cooking in a
high-rate charger for too long).  This effect lowers the output voltage
slightly, and it can make the battery _appear_ to lose capacity (because
its voltage drops to the 1.1-volt threshold sooner than it would
otherwise).  Voltage depression can be reversed by completely
discharging each cell in the battery, and then recharging it.

It's safe to discharge individual NiCd cells (1.2 volts each) all the
way to zero.  It is NOT safe to discharge a NiCd battery all the way to
zero - the first cell to be exhausted will be damaged by the continuing
flow of current through it.  So... don't try to discharge your PowerBook
battery by hooking it to a flashlight bulb or a resistor.

The PowerBook has a low-voltage detector, which detects the fact that
the battery voltage has dropped to near-exhaustion (usually 1.1 volts
per cell) and shuts down the machine.  It's a bad idea to try to use any
NiCd battery beyond this point - you risk damage to the battery, and
there's only about 1% of the usable power left anyhow.

So... the best way to exercise your PowerBook battery (to eliminate the
effects of voltage depression) is to use it until the PowerBook warns
you that low-power shutdown is about to occur... then, sleep the
machine, take out the battery, and recharge it for the recommended
length of time (but not for longer than that).  This should bring the
battery back up to full charge, without overcharging it.

>                   I have experienced continually degrading battery
>performance in other portable devices (such as tape recorders that
>always run out of juice when I need them the most!) and would like to
>avoid that with my powerbook.

Many tape recorders don't have low-voltage shutoffs, and will continue
to suck juice out of NiCds all the way down to zero volts. This leads to
battery damage.  When you're using NiCd batteries in a tape recorder,
flashlight, etc., you should remove them from service IMMEDIATELY when
the light begins to dim or the tape recorder starts to slow down.
If you run your NiCd battery down to zero in the belief that you're
preventing the memory effect, you're actually shortening its life quite
a lot.

Recharge the battery to full charge at the recommended rate (typically
14 hours, at 10% of full charge per hour), then take the battery out of
the charger.  Many low-cost chargers will continue to pump current into
the battery at a C/10 rate, and don't switch over to a low-current
trickle charge... this leads to voltage depression.


Thanks to the originator of this information

Dale parkyn@sfu.ca

PS  At a hundred dollars a piece don't mess with your batteries without
    careful consideration...