From: poggio@piedmonte.metaphor.com (Andrew Poggio)
Newsgroups: comp.sys.mac.hardware
Subject: PowerBook Power Usage
Date: 30 Aug 92 07:15:21 GMT
Organization: Metaphor Computer Systems, Mountain View, CA
Originator: poggio@piedmonte

The following is a short article I submitted to BMUG.  I hope you find it
useful.

Where Does the Power in a PowerBook Go?
or
How to get the most from your PowerBook Battery
Without Major Changes to your Computing Habits

Andy Poggio
August, 1992

Like many, I eagerly awaited the arrival of true portable computing with the 
simultaneous arrival of my Mac PowerBook.  Apple did it right -- light, fast 
and I even find the trackball very useful despite being a mouse veteran of 
many years.  But soon, the reality of today's battery technology took hold.
As I enjoyed tetherless computing in my sunny backyard, I was always
confronted with gnawing doubts about whether or not I was doing the right
things to prolong my battery charge and about how soon I would have to plug
in for a recharge.

I had read all the conventional wisdom -- let the PowerBook sleep, spin down 
the disk, run without the backlight, etc. but nothing I read quantified the 
results.  Was I better off with the hard disk spun down and the backlight 
blazing?  Would I get more battery time letting my PowerBook rest 
frequently, but never sleep?  I decided that the time had come to let 
empiricism replace theory -- I'll run an experiment and measure the results.

The Experiment

My PowerBook is a 140 with 6 megabytes of memory,the Apple-supplied 40 
megabyte hard disk, and no modem.  I ran with AppleTalk and sound off.  
During the experiment, I ran System 7 with only the Finder and TeachText 
running.  Those uninterested in the experimental details can skip to the next 
section.

In order to discover the impact of various power conserving techniques, I 
needed to measure the current drawn from the battery under various 
conditions.  I had no easy way to get to the battery terminals inside the 
PowerBook; however, it was relatively easy to get to the connections on the 
AC adapter.  (That is, if  you consider an experimental setup that included
the PowerBook AC adapter, parts of a Radio Shack AC adapter, alligator clips,
a nail, and a toothpick easy.)  To measure current, I used a Fluke Model 87 
digital multimeter.  During the experiment, I removed the Apple battery so 
that my measurements would reflect only PowerBook current and not battery 
charging current.  This procedure makes the implicit assumption that current 
draw from the battery itself would be the same as what I measured coming 
from the AC adapter.

The Results

I measured the current in milliamps.  The number of milliamps drawn from 
the battery is inversely proportional to the battery charge life -- draw
twice the milliamps and the battery charge lasts half as long.

I needed a baseline to use so that I could express current measurements as 
percentages.  For this baseline, I chose the maximum current consumption 
condition for my PowerBook:  backlight at maximum, disk spun up, and not 
resting.  With this as a baseline, here are my results:

Condition                                   Current       Percentage
                                            (milliamps)

Minimum useable condition                   210           17%
(backlight off, disk spun down, resting)

Backlight at dimmest setting                188           15%
(compared to backlight off)

Backlight at brightest setting              445           36%
(compared to backlight off)

Disk spinning                               285           23%

Resting                                     280           23%

Maximum current consumption                1220          100%
(baseline:  backlight at maximum,
disk spun up, not resting)

The Recommendations

1.  Turn that backlight off if possible:  at full brightness, it consumes more 
than one third of your battery charge and is the single most important factor 
in power consumption.  Unfortunately, my experience is that I can read the 
screen comfortably without the backlight only in sunlight.  However, I find 
the dimmest setting (which consumes less than half the power of the 
maximum setting) to be comfortable under most lighting conditions.

2.  Set the disk to spin down after a very short interval.  After running this 
experiment, I have mine set to spin down after 1 minute.

These two are the big ones -- together they can consume more than half your 
precious battery charge.  Of course, resting helps but is out of your control 
unless your turn it off.  So, leave it on unless it interferes with something
you are doing.  I have only found resting to be a problem when my PowerBook is 
being used with a MIDI instrument.

By the way, a sleeping PowerBook consumes very little power:  just 5 
milliamps or less than 0.5 % of the baseline.  But I just don't get much 
computing done with a sleeping PowerBook!