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- From: poggio@piedmonte.metaphor.com (Andrew Poggio)
- Newsgroups: comp.sys.mac.hardware
- Subject: PowerBook Power Usage
- Message-ID: <2475@cronos.metaphor.com>
- Date: 30 Aug 92 07:15:21 GMT
- Sender: news@cronos.metaphor.com
- Organization: Metaphor Computer Systems, Mountain View, CA
- Lines: 103
- 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!
-
-
-
-