Power, notebooks and solid state disks

My much-loved HP Omnibook 300 notebook went from a 4-hour battery life to 10+ hours when used with a compact flash card instead of a disk. Your laptop won't.
Written by Robin Harris, Contributor

My much-loved HP Omnibook 300 notebook went from a 4-hour battery life to 10+ hours when used with a compact flash card instead of a disk. Your laptop won't. Ever wonder why?

Me too. So here's a short course in laptop power.

Measuring power There are a lot of wrinkles in the power game, but the basic equation to remember is PIE. P (watts) = I (amps) x E (volts). The big question with batteries is "how long?" so vendors usually use either watt-hours (Wh) or, if they like the big numbers, milliAmp-hours (mAh). An amp or watt hour is just one amp or watt for one hour.

Normally you'll see a battery specified with a voltage as well as the watts or amps, so you can use P = I x E to convert one form to another. For example, the Sony VGP-BPS5A battery is spec'd at 7.4 volts and 7800 mAh. 7800mAh is 7.8 amp hours, so multiplying by the voltage 7.4 gives a watt/hour capacity of 57.72 Wh.

That wasn't too bad, was it?

We've got the power The O300 had a tiny, by today's standards, nickel-hydride battery, less than 8 Wh. But that baby would go more than 10 hours between charges, which gives a marvelous sense of freedom. But how could a 15 year old laptop last so long given all the advances in battery technology?

Easy: the O300 had almost nothing to power. Unlike your laptop the O300 didn't have a lot of power-draining features:

  • A wimpy little 20 MHz 386sx processor
  • No wi-fi or optical drive and a tiny 6 MB of RAM
  • The OS and most apps (Windows 3.1, Word, Excel, PIM, Laplink, and terminal emulator) were all stored in ROM
  • 9" LCD screen with no backlight

So when you used a flash drive instead of a hard drive, the battery life more than doubled.

Now, let's look at your notebook My Intel laptop is the Apple MacBook with the original Core Duo. Ars Technica reports the 55Wh battery in "medium" use (iTunes streaming music, medium brightness, Wi-Fi and Bluetooth on) gets about 4.1 hours of life. Doing the math (55Wh / 4.1 hours) we get a 13.4 watt average power demand. Now we can size how the new ULV Core Duo and the Samsung 32 GB flash drive affect power consumption.

ULV gets down Intel offers its processors in three power classes:

Power Class Core Duo Number Core Solo Number
25W to 49W T2000 T1000
15W to 49W L2000 L1000
14W and Less (ULV) U2000 U1000
These power cuts aren't free: the ULV clock (1.2GHz vs 1.83MHz) is lower and the front-side bus speed (533MHz vs 667MHz) is slower. Yet as we'll see, the power cuts are more dramatic than these numbers would suggest.

T2400 is hot! My MacBook uses the T2400. So what percentage does the processor use? The answer isn't obvious. Intel's SpeedStep Technology ruthlessly cuts CPU power consumption wherever it can. So Intel can give you a Core Duo maximum number (31W!) and an "Enhanced Deeper Sleep" number (3.35W). They must have their own standard workload they use for comparing processor power efficiency, but it doesn't look like it is public.

The process of elimination So we subtract the power consumption of other subsystems to estimate processor power usage.

  • An idle 2.5" 60 GB 5400 RPM hard drive idle 2.5" 60 GB 5400 RPM hard drive consumes about 3.3W, so with some seeking call it 3.5W.
  • Average screen brightness is another 3 watts.
  • Bluetooth is about 0.3W.
  • Wi-Fi power consumption depends on usage, but moderate usage is about 1W.
  • Graphics hardware is also in the 1W range.
  • Keeping all the other bits powered is probably about .5W.

That totals to 9.3W, leaving ~4W for processor consumption in "medium" usage.

ULV U2500 is cool! The newly announced U2500 is spec'd at 9.0W, or just 29% of the T2400. If the U2500 comes in at 29% of the T2500 on our MacBook, the power consumption would drop to 1.2W from 4W, for a 2.8W saving.

32-Gigabyte (GB) NAND flash-based solid state disk (SSD) The SSD is the other big win. Compared to hard drives SSDs are faster - except for small writes - quieter, lighter and use about 5% of the power of a hard drive. Let's be conservative and say it uses .5W.

Feel the power! Combining 2.8W saved by the ULV Core Duo with 3W saved by the SSD, gives a 5.8W reduction. So power consumption drops from 13.4W to 7.6W. Using the same 55Wh battery, battery life goes from about 4.1 hours to 7.2 hours.

Another new technology going mainstream soon Today's notebook backlights are almost exclusively Cold-Cathode Fluorescent (CCFL) tubes. The coming thing: LED backlights. Arranged in a string of red, blue and green LEDs behind your LCD screen, LEDs offer a visibly better color gamut than the blue-ish CCFL, crisper motion display, much greater brightness and contrast, and don't contain the environmentally hazardous mercury vapor found in CCFLs.

What isn't clear is how soon the LEDs will match the CCFLs for power efficiency. Engineers have been hard at work optimizing every element of the LED backlight systems so it could be this year.

If they can give equivalent brightness for half the power - a big if - our laptop's 55 Wh battery would now last over 9 hours. Close enough to the O300 to give me a bad case of the gimme's.

Update: Apple's Steve Jobs announced that Apple will start transitioning to LED backlights this year, and will go exclusively LED ". . . when technically and economically feasible."

The Storage Bits take Today's laptops have so many power-using systems that advances in just one of them make only a small difference. By putting together low-power CPUs, storage and backlighting we'll soon have notebook computers that truly deliver the freedom that battery power is supposed to provide.

Comments welcome, of course. And for those of you who think I'm just another Mac fanboy, the Omnibook 300, Windows 3.1 and all, remains my favorite laptop out of the six I've owned.

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