Tablet PC Battery Life: How Wireless Affects Power Consumption

by Reads (22,647)

Whenever we review notebooks one of the questions that always needs to be answered is, what’s the battery life like on this Tablet? We all know manufacturers overstate the quoted battery life for a system, probably because they test for battery life under ideal conditions for getting a high number. For example, wireless off, processor underclocked, system idle, LCD brightness set to low, no DVD and so on. So when your notebook with a quoted 5 hour battery life actually gets three hours, you’re left wondering what happened to those other two hours the manufacturer got?

The reality is, a great number of things can affect the battery life of your notebook: what tasks you’re performing, wireless throughput, processor speed, screen resolution, amount of file fragmentation on the disk, LCD brightness setting — the list goes on and on. Over the next few weeks we’re going to take a look at various laptop components, how they affect battery life, and what you can do to maximize or reduce the amount of power pull that component has on the system as a whole. We’ll keep it at a high level so the average mainstream user can understand and use this information, and not have your eyes glaze over when reading. A majority of the data presented will be based on research and numbers compiled by Intel in their Power Optimization: Furthering the Mobile Vision series of articles, and presentations we’ve attended given by Intel, but we’ll extrapolate that data to give real world suggestions.

The Affect of Wireless on Notebook Power Drain


In case you wondered what a wireless card looks like — above is the Intel 4965agn wireless card (view large image)

It’s now the year 2007 and 99.9% of notebooks out there have built-in wireless allowing you to surf the web and transfer files over thin air in the form of radio waves (802.11 a/b/g/n). Having built-in wireless means you’ve got a little chip inside your notebook that’s responsible for sending and receiving those wireless packets of data. If you’re viewing this web page and on a wireless internet connection, the wireless chip in your PC was required to work and exert a certain amount of power to download this page. If you’re plugged in, that power is drawn from your wall socket, if you’re on battery, then a very small piece of your battery life was donated to downloading this page (we hope it’s worth it for you).

So, exactly how much does having a wireless radio on and surfing the web affect your battery life? And how much power can you save simply by turning it off? We’ll investigate in this article.

Below is figure 1 that we’ll refer to a lot. It’s a bar graph showing how much power is exerted by each component in a notebook. There are five bars, each showing how much power is used based on what the notebook is doing. It’s important to note this benchmarking system was done on a reference system that did not include an LCD monitor. We’ll look at LCD power drain separately in another article (it is generally the largest drain on battery life).

Figure 1: Amount of battery drain of a notebook in various usage states. LCD power drain is not included, the reference system does not include a monitor.

 

Below is another graph that shows average power drain for four different states of wireless, which are documented below this graph:

Figure 2: Power drain of idle notebook running Windows XP with various WLAN states. Blue indicates power used by all other components, except the LCD power drain which is not included, and purple represents WLAN power drain.

  1. Power drain of idle notebook (screen power drain not included) with wireless radio Off = 0W wireless radio power consumption
  2. Power drain of idle notebook (screen power drain not included) with wireless radio On, but not connected to an Access Point (no data being sent or received) = 0.30W wireless radio power consumption
  3. Power drain of idle notebook (screen power drain not included) with wireless radio On, connected to an Access Point (data can be sent and received) = 0.45W wireless radio power consumption
  4. Power drain of idle notebook (display power drain not included) with wireless radio On and Searching for an Access Point = 1.6W wireless radio power consumption

Notice that by far the most power drain from the wireless card comes when it is searching and trying to acquire an access point. Let’s calculate a worst case scenario for how much the wireless card could affect battery life of a notebook. Meaning, let’s assume the card is perpetually searching for a network and can never find one, so the increased power consumption is +1.60W, therefore:

  • Assume average power consumption for notebook with no wireless is 20W
  • Assume average power consumption for notebook with wireless, continually trying to acquire network is 21.6W (+1.60W)

The formula for battery life (assuming a brand new battery with no wear on it) is:

  • Battery Life = Battery Capacity / Laptop Power Consumption

So if we assume a battery capacity of 53 WHr (fairly standard battery capacity) the formula for battery life with no wireless is:

  • Battery Life No Wireless = 53WHr / 20W = 2 hours 39 minutes

And the formula for wireless on and searching for a wireless network is:

  • Battery Life Wireless on and Searching for Access Point = 53WHr / 21.6W = 2 hours 27 minutes

So the difference in battery life for this worst case scenario is 12 minutes, which amounts to about 7.5% difference. Now let’s assume the 0.45W power drain for the case where wireless has a solid connection and we’re just transferring a little bit of data:

  • Battery Life Wireless Connected = 53WHr / 20.45W = 2 hours 35 minutes

Not bad, only 4 minutes, or 2.5% overall difference!

But, can we always assume 0.45W power drain from wireless? What if we’re uploading and downloading a ton of data, does that affect power drain somewhat? Well, yes…

Wireless Power Consumption Based on Data Transfer Variance

We’re going to look at a comparison of transferring compressed versus uncompressed files to demonstrate the affect the amount of data transferred has on power consumption.

You need to be aware that the more data you transfer, the more the wireless card has to work, meaning the more power it has to consume. So if you’re streaming several gigabyte video files from a media server in your house to your unplugged notebook, your wireless card will be sweating hard and doing a lot of work, thereby draining battery power faster. If your notebook is on wireless, solidly connected, and all you’re doing is checking email every 5 minutes, the wireless card will barely sip the power and have little affect on battery life.

Let’s look at how amount of data transfer affects energy used with some charts and graph comparisons.bbAssume we have 5 different files, each file can be compressed to varying degrees using GZIP:

Data Set Original Size (KB) Gzip Compress Rate (version 1.2.4) Description
Tulips.tif 1179 (1.15MB) 1.2x Med size file, very low compress ratio
Book1 751 (0.73MB) 2.45x Med size file, low compress ratio
World95.txt 2935 (2.87MB) 5.06x Large size file, high compress ratio
Pic 502 (0.49MB) 8.96x Small size file, high compress ratio
Frymire.tif 3708 (3.62MB) 14.04x Large size file, very high compress ratio

 

Tulips.tif is almost incompressible, while Frymire.tif can be compressed to almost 14x smaller than its original size. Here’s a chart showing how long it takes to download the uncompressed file versus downloading a compressed version of each file plus the amount of time it then takes to then uncompress that file once it is downloaded:

Notice that with Frymire.tif we save about 2.5 minutes by downloading the much smaller zipped up version of the file and then unzipping it. Meanwhile, the Tulips.tif file is about the same size compressed as uncompressed, and then we have to spend time and processor power unzipping it, so it ends up taking more time and power overall!

The amount of time it takes to download a file and then uncompress a file has a very direct effect on power consumption. Notice in the graph below how much less energy it takes to download the compressed version of Frymire.tif:

Several times more energy is consumed to download the uncompressed original 3.62MB size Frymire.tif, it took almost 3 minutes to download Frymire.tif, so the wireless card is working all that time.

When you download a web page, most websites (such as this one) actually send a page in gzip format and your browser will unzip it on the fly. So this type of thing is constantly going on behind the scenes as you browse the web. The more web pages you download and browse, the more power it takes.

The moral of the story? The more you’re downloading with your wireless card, the more power it consumes, so the affect your wireless card has on battery drain depends a lot on how much you’re using it.

Conclusion

For those that saw a couple of formulas and graphs and skipped ahead to this part, I don’t blame you, here are the take aways based on the data:

  • Simply turning on the wireless card doesn’t drain much power, it might shorten battery life by 2%, so if you want to remain connected to download email it’s no big deal.
  • If you turn on wireless and are constantly trying to acquire a connection, such as if you’re constantly dropping a connection and then trying to reconnect, your battery life could be 6% – 7% shorter than usual due to the wireless card having to work extra hard.
  • If you have wireless card on and are transferring lots of data, such as downloading large files or constantly browsing the web, battery life could be about 6% shorter than simply no wireless on. Consider downloading large files later when you’re plugged in.
  • If you don’t need to use the web, turn wireless off, it’ll save you at least a few short (but maybe precious) minutes of battery life!

Overall I believe that power consumption due to wireless is a bit exaggerated as a culprit for notebook battery drain. You can save a lot more power by dimming screen brightness, underclocking the processor, spinning a DVD less, or a variety of other power saving tactics we’ll look at in future articles. In most cases you’re going to have a lot more fun or be more productive when connected to the web, so don’t sweat it, turn the wireless card on and be more productive with that time you have on battery.

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