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External Batteries: How Do They Work?

External Battery Roundup: Stay Away From The Wall Socket
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Extended batteries are unlike the battery that ships with your notebook. The most obvious difference is that they come with a variety of tips, since they claim universality with each manufacturer's specific power connector.

Tip Set from Duracell's Universal AdapterTip Set from Duracell's Universal Adapter

Even within a single brand, it is possible to find notebooks that use different voltages or plug sizes. For example, Dell has a standard plug size, but the Inspiron Minis are exceptions to this rule.

The stock power adapter that ships with your notebook supplies voltage at a fixed setting. This is not so for the external batteries. In order to deliver the multiple voltages required by different devices, they must also be capable of outputting different voltages.

There are a couple of ways to do this. The easiest way is to use a voltage converter controlled with a manual switch. Some companies choose to go a step further by using a sense resistor in the adapter tip. The sense resistor drops the voltage as current crosses its path. The small voltage drops are fed to a simple comparator circuit, which is then used to control the VDC output from the power supply.

If the resistor is in the tip, there is an additional layer of protection because you cannot force an incorrect voltage over a plug. Physical switches give you more control, but there is a chance that you'll choose an incorrect voltage and damage your notebook.

The way voltage gets to your notebook is important to understanding how an external battery can affect run time. Say your internal laptop battery yields two hours and your external battery offers an additional two hours. If your notebook is at 50% and your external battery is at 100%, this doesn't mean you get three hours total. There is an efficiency loss involved whenever you make an energy transfer, and you see this happen whenever the external battery charges the internal battery. This ineffiiency is less than what you see with an AC to DC conversion if you were to use a UPS, but it is there all the same. Remember that this only applies during a transfer of energy from battery to battery. If your notebook was at 100% and external battery was at 50%, you would see three hours total.

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  • 0 Hide
    lashabane , March 9, 2011 5:18 AM
    Excellent article. I had an idea that this stuff was out there but never really bothered to look. If the 4-5 hours I get from my Asus 1215t begins to not cut it, I now know where to look. Thanks!
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    zodiacfml , March 9, 2011 6:41 AM
    I did not understood any of the technical reading especially the part about the desktop PSU.
    At one point, it is stated that AC adapters have higher voltage than the battery on a notebook so that it can be charged. Then, how can a external battery damage a notebook's electronics with a higher voltage (only if it's too high)?

    It is not stated how to set the external battery voltage correctly. What then is the correct voltage? Correct me but I believe the voltage has to be equal that of notebook battery.
  • 0 Hide
    burnley14 , March 9, 2011 1:22 PM
    It's pretty remarkable that after page 2 I could guess who the author of this article was (without looking of course) due to the thoroughness and good grammar/lack of typos. Hats off to you yet again, Mr. Ku. Job well done as always.
  • 0 Hide
    Anonymous , March 9, 2011 1:43 PM
    @zodiacfml

    it's simple really, AC adapter spit out AC, Batteries spit out DC
  • 0 Hide
    nukemaster , March 9, 2011 2:16 PM
    zodiacfmlI did not understood any of the technical reading especially the part about the desktop PSU.At one point, it is stated that AC adapters have higher voltage than the battery on a notebook so that it can be charged. Then, how can a external battery damage a notebook's electronics with a higher voltage (only if it's too high)? It is not stated how to set the external battery voltage correctly. What then is the correct voltage? Correct me but I believe the voltage has to be equal that of notebook battery.

    Your guess is actually right. The battery with its voltage set too high can damage the notebook.
    If you need to know the voltage required, you just check on your laptop AC adapter or power brick. It is not always the same as the battery.

    For instance, a Compaq R3000 has an 18.5 volt AC->DC(120w) power supply and its battery is only 14.5 volts. The voltage regulators in the laptop(in the charging system) it self cut it down to the needed 14.5-15volts to charge the battery.

    Also note that the AC adapter does NOT spit out AC it in fact spits out DC(it has a rectifier to convert AC to DC).

    As you can see by this picture(you have to click the link), The adapter takes in AC 120V and spits out DC 18.5V. AC is shown with a ~ and DC with a --_---_-- cant make it on here, but you get the point.
    http://img269.imageshack.us/img269/1950/powerw.jpg
  • 0 Hide
    Onus , March 9, 2011 2:55 PM
    Thanks for the comments on Digipower. I've placed them on my personal "Do Not Buy" list.
  • 0 Hide
    Luscious , March 10, 2011 6:29 AM
    Quite a different experience on my end testing the Energizer XP8000 and XP18000.

    For my smartphone and MiFi, the XP8000 just can't be beat. 5x runtime guarantees me 20+ hours of 3G broadband and week-long phone use. Being barely bigger than a Blackberry, I can effortlessly stash the XP8000 on my belt, and charge my smartphone while I walk.

    The XP18000, on the other hand, was a huge disappointment. Using a Toshiba NB305 netbook, it was incapable of recharging the factory 6-cell battery while powered on, and could not provide 2 full charges while powered off. For my usage scenario, that's a failure, as I plug in the external battery when my netbook hits 3% critical, right before Windows does a force shutdown, allowing me to continue working.

    Using this deplete-charge-deplete approach SHOULD allow me 14+ hours of continuous power-on time, except that even the beefy XP18000 couldn't get through 1 netbook charge. Had it been capable of providing one full charge powered on, or two full charges powered off, I would have recommended the XP18000 as well.

    http://lgponthemove.blogspot.com/2010/07/accessory-corner-3-energizer-xp18000.html
  • 0 Hide
    a_fortiori , March 11, 2011 3:05 AM
    Nice article. I wonder if these units can be used as a mini-ups for equipment like a NAS, routers and modems. It would be great if you could wire these with the NAS, and be sure that a power outage wouldn't damage the NAS. Considering that the NAS units typically consume much less power than a notebook, these should be able to cover 4-5 hrs of power outage (?) Any thoughts?
  • 0 Hide
    shineon2010 , March 15, 2011 6:12 PM
    Very good info , alot of products that im having second thoughts about.
  • -1 Hide
    junixophobia , March 15, 2011 8:24 PM
    shineon2010Very good info , alot of products that im having second thoughts about.


    Just buy an automatic inverter that works for hours with a car battery
  • -1 Hide
    junixophobia , March 15, 2011 8:25 PM
    shineon2010Very good info , alot of products that im having second thoughts about.


    Just buy an automatic inverter that works for hours with a car battery
  • 0 Hide
    Anonymous , March 15, 2011 8:33 PM
    Im missing the portable powerstation, especially the spps-200.
    i wonder how that scores against the others.
    i think they are from novuscell
  • 0 Hide
    junixophobia , March 15, 2011 8:35 PM
    sorry guys, just gettin the hang of this...
  • 0 Hide
    Onus , March 15, 2011 9:39 PM
    If you deep-cycle a car battery, you'll kill it in a matter of weeks, as its thin plates deteriorate. For deep-cycle applications, you want to get an AGM battery, containing much thicket plates; and even then you don't want to drop the SOC too much between charges, perhaps 60% is the lowest you should go.
  • 0 Hide
    Anonymous , April 13, 2011 10:54 PM
    Carrying an extra battery adds a lot of weight. And once the battery is depleted, you are dead in the water if the power socket is out of reach.

    If I am going off grid, I would rather bring a solar panel and charge the battery I already have in my device.

    http://leicadig.blogspot.com/2011/04/digital-camera-off-grid.html
  • 0 Hide
    acku , April 13, 2011 10:56 PM
    Which sonar panels are you looking at? I use Brunton myself.

    Cheers,
    Andrew Ku
    TomsHardware.com
  • 0 Hide
    agnius , April 13, 2011 11:13 PM
    I am debating between Brunton, Powerfilm and Voltaic.
  • 0 Hide
    acku , April 13, 2011 11:14 PM
    I have all three. What's holding up your decision?
  • 0 Hide
    agnius , April 13, 2011 11:34 PM
    Andrew,

    According to my calculations I would need about 25-30W to power/charge a netbook directly and charge my camera battery.

    In theory Solaris 26 could do the job, but I am not sure - I have not read any reports about it. I was planning running Targus DC adapter from Solaris 26 straight into a netbook (I had HP Mini but it broke so now I am looking at Lenovo X120e as a replacement).

    Also, I read that CIGS panels after being stored in the dark take days to reach their peak output. Solaris is CIGS, PowerFilm 30W is silicon which does not have that problem.

    But PowerFilm 30W is also considerably larger which makes it less feasible to deploy while riding. Hence I am looking at Voltaic panels which are much smaller, but I am not sure how to route power without an intermediate battery from them (because they are 20V).

    I am stuck in this decision loop while looking for more information.
  • 0 Hide
    acku , April 13, 2011 11:38 PM
    Are you only trying to charge the camera and netbook at the same time? I assume the camera charger requires an AC outlet?
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