# connecting a battery to my desktop



## dezaman (Jun 15, 2007)

hey guys, im the new guy, Derek, ive been forum trollin for a while now searching for the biggest it question i could ever ask, after the long wait for the r600 to be released (x2900) im finally going to upgrade to a new computer, fx62 with sli'd 8800gtx's for the sake of being overkill, instead of buying a standard case im building inside a metal suitcase, and the idea is a really cool idea but if im making it look like a laptop i may as well try and make it mobile, i want to connect a battery to it, i cannot find a single forum that has done this, and so possibly this an imposible feat, but if desktops can be connected to cars using inverters why cant we somehow connect a pc to a laptop battery, hell if the laptop battery wont cut it i have no problem carting around a computer connected to a lead battery from a car, i am just dying to know if its possible and if so some methods for doing it, it would be really cool if it could be done, thanks Derek


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## cwwozniak (Nov 29, 2005)

Hi Derek and welcome to TSG.

A few things to consider ...

1) A notebook or laptop is designed to run as efficiently as possible from the ground up. When running from an external power adapter, the entire PC may only be pulling under 100 watts of power while recharging the battery at the same time. Your new ATI X2900 video card alone needs up to 230 watts of power. Add in a mobo with a real fast CPU and RAM plus hard drive and you could be looking at 500 Watts.

2) A notebook or laptop motherboard is designed to run from a single DC input voltage that can vary quite a bit. A motherboard on a desktop PC needs several positive and negative voltages with tight tolerances for proper operation. You would need to design and build a DC/DC power converter to take your single battery voltage and convert its output to multiple stable voltages for the motherboard and drives.

3) You could simplify your design by using an off-the-shelf high-efficiency power supply for the motherboard power and run it from an off-the-self battery back-up UPS. Be sure you double check on the size and weight (and price) of a UPS that can crank out 500 watts for your desired run time between charges.


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## kiwiguy (Aug 17, 2003)

Given the current you are going to need, and that the only practical and economical solution is to use a 12v inverter, you need to be aware that a full-size car battery will give you only about 3 to 4 hours running, and if you run it for that period each time the battery will die after only about 10 or 20 cycles.

You would need deep cycle batteries and a hand truck to move them otherwise.

You sure will not be able to use laptop batteries for that power dissipation.


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## dezaman (Jun 15, 2007)

so ok, i understand about the power issues so lets change a few things, instead of the high powered parts before hypothetically lets say im using the new sempron 2100+ 8 watts with low voltage underclocked ram with integrated gfx or if thats still to much an itx board, is it possible without the use of a very bulky ups to connect a li ion battery to the psu of a desktop computer


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## cwwozniak (Nov 29, 2005)

dezaman said:


> is it possible without the use of a very bulky ups to connect a li ion battery to the psu of a desktop computer


Not with any Desktop PSU I have ever seen. They need 120 and/or 240 Volts RMS of alternating current. A notebook type battery pack may output 6 to 12 volts of direct current.

EDIT: Found one company that claims they have UPS in an ATX PSU form factor that runs off of a 16.8V Rechargeable battery when the AC power goes away. The but do not mention how long it will run off of the included battery or if you can add a larger external battery.

http://www.amtrade.com/ups/small_uninterruptible_power.htm


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## kiwiguy (Aug 17, 2003)

I agree with the above. LiOn direct connect is out. Laptops that use these batteries then convert the (typically 14v) output to many different voltages for use throughout the PC.

A standard PC PSU will not do that, it rectifies the mains to about 180v DC then feeds that to a switchmode unit to transform it into +3.3, +5, +12, -12 etc

You are trying to make bacon and eggs without the bacon and with no eggs.

You would need a custom PSU at best. Even then, you are not using "low power" components as are used in a laptop, any LiOn laptop battery life would be tens of minutes rather than hours. Recharging also requires specialised conditions.


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## cwwozniak (Nov 29, 2005)

kiwiguy said:


> it rectifies the mains to about 180v DC


Is suspect that applies for the 240 VAC mains in your part of the world.


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## kiwiguy (Aug 17, 2003)

Yes, we have "real AC Mains" here, actually 230 volts. Not that woosy 120v that you have in the US....


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