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I don't know if this question is suitable for ServerFault, but here it's anyway.

I brought what's called power-inverter connected it to a battery charger to test how many amps my computer uses, so I found out it uses 10 amps (per what I don't know).

so anyway, I bought a 100amps car battery, connected it to the inverter and now it's capable of running my computer for 1.30 hour only.

The inverter itself is 1200 Watts (I tried 1000 watts inverter & it got really hot it's cables nearly melted down).

I am running AMD 1090T, 16G DDR 3 RAM, 2X 1 T.B Green Energy W.D HDD on windows 7 x64bit. ThermalTake PSU 600

What I'm doing wrong? I need something that will last for 5 hours at least instead of an hour & half.

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closed as off topic by Tom O'Connor, TomTom, Dan, dunxd, Jay Apr 3 '13 at 10:57

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Vote to close- this is NOT a professional capacity setup. Want to build unstable and dangerous usv - go to superuser.com. A pro admin will tell you to buy a USV. –  TomTom Apr 3 '13 at 10:53
@TomTom I agree with this - no professional would attempt roll their own. And for good reason as you've discovered with melting wires - you may not realise this, but that's a rapid way to burn your house down. Imagine if it did that while you weren't around? –  Dan Apr 3 '13 at 10:57
I'm not sure I agree. I don't know where the OP lives and works, but it may well not be the US/UK/Europe. I spent a few weeks in India a few years back, and found it was fairly universal to run UPSes (because of very bad power), but that - unlike UK/US practice where the inverter and battery come as an integrated UPS - Indian practice was to source inverter and battery separately, and to use whatever size battery was needed for the job (often, a truck battery). Under such circumstances, a request for sizing guidance is no more OT than one for help sizing a DB storage array. –  MadHatter Apr 3 '13 at 11:23
I don't understand the reason I've got this harsh responses, I am using this during power outage when I am present (not running it while away). The cheapest UPS which runs for 10 minutes only would cost much more than my setup & is beyond my budget. –  ahmad Apr 3 '13 at 13:02

2 Answers 2

Give or take a factor of root 2, at 12V, 1200W is 100A (power = current * voltage ).

A draw of 1200W will therefore exhaust a 12v 100Ah battery in an hour.

A draw of 600W (which the server is rated for) will exhaust it in two hours, but given losses in the inverter, I don't think an hour an a half is unreasonable.

If you want five hours, which is 3.3 times the uptime, you'll need a 330Ah battery.

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Don't forget, that the faster you drain a lead-acid battery, the smaller capacity it will also seem to have. This is because the cells at the end (cathode & anode) will go low-voltage quickly, while the cells in the middle of the battery will still be close to their starting voltage (AFAIK, 2V in most lead acid cells?). Time is required for the cells to even-out. So what I'm saying is you usually need to make the battery bigger again, with a large load to cope. –  Snellgrove Apr 3 '13 at 10:51
An excellent point, thank you for that. This stuff is tricky to get right, which is why APC is such a large company. –  MadHatter Apr 3 '13 at 10:53

so I found out it uses 10 amps (per what I don't know).

Ok, this depends on where in the world you are and what voltage you operate on:

230 Volts = 2300 Watts. Or, the equivalent to a domestic electric oven.

110 Volts = This is still high, 1100 Watts. What's at least 2 graphics cards working at full speed.

As you can see, this missing information is important can create some confusion.

The inverter itself is 1200 Watts

Yeah, as per calculations above.

What I'm doing wrong?

I fear you have a slight misunderstanding of the physics involved:

Watts is energy use. Amps * Volts = Watt (unless you get really tricky with 3 phase).

Car batteries are 100 Amps (strength) but LOW VOLTAGE - normally around 12v.

Personally, in conclusion, I would advise consulting an expert. It is possible - I have a 15kw USV in the basement. But it costs.

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During my test as I wrote, The inverter itself is 1200 W, the charger output 12V, and it uses 10 amps. so as per answers above I'll need to connect 2 more 100 amps batteries to get it working for 5 hours. –  ahmad Apr 3 '13 at 13:07

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