@aspringv thanks for the links, i will go trough them as quick as possible.
@evanc i realise now that deep cycle batteryes are the obvious choice but there is the price issue, for example a medium 40A car battery is way cheaper than the deep cycle one at the same capacity ( less than half here in my country ) so you see it's worth trying to make do with the car one...
@FoMoCo i think you might be right, but i am not worried by that for 2 reasons: 1-I will never let the UPS's battery backup the PC more than 5 to 10 mins max, as soon as the line drops i instantly hit the shutdownd command on the PC, so in fact it only has to cope with less than 5 mins... and the 2'nd reason is that as i sayd i plan to modify completly my UPS, i will design a new converter using a ferrite core and make sure to get as high eff as possible from it, it is much work i know but i am not worried by that.
@Mooly again i am in your debt, your imput is greatly appreciated. Now i went on and checked some numbers, first of all my PC main unit is something like this:
-The power supply is a HW-ATX2.2-V1.2, i do not have much info on it, it was in a verry sorry state when i found it, just a board withouth any cables power devices heatsinks, case... etc, i have reconditioned it cus i liked it's architecture and put it in some case i had, it's performance was real nice, so i have put it in my PC and it does it's job verry well.
-The main board is Biostar G31M+;
-An InteL(R) Pentium(R) Dual CPU, E2180 @2,00GHz;
-3Gb of RAM;
-An onboard VGA;
-A Compro DTV TV-tunner;
-A Genius sound board;
-2 HDD's, one 80Gb and the other 320Gb, both SATA2 interface;
-An Assus DVD-writter;
And that's about it, along with the main unit i got an Fujitsu-Siemens LCD display 19"-wide, i have just measured about 100W of power consumption from both of them combined with the PC on idle ( well not exactly idle, a net browser and some advanced sound processing software running ), i am no gamer so i do not anticipate more than let say 200-250W of power consumption, i usually run easy stuff. Now as for my UPS's converter, it runs at a low freq and as i thought it's eff is verry low at about 50%, i am not sure what is the reason but as i sayd it does not matter, i will design a new one to make sure i get at least 70% eff ( i am hoping for more but it remaines to bee seen what and how ). Now judging from your examples and allowing for a 70% efficiency the battery would have to provide about 350W of power, let round it up to maximum 400W ( i doubt it verry much it will go any higher ) so about 33A, if i choose a 40A car battery it would be about 1 hour and 20 mins for the battery to compleetly discharge, running it for 10 min would mean about 12% discharge, and at my 5' stated would be less than 10%, am i right?