For a UPS, which method is best

Discussion in 'General Electronics Chat' started by R!f@@, Dec 8, 2010.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
    I have been thinking (again) when my UPS battery died..
    It's been a few years now after the battery swap.

    My idea is to use a battery in a way that when ever I want , I can use what ever size I can use during that time. (due to availability of battery options here).

    The method may have been used.
    I need some comments on this. That is this method feasible enuf.

    I will be using a standard battery, AH will be decided after calculating actual current draw.

    Since a supply has 12V, 5V & 3.3V. The PG and the rest is quite easy to adapt.
    The 12V will be taken directly from the battery.
    The 5V & 3.3V will be stepped down by DC to DC converts.
    Since High efficiency converters can be designed using latest components. This Loss will be negligible.

    What do you think. The converters can be built into standard ATX PSU casing. Depending on the Power A SMPS Offline charger can be built into in too. All it needs will be heavy duty battery Connection. Battery wires will also measure on how far the battery is placed.

    To keep the battery life at best, charger can be automatic. Say during power down charger charges. And if long term PC operation is needed, the charger can be used to top up the battery as the terminal voltage drops.

    I like to know any draw back that this idea will produce.
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Looks like you are trying to power a PC, right? So here are some things that come to my mind:
    First, why not use the ups just for the brief moment until the genset starts (automatic start and transfer switch)?

    I don´t think taking the 12V straight from the battery is a good idea. It most of the time is not in the 12±0.5V or so range, so the motherboard will have hard time filtering it, keep in mind pentium 4 and newer processors are powered from the 12V rail. The noise induced from the two other SMPS will be pretty bad, and the noise from the charger even worse, which may result in resetting the PC ocassionaly.

    I think better idea is going for 24V battery, using a stock PC PSU and rewinding the primary to suit the voltage. If the transformer is not too hard to rewind, that seems the easiest solution. Then you just have to maybe change the transistors in the halfbridge and tweak the feedback and it should be ready to go.

    The charger should have some way of knowing if the computer is running, so that you can either just charge, charge and power the pc, or just power the pc. That could be the harder part.
  3. tom66

    Senior Member

    May 9, 2009
    The problem is a 12V SLA battery has a large operating range. It runs from 11.5V to 13.8V usually. I've done some tests using my modified computer power supply on an old 5GB hard disk with 13.5V. Although 13.5V doesn't damage a hard drive, the motor spins faster, causing the drive to reset often as it loses track of where the disk is, and obviously this means the operating system probably couldn't boot (but I didn't dare to see if a motherboard would run on 13.5V.)

    What you want to do is to use a car power supply for a computer. These are usually capable of outputting 12V, 5V, 3.3V and -12V - all the voltages required by ATX - from a variable 6V to 30V input.

    Or you could use a boost converter to boost 12V to ~300Vdc and put this into a power supply. Most isolated SMPS computer power supplies will accept DC, although you can't use the 115V mode as that uses a voltage doubler. PFC supplies may work.