Stupid question: why not using a 24 V battery plus a rectifier, and then use a couple of 7812s to have 0, 12V and 24 V levels. With these level you:
use the 12 V as the 0 V for your computer.
your 24 V level, when the reference is 12V, becomes 12V
use a 7805 connected to 24 V as Vdd (+) and 12V as reference and there you have your 5V
an LM 338 connected between 24V and 12 V (with appropriate resistances) to get 3,3V
Your 0V on the battery becomes your -12V.

If you feed all these directly to the motherboard and peripherics (bypassing the PSU) you have the most efficient, cheap and simple way of ensuring that your computer will have power even when the lights go out.

 

yeah, except that you can only get a few watts out of those kinds of linear regulators, and they're horrendously inefficient for dropout voltages that high (12v dropout from 24v is already 50% efficiency at best...). PC's generally require 100-200W or more. You do the math.

This is why switching regulators are used for high power loads - which is precisely what the DC-DC ATX power supplies I suggested do.

Also, you probably could not get away with the "virtual ground" scheme you have proposed - the positive 12v rail of a PC is going to carry much more current, and thus your 12v regulator acting as virtual ground would have to SINK current - which is not how these linear regulators operate.

And on top of all that, ATX supplies need to do more than just provide voltages. They also have to deal with some control lines that let the computer control the power. Not a big deal, but just one more reason why it would make no sense to reinvent the wheel in such a terribly inefficient way, when existing DC-DC supplies can do the job better, much more efficiently, and be smaller and cheaper.

__________________
EEgeek.net

 

In practice it wouldn't last for ever because there will be losses in the form of electromagnetic radiation.

Well the losses will be lower at a lower frequency.

Anyway what's normally done when AC power needs to be stored is it's converted to DC (using a transformer and rectifier) then converted back to AC (using an inverter). Un fortunately the overall effenciency is quite low, it can be less than 60% once the losses in the battery, rectifiers and inverter are taken into account.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.