computer power supply with built in battery backup

This from the layman's perspective is wonderful as a project since even if it can't be done i'd love to know WHY things are the way they are.

Hi Matt,

The APC supplies are set up like this because desk top computers and monitors can not run directly off a single DC voltage.  The ATX power supply creates + and - 12 volts, + 5 volts, and perhaps + 3.3 volts. The monitor probably runs on yet another voltage and if you have any other accessories they may also have different voltage requirements. By having the APC turn its battery backup power back into 120 VAC the individual power supplies can do their jobs of producing all the different required voltages.

John

Informative!

Yes thank you I know that part.  They make 12 volt DC-DC power supplies for computers.  I want to avoid the power loss of conversion and only switch to DC once like a Laptop does.
In searching I think I may have found some products that would fulfill my needs. 
OpenUPS to regulate charging and pulling from Battery.  http://www.mini-box.com/OpenUPS?sc=8&category=981
and this 250W DC-DC power supply to take the DC from the openUPS and step up and down for the PC.  http://www.mini-box.com/M4-ATX?sc=8&category=981
Combine that with a LiPO battery pack and it's awesome.

Another approach is to move some tasks to low-power platforms such as single board computers (SBC) - like the Raspberry Pi or BeagleBone Black. Or even a router if you can run things on it (some allow this), or your network storage. They need a single DC low voltage power supply.

Maybe nothing else is critical and can be rebuilt.

As an example, I wanted something guaranteed to work for a couple of hours in the event of power outages (it was an experiment that needed to run for several months uninterrupted), so I provided a 12V supply to a router and to a single board computer. My supply came from eight AA alkaline batteries : )

No need to worry about charging them, and they would have powered my devices for an hour or two. I used a relay to switch them : ) didn't require MOSFET switching either. The relay was quick enough combined with a bulk storage capacitor. Nice and simple. I could replace the batteries at any time (e.g. once a year, or after an outage) provided it wasn't during an outage.

Google took this approach with their early server builds.

http://www.cnet.com/news/google-uncloaks-once-secret-server-10209580/

There have been a couple of products on the market that do appear to achieve this sort of functionality. Something like this:

eNSP3-450P-S20-H1V - ATX Power Supply with UPS Function

The PSU unit replaces the existing ATX PSU and the battery(s), are designed to mount in a standard 5.25" drive bay.

I've been interested in this idea for a while myself as I've generally found the external UPS solutions to be notoriously unreliable whereas in comparison the laptop type mains/battery arrangement appears to be very reliable.

Hi Dave,

It looks like an interesting idea. At least it would conserve volatile memory until the unit could be shut down. Of course if you wanted to be able to see what you are doing you would still need power for your monitor.

John

Hi John,

For me it's more a case of covering two scenarios:

1. surviving the short blips that are long enough to cause a reboot
2. clean shut down of the system if the power is off for more than 30 seconds, as in most cases that means its going to be off for an hour or so. So saving files, suspending open virtual machines then a shut down.

So for me the monitor issue is not necessarily that big an issue, I'm not looking at continuing to work as normal through a significant outage.

These days though the chances are you could still remote in from a tablet if you really needed a terminal and grab any vital files you might need before powering down the desktop.

More often than not though I've found that the external UPS unit lets you down when you need it the most anyway. Out of the 40 or so UPS devices I've worked with, only the original APC RS1000 and RS1500 units still managed to perform to spec' after a year of use. In comparison I have a 7 year old laptop still working on its original battery.

Hi Dave,

It sounds like you have done a good job of analyzing what will work best for you. Personally I took an old APC and installed an external car battery. I have voltage and current monitors on the battery and I test it for proper operation monthly. I also have a separate APC on my Modem and Router which is tied to the local cable service. In the last couple power outages I was able to continue to work and monitor the weather on the computer without interruption. Here are a couple picture of my setup.

John