computer power supply with built in battery backup

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

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 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

Hi John,

That's a nice set up you have there.

I like the idea of separating the electronics from the battery. I currently want to do something like this for a USB / coaxial power bank type scenario as well, as I've not been having much luck there with off-the-shelf products lately either. Was thinking of using some deep cycle leisure batteries as the power source as used in caravans and motorhomes for standby power.

The last significant power outage here took out the city's main substation (it is next to a river which burst its banks) last winter and left the city without power for 30+ hours over a weekend, then for another 10+ hours the following afternoon. The cell phone network was disrupted as were VDSL network services as the fibre convertors in the roadside cabinets require power. Landlines remained up as the telephone exchange had back-up supplies and I believe ADSL services stayed up for those that haven't yet moved to VDSL. It also took out many of the modern radio services. So it was dark, raining heavily, main roads were flooded, cold and for many people no access to telephones, information or entertainment. 

I thought I was better prepared with my solar charged standby 12v golf battery and mains inverter however I was to find out the hard way that it had partially dried out (downside to maintenance free batteries) and had little charge left in it but just enough for the battery indicator to remain green. Perhaps a similar reason behind a number of my UPS battery failures. The inverter automatically shuts down on low voltage to leave enough juice to still start the car it is intended to be connected too.

Rather than attempting to keep a desktop running throughout an outage like that, I was thinking more of just being able to keep something smaller like a tablet and a few accessories running which would at least allow for access to ebooks and videos, and be able to access data from USB drives, along with some network connectivity if it was available.

We used those type UPS in a former vocation.

If I recall they feed the mains directly through to the output via a transformer, that can compensate for +- xx%.

When the mains finally drops it then starts the inverter.

Much better than other UPS that are always driven from the inverter ...

Hi Mark,

Yes, this unit passes the mains through to the units when mains is available and charges the battery to 13.6 volts. When the mains drop down it switches on the inverter quickly enough so that the interruption is not relevant to the computers power supply. I was not aware that there was a different approach than this.

John

I was not aware that there was a different approach than this

Most UPS use the mains as a battery charger, and the inverter runs the load using the battery (which is being float charged normally)

Mark

What are the main pros and cons between the two methods ?

I guess that most of the inverter circuits are not true sine wave output ?

Conversion losses whilst on mains supply and audible noise from the inverter perhaps ?

I've heard of some talk of a move to DC power distribution recently though which could start to change things in this area.

I'm certainly not the expert, but some observations are :-

(I'm fairly sure they were referred to as SPS v UPS.)

What are the main pros and cons between the two methods ?

The SPS John has, will raise or lower the incoming mains to keep a constant output.

The inverter is not always working at load, so in theory the heat from the inverter is gone. (and maybe any noise as you say)

There is a small delay in starting the inverter but they claim less than 20mS which is a cycle.

The others are full time UPS inverter, with no delay.

They have no issues with phasing where the inverted waveform needs to be sync'ed to the incoming mains.

I would expect that the full time UPS will have more harmonics and possibly a higher emc than john's version.

Both units tend to use a multi step type waveform, so they are much closer to the full sinewave.

However the modern electronics is generally a rectifier, so it doesn't particularily care.

Our 100KVA units at work have a string of batteries to give the voltage needed for the inverter to supply 3 phases at 230v.

DC Distribution

I thought they tried that first ...

it has big problems with distribution.

The first is I2R losses in the cabling.

The next largest headache will be converting it down to something the user can apply.

There are also issues with magnetic attraction to the distribution wires.

Many of the electrical switches cannot handle the currents.

AC ones depend on the return to zero voltage to break the arc cycle during switching.

Sorry but I'm not seeing the benefits of a DC distribution system.

Mark

There is a brief article on DC power distribution by the IET here:

Low and extra low voltage direct current power distribution in buildings - IET Electrical

I first heard about it about a year ago when an electrical engineer took one look at my office, saw around 50 wall warts in use on my desk and commented along the lines of 'we need to get your new office onto a DC power distribution system'. He seemed to think that like it or not, it was coming as the majority of items around the home and office are primarily DC supply devices, and that there was a need to start removing all the wall warts that we have become accustomed to.

However I'm curious as to how it would look in reality and whether or not it would just end up replacing all the AC-DC wall warts with DC-DC wall warts.