computer power supply with built in battery backup

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 ...

I believe that there may be some 12v supplies designed to run a PC from a car.

This would in essence save the "mains charging a battery to generate mains, which is then converted to lower voltages".

The 5v is easy, along with the others voltages, and the -12v is usually for the serial port, and is very low current, so could be a small DC-DC converter.

Mark

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.

I'm away at the moment so haven't read the link.

If all the wall warts were the same voltage then there is some savings, but otherwise you'll need converters.

You also have the issues of ripple being introduced to the other loads.

Cheers

Mark