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12v UPS Answered

With most UPS's there is electronic circuitry to reduce the home voltage to 12 or 24v to store in the battery, and then another circuit to raise the voltage back to the home voltage.  I know there is always a loss when attributes of electricity are changed. This seems very wasteful to me. I want to build a UPS for my notebook computer, that I use as a home server.  The power adapter for the notebook creates 19 volts but the notebook itself uses 12 - the other 7 is perhaps used to charge the battery while the computer is on.  I would like to put together a UPS made of 12 volt Ni or LiFe packs - would most likely be 13 volts but I don't think that would be a problem to avoid the double switching of the power.  Can anyone point me in the right direction?



Yes, you are right. First of all, the battery is shot - powers the computer for about 15 minutes. The area that I live in is subject to power "infrequencies" where the power will drift on and off over more than 12 hours. I leave the notebook on 24/7 with the screen powered off, so it uses less electricity than normal. A new battery would probably work for about 5 hours. I also have an ssd in it and have removed the DVD drive to reduce power consumption and help with cooling. I would like to be able to power both the notebook and the router that I use - also a 12v device for 24 hours. The internet does not go down when the power goes off - not sure why but it is my experience. I use the internet for business - communications with the US and have to deal with a 12-16 hour time difference. A long story...but that is what brought me here.

You can most easily just replace the 18650 cells inside that pack. Ternergy 2600mAH 18650's cost about &6/cell last I checked. I use these in my 2000 lumen flashlight and it lost a long time. Do not but crap quality ones. Very high quality ones like AW's or Panasonic 3100mAH cells can cost $20/cell.

Well a 12V battery isn't going to work. Pull the battery from your laptop and see what it's rated at. I'll bet it's more than 12V. You'll need at least the rating of the battery to run the thing.


3 years ago

If you are powering a notebook computer, there is to need for a UPS. They are simply to allow critical electronics to work a few minutes after power is lost so they can shut down softly. A notebook will still work 30 minutes - 1 hour, maybe even longer after power is lost. You can always upgrade the 18650 cells inside the battery pack for better battery life.

Also, the actual operation of different types of UPS supplies are different. The 3 main types are offline/standby, Line interactive, and Online/double conversion. The UPS you describe most closely resembles the third one. It converts mains to a low voltage, generally something like 32V-48V to charge a battery, and then convert that back into AC though an inverter. Doing all these conversions is not only wasteful, it is also generally not reliable, thus defeating the whole purpose of the UPS! However, in some special cases, where ultra sensitive equipment is very sensitive to input voltage, and the available power is is very poorly regulated, This may be the only option. They are generally very expensive and difficult to engineer for reliability.

Instead, most consumer-grade ones will be a designed around a offline/standby topology. These basically have a relay inside that when power is available, it will connect the output directly to the input mains, and the unloaded battery can charge up and sit around on standby. The inverter output will remain inactive. Once power is lost, however, the relay will switch to the inverter, which powers on and powers whatever for a few minutes until the battery dies. The output on these is still susceptible to noise and interference on the mains, poor regulation (like frequent brownouts), etc.

So the second topology, Line interactive, offers a decent middle ground between the 2 types above. It has a adjustable autotransformer inside which will compensate for brown outs and over-voltages. When power is lost completely, though, the inverter will kick on and the relay will switch to that. It is quite ingenious and offers the best of both worlds.

For more details, watch this (albeit long) video by EEVblog. https://www.youtube.com/watch?v=Fj7e3WGUKO8

Did you ever consider a car-PC?
They are designed to run on 12V and it is fairly simple to use a standard battery with an electronic charger.

Why do you need a UPS for a laptop? It already has a battery in it. All you need is a good surge protector and your set.

I could buy a large ups and be done with it. The power demands are quite minimal, but I am also interested in using this project to learn a little bit about electricity. I would also like to be able to repair this if something goes wrong, and I think the best way to do that is to make it myself :)