Being of a I.T. bent I have an attic littered with computing technology going back twenty years or more. Among that stash of crap is an old uninterruptible Power Supply (UPS) which once held a small server alive during brown outs and power cuts. As the cost of replacing the battery in a duff UPS is often about the same as simply buying a new unit, these things are often easy to get hold of.

The UPS I have is an APC SmartUPS 620 and it's battery is long since dead (and recycled), however as it's job was to convert the 12V DC from its battery into 240VAC to power a computer during a black-out, it stands to reason that with a bit of re-jigging it can be given a new lease of life as an inverter with the ability to convert the output of my solar panel battery into mains voltage for running low power mains applications (I'm thinking laptop and phone chargers, etc.)

Inverters can be purchased at relatively low cost and as I have wired automotive sockets around the house I could buy an off-the-shelf model such as this fella from CPC, but as I have a perfectly serviceable piece of equipment already which can be made to work with a bit of modification, I'll take the do-it-yourself route (it's in the spirit of recycling after all).

I suppose I should start off by stating the obvious. Messing around with high voltage electricity isn't for the faint hearted and I will be modifying the use of this UPS in a way that it wasn't designed to be used for (and which the manufacturers wouldn't approve of).

The first thing to check is that your candidate UPS will 'cold start' - i.e. that it will actually power up without a mains input supply to begin with. What we have to remember is that as well as being designed to convert a battery output to mains, the UPS is also designed to keep a battery charged. As the UPS will eventually have it's battery terminals connected to the output of the solar regulator I don't want to go hooking it up to the mains as it would attempt to charge the regulator output which could cause damage to the regulator or the UPS - or start charging the solar battery from the mains which defeats the whole objective of hooking it up to a renewable energy project!

So, does the UPS cold-start? If not, can it be made to with a bit of modification? You may find your UPS has an AC-DC circuit inside that brings the input mains voltage down to 12V for it's control circuitry in which case it might be possible to frig it by connecting your battery at this point thus fooling the thing into thinking it’s got a live input. I opened up my UPS and was quite taken aback at the amount of discrete components inside so if you don’t have a cold-start feature you may have your work cut out. A search on the web found that my beast would cold start if the power button was held in and then pressed again when it started beeping.

With the cover removed you can see the back of the circuit board - look at the size of it and how populated it is! What a minefield!

If you can cold-start your UPS then modding it into an inverter is pretty easy…

Step 1 is to take the existing battery connections and make them suitable for hooking up to your battery – in my case this means fitting a flying lead with a cigar lighter plug on the end so it can plug into one of my automotive sockets.


Step 2 is to remove the buzzer (or to quieten it down). Remember the UPS is designed to work as an inverter only when there is a fault condition (i.e. when the mains fails) and all the models I have ever come across make a bleeping or buzzing alarm tone when they are active. You’ll have to fish around inside the UPS to find where the buzzer is and then either remove it or silence it in some way unless you want it to sound off while in operation. I do want mine to make a noise (for reasons explained later) so mine has been ‘softened’ rather than silenced. Cover the buzzer with silicon sealant or Blu-Tac for a quick and easy way to quieten it down or maybe fit a resistor in series with it for a more technical approach!

Step 3 is to make a converter cable to change the presentation of the inverter output (IEC ‘kettle’ sockets) to a normal mains socket. This is easy to make and just involves wiring the live, earth and neutral socket connectors to an IEC socket – just make sure you get the pin outs the right way around.

Step 4 is to Earth the AC input for safety reasons. We don't want any AC to flow into the UPS so only wire a single Earth cable from the Earth pin of the mains plug to an Earth pin of the IEC plug.



Above: The conversion cable to change the presentation of the UPS IEC socket output to a normal UK mains socket. Also shown is the Earth safety lead and a mains socket tester for checking everything is wired correctly before anything expensive is plugged in!

And that's about it. Just a few points to note...

The UPS is designed to run a high drain load for a short time (such as a computer for fifteen minutes to give it time to shut down cleanly in a black-out). Using it as an inverter for long periods of time may cause it to heat up so keep vents clear, ensure decent airflow around the unit and, if necessary, look at fitting a fan to the case. Certainly don't go leaving it on all the time (besides, mine takes about 250mA from the solar battery without even having any load connected). This is the reason I left my buzzer in - I want it reminding me that it is on so I don't forget to shut it off when I'm done with it. Besides, I need to hold the power button in until I hear the tone and then push the switch again in order to cold start it so removing the buzzer would make switching it on more difficult!

To be honest, converting a UPS in this way has questionable benefit. It isn't particularly efficient when running, trips out easily, doesn't handle certain loads, is large, heavy and has a noisy 'hum' when on. A proper automotive inverter is probably a lot more sensible if you're considering such a project and being smaller and lighter you could swap it between your house and your car.

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