12V DC-to-DC UPS
Well, if you are sensible you should not have any trouble working with electricity. The difference is that low voltage is safer to work with because the voltage is low. I mean you can put ur hands across the battery terminals and you wont shock. Try that across the mains with 240v and its another story. Remember that the mains can supply 240V at up to 20A in most homes. This is enough to fry you multiple times over. Fortunately most homes have working Earth Leakage Relays that trip from 30ma upwards. that is supposed to be enough to give you a good shock but not kill you. However if you got stuck on a plug without the earth leakage working to trip, you are quite likely to die.
Except if you're one of these guys:
https://www.youtube.com/watch?v=ganoF0akVig
For anyone still looking for a solution like this - it will cost a little bit more than a R300 - R500 "cheapie" 240V UPS, but will run your 12V electronics for around double as long - build a cheap 12V DC Battery "UPS" for R400 - R600:
Solsum F 6.6 / 8.8 .. Solar charge regulator: R300
12V Sealed Lead Acid Battery** or old car battery (less efficient): R0 - R150
15V Laptop Power Supply: R150 - R350 (It needs to spit out at least 14.4V for Lead Acid battery.*
Hook everything up and let it power your device - the 15V PSU connects where the solar panels normally would. Using simple math you can calculate how long it should run. Or you can just run it and see how long it runs... and if you like you can reverse-calculate the efficiency of your PSU and charge controller.
I would expect efficiency as follows, assuming I'm running an ADSL modem, or a 1A 12V media box, or wireless access point:
Above DC-DC solution with a 7AH battery: 6 Hours, 3-5 years.
7AH battery with PSU and no charge controller: 6 Hours, 3-12 months.
240V UPS with 7AH battery: 3 Hours, 1-2 years.
(I would love someone to test this!)
The reason the DC-DC "UPS" is a better setup than a 240V UPS, is because all cheap 240V UPSes give a square wave, or modified sine aka. still very square wave - that doesn't convert as efficiently as a pure sine wave through an induction transformer, which in itself is already only 60% efficient and with the square wave probably only 40%-50% and will heat up a lot more. The very small power supplies are switch-mode supplies, and approach 90% efficiency... but the 240V UPS is already only 80% efficient.
* To be tested: A 12V PSU might work - as the spec sheet does mention the word "Boost" so it might boost the voltage so as to properly charge the battery. But I doubt that it will work because 12V solar panels give off 18V+ quite often.
**DO NOT** USE A GEL CELL / AGM / GEL BATTERY with this charge controller - those require a lower float voltage and they will swell, gas and break. Unless your solar charge controller is of course is made for this kind of battery. You can use a Lead Acid battery on the controllers made for gel batteries, but it will never properly charge so will be less efficient.
A cheaper version of this, is, of course, to connect a power supply and your device in parallel to the battery - while it would seem that this works, it will stop working much sooner than the DC-DC solution - and I would urge you to avoid doing this for several reasons: Your battery will never properly charge - or will over-charge, gas (release potentially explosive H2 gas) and become a hazard, because it's almost impossible to get the voltage perfectly right unless you use a voltage regulator on both your load and your power supply - and configure these to balance the voltage perfectly.
Another variation, is of course, to open up your cheap UPS, and run thick copper wires to a car battery or bigger battery bank. The problem here is that if you accidentally short anything, and short-circuit the battery, you will have a fire on your hands within seconds, as the copper wire will heat up very quickly depending on its thickness, and ignite its sleeve - so you will need a fuse or a quick way to cut or disconnect all the cables - and nothing that can catch fire within range. But another problem with this is that the cheap UPS transformers are only made to run for 15 minutes max. And even then they approach 100C+ and will burn you if you touch them. I learnt this the hard way when my cheap UPS connected to a car battery ran for almost 3 hours when my pre-paid electricity ran out, and the transformer burned its way through my plastic UPS chassis and through my carpet. While I wasn't at home. Thankfully nothing caught fire.
