PV providing power for a small pc, with mains powered 12v backup - Page 1
Forum section: electronics / projects

author: lk
author_rank: Regular Contributor
timestamp: August 01, 2012, 03:31:03 pm
content: Greetings,I got a new little machine for running my cctv system, and when i realized that it was only drawing around 40 watts from the wall, i realised, i started thinking about how to run this thing off a PV panel, i can easily supply it from a picopsu either the 12volt version or one of the ones that have a bigger inputrange.I would need some circuity that would be able to swap between PV and 12volt plug when needed, but i have not really been able to find anything the remotely, possibly because i don't know the right words for what I'm looking forMy guesstimate is that i would need 7-8 amps on 12 volt, maybe it would make sense to run it on 24volts so i have to deal with less current.Anyone that can send me in the right direction?-lk

author: Simon
author_rank: Unknown
timestamp: August 01, 2012, 06:30:16 pm
content: The simpler way might be to run off the battery (that you will actually be doing most of the time anyway), and keep it topped up from the solar panels. If anything you want an emergency cut in from the mains.

author: Chet T16
author_rank: Frequent Contributor
timestamp: August 01, 2012, 07:08:07 pm
content: Is this relevant to you?https://www.eevblog.com/forum/projects-designs-and-technical-stuff/battery-psu-swapover/The board i made is designed to switch a solid state relay on a 12V PSU when the input voltage (from a solar panel/battery) drops too low

author: lk
author_rank: Regular Contributor
timestamp: August 02, 2012, 07:50:08 am
content: Thanks for the replies,Simon, i did not plan in the first phase to use batteries, may an option down the road, but i would need a significant amount of batteries to drive 80 watts for 16 hours a dayAda, so i would need the automatic switchover, in any case.Chet T16, it sounds very much what you have there that i want, i need to study your schematics a bit more in detail, so understand if it can handle 8 amps at 12 volt.-lk

author: Simon
author_rank: Unknown
timestamp: August 02, 2012, 07:52:16 am
content: Without a battery or mains backup your system will not work. There is so much variability in the suns intensity or availability at all that your system will be unreliable without a reliable source of power.

author: lk
author_rank: Regular Contributor
timestamp: August 02, 2012, 10:04:11 am
content: My OP might not have been clear, but the intention was to run off PV and then switch to a mains fed power souce when needed, and what i was fishing for was ways to do the switchover in an automated fashion.-lk

author: Simon
author_rank: Unknown
timestamp: August 02, 2012, 04:52:58 pm
content: well you will need well over speced panels to guarantee a minimum of continuity (small clouds passing over ect.) and in order to make reasonable use of the sun you want a fair amount of panel power and a battery to pick up during the small gaps. As soon as the battery voltage goes below a minimum you cut in the mains. Use a comparator circuit and relay and have some hysteresis or it will oscillate as the battery recovers a bit when the power draw stops. If you try panels only it will be very unreliable unless you have much more panel than you need in which case you should conserve it in a battery. Your other option is an off the shelf grid tie inverter, I know it's adding another loss but you'd have that with the battery anyhow

author: lk
author_rank: Regular Contributor
timestamp: August 03, 2012, 06:58:59 am
content: Hi Simon,I see what you mean, but i have a feeling that i will want to have a micro controller over this, to keep track of time of day. eg if i run out of battery power at 10 in the evening in December, and if the voltage for some reason recovers on the battery there is no reason, to switch to the battery input again.And a relay was what i was asking for in my OP. Not until i looked closer at one of my own relays, i realized that i could use them to switch between two sources, previously i just used them as on off switches.I did think about the grid tie inverter, but as i will be getting a 6 KWH PV setup connected to the grid, within a couple of months. So its properly best not to do anything that, upsets my utility company, this is why i want to run it off ~12 volts. So technically i dont need to do this, but its fun to go an ponder about.as far i could see from the datasheet the relay im using has a switch time of 1ms, which makes me think that i might need a C/L filter on the output, to catch that 1ms power loss.-lk

author: Simon
author_rank: Unknown
timestamp: August 03, 2012, 08:33:53 pm
content: Quote from: lk on August 03, 2012, 06:58:59 amNo, when you take the load off the battery voltage will rise slightly, so you switch your load back on ad it drops again, so the load will get removed and the voltage will go up again. It will oscillate. Not good.

author: yyrkoon
author_rank: Newbie
timestamp: August 09, 2012, 04:57:40 am
content: Quote from: lk on August 01, 2012, 03:31:03 pmRun panel through a charge controller to batteries. Run off batteries continually. Batteries need to be "exercised" ( lead acid ) so this works out good. To run an 8A load for 16 hours continually. You will need approx 813 AH ( amp hours ) worth of storage.On the cheap, wal mart brand deep cycle batteries go up to 200AH each. If I remember correctly. They cost ~$150 each. So you would need 4 if you fudged it a bit ( I show you'd need less than 7A for 80W ). Now do also keep in mind. Kind of an unspoken rule of thumb in the offgrid community. Would be to calculate your storage needs, then double it. But for your case, I think it will be alright as is.Now . . . off the top of my head, I am not sure of the math for calculating how much power in you're going to need. You could take say like 60A as rough guess, and divide into 813 where you would get 13.55( hours ). However I know this is wrong. Since we here have 2100 ah worth of storage, and peak at around 105A during the day. Just a rougher guess on my behalf I would say that 30A would be a nice round figure. Where you could buy an inexpensive Trace/Xantrex C40 PWM based charge controller. 20A may even work out for you. Locale will play a big factor here. As well as how clear the sun is into the panels the. Shaded, not shaded, cloudy, etc.Now the cost . . . which is the big deterrent for most everyone. Assuming $1 per Watt panels. 2x 200W $400. Xantrex C40 charge controller. ~$60. 4x cheap 200ah batteries $600. Suitable wire( welding cable etc ), panels stands and/or self tracking stand. Will cost a lot more. If you require an inverter . . . more cost still.Anyhow, as you can see it can be quite expensive. Assuming you do need an inverter, self tracking panel array. You can expect to pay at minimum $2000-$3000. Then if you purchase good quality RE type batteries ( highly recommended ). Expect that figure to be closer to $4000. Then, you do need a proper place to store them, which hopefully keeps ambient temps within an acceptable range . . . well ventilated . . . LOL I know it sounds ridiculous, But take it from me. all of this is important. Having lived completely offgrid for close to 4 years now you tend to pick up a few things.Anyhow, if you have more specific questions fire away when ready.

author: yyrkoon
author_rank: Newbie
timestamp: August 09, 2012, 05:48:15 am
content: Ah, forgot to touch on the 12v vs 24v issue,Overall 24v is more efficient. Less line losses if going long distances. Cabling is/can be cheaper( less copper )However, one caveat would be battery cost. Battery cost is one of if not the biggest cost in any RE system. It is also the only reoccurring costs if your equipment is well maintained, and good quality. So, as an example. You halve your current requirement for total power output, So if you use 12v batteries there is really no added cost. But, if you use 6v batteries, you're going to need at minimum 2 just to reach your voltage requirement. 2V batteries ( which is what we use ) you would need 12 minimum. Our batteries cost $326 each . . . So cost of admission for just batteries alone would be approaching $4000. Versus $2000. So there can be some trade-offs, where good/bad is concerned.I would recommend that you research all you can before diving into something like this. Price per kW as it stood a few years ago. You could never beat your local power company in cost moving to solar / wind ( here in the USA anyhow ). However, there can be a lot said about being completely liberated from your local power company. More than the satisfaction of being the only house in the neighborhood with lights on during a brown outAnd once again . . . apparently I missed the meaning of your post. Got it now, after reading all of the post in the thread here.Why not run off the batteries continually for the computer you speak of. Have a voltage sense device watch for a certain voltage on the batteries, Where it could then kick on a charger powered by the mains until either a) you're getting enough solar power coming in, or b) the batteries have reached a decent SoC? I do not know of one off the shelf, but it would be very easy to implement. Could even be a fun project. Personally, I would have the mains kick on to charge the batteries before they hit 70% depth of charge. But do some research on lead acid batteries to see what I mean.Again, lead acid batteries have electrolyte in them that needs mixing. Which short of picking them up and physically shaking them yourself. Is done by charging the batteries enough to a point where they start bubbling. This also helps keep the plates clean.For your 6kW array, 24v is going to be better. If that turns out to be an option. 48v would be even better yet.