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Failed on first attempt at off-grid. questions about voltage.

So I bought a 45W photovoltaic kit from Harbor Freight which comes with its own charge controller ($139 on sale), a used deep cycle battery with 135 amp-hours, and a 1.6 amp DC pump that moves over 300 gallons per hour. Yeah I know I should have done my calculations ahead of time but here they are, after the fact.

1.6 amp X 12V comes out to 20W per hour times 24 hours for (roughly) 480 watt-hours of consumption per day. At 45W the solar panels would need 11 hours of full illumination/full output per day to keep up with the pump's consumption, and I'm not getting that. So starting with a fully charged battery, it goes for about three days before the battery drops to the 11V cutoff where the charge controller kicks off flow to the pump.

Not a lethal problem, since I have back-up pumps running from the grid, but I'd need to add more solar panels to keep up with my pump's consumption. I can do that (if I can find space for them)--but here is a question that is more theoretical for those who know about electrical stuff.

My pump is rated to work from 5W-13W. If I connect the pump directly to the battery rather than to the charge controller with the 11V cutoff, the pump will continue to work as the battery discharges to a lower voltage, and at this lower voltage presumably the pump's draw will be lower as well. At some point, this lower rate of consumption will be equal to the feed coming off the photovoltaics, right? Would there be a problem with this? Is it hard on a 12V battery to keep it fluctuating at a lower voltage--say between 7-10V? Will operating at a lower voltage affect the pump's expected longevity?

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My situation is really not terrible, considering the less-than-careful approach I took. To be honest, it was largely an impulse purchase-- I needed a sunshade for my IBC fish tank and heck, these solar panels were not that much more expensive than the corrugated roofing plastic stuff I used on my chicken coop (how do they get away with charging $29 a sheet for that stuff?). A lot more interesting as well and a good reason to revisit some of my college physics.

Surprisingly, it looks like I'm actually quite close to ins and outs matching up, since it took three days of only partly sunny April weather to discharge the battery with the pump running 24/7, and the thing comes back up to full charge in a day of moderate sunshine (with the load removed). I think that full summer sunshine in California will get me pretty close to equilibrium--we'll see.

It seems to me that with another 45W of panels I'd be able to cover the load of my 1.6 amp DC pump running 24/7 with sun as good or better than what we're getting now. Roughly 480W needed per day (1.6a X 12V X 24 hours) and more than that provided by 90W of panels--we're getting about 7 hours of good sunshine per day for 630 W, and this allows for 25% loss to various reasons. Does this sound right? 

I understand that I'll be back on grid power when I'm getting less sun than this, and that it's more typical for people to want to operate on a scale similar to what others are doing--e.g., oversized solar arrays and battery banks for relatively small loads, which allows for the kind of cushion needed to keep the tank and garden alive during bad weather and the short days of winter. I'm not worrying about that here. I'm actually kind of interested in how close the theory (specifically predictions about sustainable loads with X amount of sunshine) matches up to the reality of a working system. 

Jeffrey, I feel your pain as to the GH panels. I need enough to cover our 24' GH roof this summer. I did find a local steel panel co that has a seconds pile and a 15.00 panel they let go for about 3 - 5.00 and we are hard pressed to find the defect. So, if you can find a local co that makes the clear panels you may find some great deals. I'm looking in our area with fingers crossed as we speak.

In the future you might consider 6v golf car batteries as they are far superior to the 12v marine/camper types and designed for more abuse and golf courses get rid of them at scrap prices at no less than 50% life as they don't want complaints from customers. It goes without saying that you need 2 for 12v. They will handle deep discharges better but still like to be kept charged. Our golf car has 9 yr old bats that are finally showing their age and I was on jobs with it for years on an island in ME where I had to run them so dead I would have to help push it back to a plug somewhere. 12v types would be worthless long ago. Also if you beat on a dealer hard enough you can get brand new blems at 50% off. They will deny they exist at first so stay the course. Trust me they have them or can get them. My dealer in ME totally denied their existance and I left there with 4 for my UPS unit. That is a 24v system.

There are ebooks on the subject of getting high grade solar panels from state maintenance garages at scrap prices because like batteries they have a useful life span (figure about twice the life of batteries in gen) and being gov agencies they change them out as soon as they exhibit degradation but are still far superior than HF's hobby panels. You can see them everywhere now a days on the highway powering signs, etc. Takes a little brazen foot work but I think you'd be amazed at the deals you can find now that you are educating yourself on the subject and now "have a clue" So stick at it.

Personally, like I said I am pursuing the hydro route as I did buy a farm with a 30' drop in the year round stream running thru. I have 200' of 2" pvc already run to make up for the lack of drop (50' ideal for 1") so not sourcing panels right now but I would grab them if they happen to come my way My daughter has been installing private systems all over the Boston area for some time now. (For very rich clients)

What I am saying here I guess is: you may achieve "payback" if you can get the equipment at wholesale or scrap or even blem prices. My brother in Texas does (achieve payback that is) but he gets gov and private ind grants. Must be nice. Typically the only way. For you and I: we must scrounge. And read, and read, and learn. I dig your enthusiasm.

Thanks Jim. I'm taking notes.

I live next to a golf course--maybe some good will come out of this location after all, since I don't give a rat's ass about golf. It would be nice to have it pan out as a source of inexpensive batteries.

The finagling of government cast-offs will be harder. I'll have to look for those ebooks, but the bottom line is I'm not a great schmoozer. I might have to enlist the help of my car salesman buddy.

Glad to see you're recirculating water and getting power from hydro--it would be easy enough for you to use the stream to keep up the water quality for your fish, but that would degrade water quality in the stream (a la salmon farming).

Jeff

Well Jeff you hit the nail on the head there solar is expensive lol I live off grid with my family and have for 19 yrs. It can be tough to understand the point to it all lol. Just remember its great thing to work towards and really isn't about saving money once you get passed that then you can make more sense of it. Solar is about being self sustained and not drawing off the grid envision 30% of the world doing that. From my experience I will tell you this. When you add the hard numbers up of panels and batteries you never seem to get what you think you should be getting. also remember that a battery can absorb power for about 5 hrs a day so figure that in when you crunch the numbers. Also try keep your batteries at 50% and above if you are constantly going below that this will greatly reduce the battery life. Don't give up bud :)

Jim, It's been a while since your post but you got my interest with the hydro comment. How does that system work?

Jim Fisk said:

Jeffrey, I am all for experimenting and hobby solar and wind and hydro. I too love it but I do not expect wind or solar to be money saving or offer a payback. I am however tired of the Clickbank scams  that promise 50% savings after spending only 200.00 due to a "Weird Trick" and building your own panels, etc. I am a Master Electrician in 2 states and have seen all the promises that the unaware fall victim to every day. If people go into these things with their eyes open (read knowledgeable) they won't be disappointed and can have a lot of fun with it, when you're a geek like me and probably you. But when I see people expecting a "payback" versus on-grid I know they are drinking the Koolade. I want to see private money fund the research and not our government picking winners and losers with our money.(read campaign finance payback) Odd how most go belly up AFTER they get billions from us and then give out huge bonuses (more than most of us see in a year) as they lock the doors. Aren't you getting tired of it?

Great as a hobby, great if you have no choice, great if you built a nice country home only to find out the electric co wants 15 or 20K to run the lines and hook you up. But to beat the price of our cheap grid in general. Don't hold your breath. Now if you want to talk small hydro, I'm all ears. A 1" pipe dropping 50 feet VERTICALLY not length, will produce all the power you will need for an efficiently run house. Why, because it runs full out 24/7. Not a sunny day here or a windy day there.

Sorry if that offends you. Not meant that way. I just don't like seeing people swindled.

And Fox News, you mean the people who at least tell half the truth as opposed to the rest that tell about 10%? Not a fan of any of them.
There is a great deal more to say on this solar thing like where to buy panels at wholesale, 6v verses 12v bats, how to get 6v bats cheap, etc. and I'll be part of that discussion any time as well. But let's keep it real.


Jeffrey Ihara said:

Evidently it doesn't take much to trigger a FoxNews-flavored tirade out of people these days. But I have no desire to derail the direction of this discussion thread and no intention of deep-sixing my experiment with solar technology.

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