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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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lots of problems... lower the voltage raise the current draw!

battery voltage less than 10 really hurts the battery and related equipment. Sorry for the bad news. I also found that my solar charger did not deep cycle my batteries.

Another thing you could try is you don't need for your pump to run all night long. Run it from 6am to 6pm. Keep it hooked up to the controller, what ever you do. Like Terry said, it will ruin your batteries if you don't. Perhaps another  used battery would be available as well? Just make sure it's similar to what you currently have (agm to agm, etc).

Eh, I figured as much. The real world hardly ever conforms with the theoretically sensible. But aren't deep-cycle batteries made to be discharged to much lower voltages? 

So at 6V the pump would be drawing more current than at 12V? What if the pump's specifications say that 1.6 amps is the maximum? If there is a 1.6 amp max on the pump, wouldn't it just pump slower as the wattage went down with voltage of the battery?

There are units out on the mkt called power plants fancy inverters battery bank at minimum in series 2 I prefer 4 with auto switch over is the best

If ever you decide to have a backyard Off the Grid Grow Room Delivered ready to go contact me

Heather--good thought. I ran my pump during the day-only while I was cycling, and the plants were fine with that. But now I have stocked with catfish, who eat like pigs at sundown. Peak metabolism/O2 consumption hits at around 6 hours post-feed, so I have been reluctant to turn off their circulating water at night.

As for adding more batteries to the bank, the way I see it this would only give me more amp hours, while what I need is more watts from the solar panels. If I put enough photovoltaics in, then I could use a larger battery bank to store the extra energy once my watt-hours produced > watt-hours consumed.

Thanks for the quick help!!.

deep cycle batteries means that they can be discharged to about 50% (NO MORE THAN THAT) without any issues. If it goes below that, the battery will fail to hold a good charge. Your pump would pump slower as the watts go down but you will also ruin your pump that way as well. I have a bilge pump running for 15 minutes each hour (12 hours the sun is up). very cheap, very good pumps. Just an idea for you.

Heather--my beds are on bell siphons so they need constant flow. I know I could convert to timed flood and drain, but that's not an option that is immediately attractive. The other question is whether or not the intermittent flow will be adequate for my fish tank. I figured I was cutting things at the low end by having just under one full volume of water change per hour.

But apart from the extra wear and tear on my pump, what I'm hearing is that if I could somehow re-set the cutoff in my controller from 11V to 7V (still above 50% discharge), I should be okay?

STOP !!!  50% duty cycle is NOT 6 volts, its 50% of the stored energy in the battery! Ex. 12 volt battery with a 120 amphour charge can be safely used down to 12 volts with 60 amphour charge.  You are going to need a battery load tester or a hydrometer to check the weight of the water in your batteries

I'm reading the voltage from the charge controller display, and it drops when I turn the pump on, and it continues to drop as the battery runs the pump. Before turning on the pump it's at 13.1V, then it drops to 12.4V with the pump on. Overnight the voltage will drop by about three or four tenths of a volt, and the next morning it may be down in the high elevens, only to come back up when the sun starts shining on the solar panels. The charge controller is set to shut off devices drawing from the panels/battery when the voltage drops to 11V.

If I'm getting this right, the voltage I'm seeing does not reflect the amount of stored energy left in the battery, which will always remain at around 12 volts, and I need a different device to tell how low I have run my battery down.

Will something like this work?

I keep a 1-amp charger on my batteries all the time just to ensure that I have a full charge in case it is needed.  The charger comes on automatically at night and during cloudy days.  Solar can be a tough go - short day charging is much different from long days.  Short, cloudy days. . . well, you know.  I'm using three 195 watt panels and two batteries.  

Ill have to agree with switching to some kind of timed flood and drain and/or running only partial days. I havent seen too many people with setups running 24/7 off grid that make a whole lot of sense expense wise for what is really needed. Of course some applications may be more reliable with 24/7 pumping.

You do not necessarily need to change over all your plumbing. Drill a small 1/4 hole in the base of your standpipe or bulkhead if you can. You can let the auto/bell siphons work as they do now. When the pumps cut off the beds will slowly drain anyway. As you point out though you are cycling your water once and hour now so on a timed floor you may need a bigger pump. Hard to say if that will solve your problem or not but usually 24/7 aeration + a timed pump is drawing less power then pumping 24/7. 

Here's what I'm not following. George has three 195W panels, for a total of 585 watts per hour. If the draw from my pump is 480 watts per day, then if I had George's panels, I'd get what I need from less than a single hour of sunshine per day, and given the amount of sunshine I have in southern California, that would seem like overkill, even in the winter.

Divide 585W by 12V give him 48.75 amps of current per hour from his solar panels, so this should fully restore a bank of 2 120 amp-hour batteries that have discharged to 50% in less than 2.5 hours of sunshine. Is this correct? 

Sorry if I'm so clueless--I actually have a PhD in biology, but I have never tried to understand electricity until now.

My problem with switching to timed flood/drain is not a plumbing issue. I already have a small drain hole in a second long standpipe I put into my beds to make for easy and complete draining--just pull the pipe out and the bed empties--this was my idea (and I like it). If I switched to timed flood and drain with 15 on/45 off I would need four times the gallons per hour relative to what I have now, just to maintain a single water change per hour. My set-up won't support this. I pump from the sump to the fish tank, and from there solids lifting overflow moves water/fish waste to my grow beds. In order to get 4X the flow out of the fish tank, I would need to raise the level of the tank higher than where it is now, and it's already close to the top of my IBC. Besides this, having a bigger pump running intermittently wouldn't be saving me much in the way of energy consumption unless I were to be cutting back substantially on the amount of water being moved.

Until I get more panels, I'll just put my system on grid power every third day, allowing my battery to charge. This whole solar thing is a (kind of expensive) experiment anyways. I figure I would be saving a total of maybe $30 per year if I were to get the whole thing off grid(which I haven't), and it'll take more than a lifetime to have this recover the costs of the panels, batteries, pumps, and various accessories I'll be needing to buy in the meantime.


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