Grid Backup For PV System

5 Posts
Oct 8, 2008 03:39 pm
Grid Backup For PV System

I'm building a system to run a 30 watt pump and some LED lights for a Koi pond. It needs to run 24/7 or else fish will suffer.

Although we've got fairly high insolation values (4-5) in Southern California, there are times in the winter and spring when the sky is overcast for days, sometimes weeks near the coast. When the batteries are low during these times, it seems to make sense to use a backup charger powered from the grid.

It seems like one way to do this would be to use a Solar Converters Inc. relay board to connect the 120v charger. This would allow setpoints that enable and disable the battery charger when the panels don't produce the energy needed to run the pump.

Any thoughts and ideas would be appreciated, especially if I'm misdirected with this approach. Any equipment recommendations are welcome, but I've already purchased a Morningstar charge controller for the four 65-watt panels that I'm building myself using eBay solar cells.

Regards,

Chris Kreymann

 
Oct 10, 2008 07:37 pm
Re: Grid Backup For PV System

Just out of curiosoty, which variety are you raising? Hybrids? Long finned? Nishikigoi, maybe Tancho?
Can I assume that the 30 watt pump and LED lights is at 120vac? If so...
30 watts/120vac = .23 amps
30 watts/12vdc = 2.5 amps
2.5 amps X 24 hours = 60 amp hours
60 amp hours X 7 days = 420 amp hours
8 - 6 volt 220 amp hour "golf cart" batteries wired as 4 sets of 2 in series/parallel will provide 880 amp hours at 12 vdc. So, even if it were to go for 7 days of power outage, it would only be half "empty" or, still half full. That depends on you.
Of course there is the possible need for a small inverter but it would only change things by about 15%.
All you have to do is come up with a way to recharge after each discharge. If you have the grid just plug in a charger.
http://www.hajiwaru.com/img/jinmengyo.jpg
 
5 Posts
Oct 13, 2008 02:09 pm
Re: Grid Backup For PV System

Hello Thomas,

Thanks for the reply, I really appreciate it.

The Koi aren't anything fancy, they're hybrids from the fish store. My son and I developed an interest and, after much research, I decided we need to start modestly and develop our skills before we accept the responsibility of caring for these fascinating fish. The ones we have (4) are healthy, growing quickly, and producing eggs in the pond.

I understand your load calculations, and appreciate the confirmation of my own which are similar. Just like with the fish, I was thinking of a more modest solar setup on a budget. We're starting with a small but scalable system, so I'm reluctant to invest hundreds of $$ in battery storage capacity yet.

You said "All you have to do is come up with a way to recharge after each discharge. If you have the grid just plug in a charger."

Right now, my design provides for 3 days of storage at a 20% discharge to extend cycle life. That's doable, but I worry about being out of town when the weather isn't solar friendly. That's why I'm interested in some way of "plugging in a charger" from the grid automatically.

So, how can that be done? My first thought is to install a voltage sensor that's set to recognize a sufficient voltage drop across the batteries to close a relay that starts the charger, and then disconnects it when done. Let's assume it's set to a DC battery voltage that represents a 70% discharge rather than the 80% setpoint for the panels.

Someone in this forum also mentioned that they connected a DC charger across the PV panel input terminals on the charge controller. Doesn't make sense to me because the A/C charger will provide more charging current than the panels, thereby defeating the whole purpose.

Maybe I'm overthinking this whole thing and should just build it and then see what happens. Being an engineering type, I'm prone to endless iterations of design work on paper. This can frequently lead to a "disconnect" from the obvious 8-)

Thanks,

Chris

 
220 Posts
Oct 13, 2008 07:23 pm
Re: Grid Backup For PV System


 hi chris,

 a low tech low budget solution might be to use a plug in timer with a battery charger and set it to top off your battery bank every night or every few days (weekly timer) for a few hours. you can watch how things run and adjust the timer accordingly.

good luck with your project, sounds like fun.

do let us know how those home assembled panels turn out?

all the best, dave
 
5 Posts
Oct 13, 2008 08:09 pm
Re: Grid Backup For PV System


 hi chris,

 a low tech low budget solution might be to use a plug in timer with a battery charger and set it to top off your battery bank every night or every few days (weekly timer) for a few hours. you can watch how things run and adjust the timer accordingly.


all the best, dave

Hi Dave,

Thanks much...there it is: the obvious solution. Let's see if I got this right: With a decent 3-stage DC charger on a timer (set for nightime), the charger should sense the condition of the battery bank and switch itself(bulk/boost/float) as needed. That shouldn't require any additional intervening charge controller, because the charger also behaves as a controller (wouldn't ever float, though, because the bats have a 24/7 load). Very cool, thanks!

As for the panels, I see lots of posts that discourage DIY and I agree EXCEPT when you're just getting into the field with an obsession to learn and save money. I'm a hobbiest that builds furniture, audio tube amps, circuit boards, etc. None of this is a paying proposition except when you do it as a hobby in the sanctuary of the workshop in your own garage.

I'll be sure to share my experiences because there's not much on the web for people like me and the stuff I saw was not built for longevity. I'm thinking sheet aluminum, conformal coatings, optical silicone and glass.

Best,

Chris

 
 
220 Posts
Oct 13, 2008 08:41 pm
Re: Grid Backup For PV System

 chris,

 fine business. yes i'm all for poking around with different things, idle hands and all that.. recently i've even had to buy new electronic components just to rob the needed parts for whatever project was at hand! seeing that my local (audio shed) has all but gone out of the parts supply business. so that comment about home built was in no way meant to be flippant. just the opposite to be sure Smiley

  happy hunting, dave kb1mzf
« Last Edit: Oct 13, 2008 09:24 pm by david ames »
 
5 Posts
Oct 13, 2008 09:23 pm
Re: Grid Backup For PV System

dave,

not taken as flippant at all...thanks again for the good idea.

cheers,

Chris
 

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