Sizing a system for electric fencing?

[PACrofter]

I'd like to power a fence charger and a few lights in the barn, which doesn't currently have power. I'd also like to force myself to learn more about solar. I've done some reading on the subject but it's just not clicking.

What I'd like to do is power this charger with a solar setup:

http://www.valleyfarmsupply.net/prod...hp?prod=G39600

The specifications sheet says it takes 1200 milliamps at full power. Is it the case, then, that I can use the formula W = A * V to see that a 12-volt system would translate to 14.4 watts (1.2 amps X 12 volts = 14.4 watts)?

If this is right, presumably this is not drawing power all the time, since the charger pulses (I think) once per second. How do I translate this to the total amount of power that I'd have to have available to the charger?

I realize that setting up a solar system may seem like overkill, but this is also a learning experience for me so I'm willing to invest in it.

Any thoughts or advice would be appreciated!

[Jim-mi]

Do two systems.
Get a 'solar' fence unit and be done with it............

[PACrofter]

Originally Posted by Jim-mi

Do two systems.
Get a 'solar' fence unit and be done with it............

Is this supposed to be helpful?

I guess I should have specified that I was looking for "thoughtful advice".

[12vman]

Sounds like a great "Git Yer Feet Wet" project!

How many lights do you plan to install? Will the fencer, battery and lighting be in the same area/building?

[Jim-mi]

Yes it is .. . . . .sorry you don't see it that way............

[WisJim]

The easy thing to do for a fencer is as Jim-mi suggests and get a solar powered fencer that is designed as a system. If you want to use the referenced fencer, you will need to get the actual power consumption of the fencer, either from the company or by measuring it in use. If it actually uses a constant 1200 ma (roughly 15 watts, as you calculated) that is 15 x 24 hrs or 360 watt-hours per day. Where I am in western Wisconsin, 100 watts of PVs will produce about 400 watt-hours a day (my 1.5kw PVs produce an average of 6kW-hrs a day, averaged over the year, not including battery and inverter losses).

So, a 100 watt panel might be enough if there were no battery losses. To be on the safe side, I would get a 200+ watt panel, a charge controller, and a pair of golf car type batteries such as the T-105, and be able to run a light on occasion, too. If you got a charge controller bigger than minimum needed, you would be able to expand in the future by simply adding more panels, one at a time if wanted. T105 batteries are 6 volt 220 amp hour if I recall correctly, equally 2640 watt hours (12 volts times 220 amp hours for a pair in series), and using 20% of your battery capacity gives you a safe load of 528 amp-hrs each day. No sun for a day? your batteries will discharge more, shortening the life of the batteries. You don't want to regularly discharge lead acid batteries below 80% of a full charge (in other words, you don't want to use more than 20% of the capacity of the batteries on a regular basis).

Jim

[PACrofter]

Thanks, WisJim. I thought I got the math right but if the basic principles were off, the math would be irrelevant! I appreciate the explanation as well.

I would definitely like to expand in the future, so I'll go for a controller that's got spare capacity. As 12vman points out, this is really a 'getting my feet wet' process. At this point I figure I'll need to have a few shop lights, but I'd go for LEDs to minimize the draw.

The back of the barn is almost perfectly aligned to the south, and there's an overhang on the back with an angle that makes it just about right to mount flat on the roof of the overhang - we're at 40 degrees north latitude and the overhang is about a 50 degree tilt. I can build a battery box on the inside of the barn within feet of the panels to minimize voltage drop, and the charger would go on the wall within a few feet of that. Sure beats running a line from the house.

One question for you, based on the information you provided: why do you recommend going no lower than 80% charge on the batteries? I've seen elsewhere that you shouldn't go below 50% charge. Is there some sort of 'sweet spot' at around 80% charge?

Thanks!

[TnAndy]

Originally Posted by PACrofter

Is there some sort of 'sweet spot' at around 80% charge?

Thanks!

All batteries have a certain number of DOD ( depth of discharge ) cycles.

At 20% discharge, it may be 1500 ( for example )....at 50%, it will cut considerably.....maybe to 800....and at 80%, maybe 300.

Doesn't matter WHAT the actual rating is ( and that is statistical average from a large number of them tested....your mileage may vary ), the point is: The deeper you discharge, the less number of times you can do so, and thus, get closer to the day you have to replace them.

Different folks drive their cars differently, and use their batteries differently as well.

It's also a balancing act between how large a bank you want to install versus how much DOD you're willing to accept. Also, you have to factor in with the size of the bank, how much of your solar input is simply going to kept up the self discharge of the batteries.....make your bank TOO big, and it may take too much of your system output just to maintain batteries.....and when you DO discharge too large a bank, you may fail to top them off properly before you start discharging them again, thus keeping the batteries in a permanent state of low charge, contributing to early failure.

It's always something...

[PACrofter]

Thanks, TnAndy - that's helpful. I guess I'll need to find my own balance between paying up-front for the batteries or setting up a longer-term system. Hmmm...sounds like I have some homework to do.

Thanks!

[12vman]

Figure your total load(s) in a 24 hr. period. Add 20% for safe measure. Multiply by 4 for battery sizing. Stay constant with your loads and you won't discharge any less than 20-25%.

Same for panel wattage. Figure your load(s), add 20% and divide by 4 or the amount of prime charge hours in your area.

[mightybooboo]

Originally Posted by Jim-mi

Do two systems.
Get a 'solar' fence unit and be done with it............

That is INDEED excellent advice,go look at solar fence chargers,prices reasonable and you can see the specs.My solar fence charger 20 years ago was less than 100 bucks and did 1.25 acres like a champ.And still works to this day.And powerful enough to do much more,its my lot size that dictated what I did with it.

So as Jim said,its the way to go for fencing,then do your other solar as you want/need. I LOVE the solar fence on its own system for price,quality and product life.