Will this panel/battery setup work for DC fence charger?

MO-NH
MO-NH Registered Users Posts: 9 ✭
in General Solar Power Topics #1
Hi all,
I'm fairly new to solar power and have a question about an electric fence charger that I want to run off a solar panel and battery.

Here are the details:

Solar panel: Coleman 18 watt amorphous solar panel with 7amp charge controller.
Battery: small 12 volt 8 amp hour battery. http://www.cabelas.com/product/700551.uts?WT.tsrc=CRR&WT.mc_id=crrdtfd
Fence charger: Zareba B10M http://www.zarebasystems.com/store/electric-fence-chargers/b10m

This would be used to charge a quarter acre hog enclosure from early May to late September. We're located in northern NH.

So, what do you think? Seems to me the solar powered fence chargers that I've seen are pretty small so the battery inside can't be very big, and the solar panels seem pretty tiny on them... so I'm thinking these components I already have might work. Will this work? :)

I do have a spare regular sized 12 volt, 90 amp hour trolling motor battery that I could also use if a bigger battery is a must.
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Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Will this panel/battery setup work for DC fence charger?

    welcome,
    seems ok to me, but i'm not all that familiar with electric fences. during long cloudy periods you may want to check on it to be sure it is still charging the battery high enough. if you find it to be low then you could sub a ups battery there and charge the fence battery with a small charger. if it's an agm then be careful of the voltage going too high. (usually around 14.4v or so)
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
    Re: Will this panel/battery setup work for DC fence charger?
    MO-NH wrote: »
    Solar panel: Coleman 18 watt amorphous solar panel with 7amp charge controller.
    <snip>
    I do have a spare regular sized 12 volt, 90 amp hour trolling motor battery that I could also use if a bigger battery is a must.

    Welcome to the forum. My first solar fence charger system (back in the 80's) was for pigs. I ruined a few batteries back then, but that was when batteries were cheap and solar panels were expensive.

    You will definitely need more panel, and maybe a bigger charge controller if you use the bigger battery. Can't say for sure without knowing what the fence charger draws. Your charge controller can handle a 100 watt panel. Your 90 AH battery needs at least a 65 watt panel just to get it charged. I would aim for the 100 watt panel because you don't know what the load is.

    btw, an outdoor battery in northern NH, even in summer, really wants a charge controller with temperature compensation.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • MO-NH
    MO-NH Registered Users Posts: 9 ✭
    #4
    Re: Will this panel/battery setup work for DC fence charger?
    vtmaps wrote: »
    btw, an outdoor battery in northern NH, even in summer, really wants a charge controller with temperature compensation.
    Thanks for the info. Anyone recommend a place where I can find a particularly good buy on one of these type of charge controllers?

    vtmaps wrote: »
    You will definitely need more panel, and maybe a bigger charge controller if you use the bigger battery. Can't say for sure without knowing what the fence charger draws. Your charge controller can handle a 100 watt panel. Your 90 AH battery needs at least a 65 watt panel just to get it charged. I would aim for the 100 watt panel because you don't know what the load is.
    Thanks.

    As for what the fence charger draws, all I could find is this on the zareba website:
    6 or 12 volt battery with pulsed DC output (1-second intervals). Output can be doubled with 12 volt battery. Battery not included; battery life: 12 volt approximately 3 to 4 weeks; 6 volt approximately 4 to 5 weeks. Automatic 6 to 12 volt battery switching. Output voltage no load is 8600 volts.



    Can anyone help me understand why the self contained solar fence chargers on the market have such tiny (5 or 10 watts, maybe?) panels, and the internal 6v batteries that they sell for replacement are only 10 amp hours. How can these possibly function as advertised considering what I need to run this small DC charger?

    I've read that these DC chargers run 2 to 3 weeks on a fully charged 12 volt 75 amp hour marine battery. Where am I going wrong in my math calculating that an 18 amp panel could recharge one of these batteries over a 2 to 3 week period?

    I still don't have a grasp on the math for this solar stuff :p - anyone have any idea how long it would take to recharge the small 8 amp hour 12 volt battery if it were run down to 50% capacity? And does anyone know roughly how long it would take the above DC fence charger to run that 8 amp hour battery down to 50%?

    Thanks for the patience 'splaining this stuff to my pedestrian mind. :-) :blush: 8)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Will this panel/battery setup work for DC fence charger?

    You are getting into the "solar mess" where there are multiple rules involved for long battery life. A few of them are:
    • Don't let Lead Acid batteries set for long periods partially charged--Batteries sulfate faster. My recommendation is to to keep your battery between 75% and 90%+ state of charge.
    • Lead acid batteries taken below 20% state of charge (taken to "near dead") can be permanently damaged (or at least have lost a big chunk of their useful life).
    • Batteries should have some sort of charge controller when panel charging amps is greater than ~1% of battery AH capacity to prevent over charging (and venting for sealed type batteries).
    • Lead Acid batteries need a minimum rate of charge. We use 5% around here for multiple reasons (of 20 Hour capacity. A 100 AH battery should have 5 amp minimum charge as an example). So, large batteries need larger panels.
    • Load needs solar panel of minimum size. Say your fence charger uses 1 watts of power 24x7, you assume 50% charging losses (conservative), and you have a minimum of 3 hours of "noon time equivalent" sun per day:
      • 1 watt * 24 hours per day * 1/0.50 system eff * 1/3 hours of sun per day = 16 watt panel
    • A 16 watt panel at 5% rate of charge would support a 12 volt battery of:
      • 16 watt panel * 1/14.5 volt charging * 0.77 panel+charger losses * 1/0.05 rate of charge = 17 AH battery bank
    • If you install a "big/over-sized" battery then you are "forced" to use a larger solar panel. Say a 90 AH 12 volt storage battery:
      • 100 AH * 14.5 volts * 1/0.77 panel derating * 0.05 rate of charge = 94 watt panel/array
      • Note that this is assuming you are using 25% of the battery capacity every day and need to fully recharge. In this case you are using 2% or less of battery capacity and can use a much smaller solar panel (maybe even 20 watt or less panel, especially if using low self discharge batteries like AGM).

    In the end, our rules of thumb assume you are "deeply" cycling the battery by 25% to 50% every day... And in this setup, the fence charger is drawing very tiny amounts of power.

    So, we are left with two options. A really tiny battery with tiny solar panel and a few days to a week of stored power.

    Or a really large battery bank where the fence charger load is basically nothing, and the solar panel is sized as a battery maintainer--Mostly just keeping the battery from self discharging (self discharge being a "larger load" than the fence charger--if the battery is big enough).

    AGM batteries have lower self discharge, so can last longer without sun and/or with a smaller solar panel vs flooded cell (AGMs can go 3-6 months in storage, flooded cell about 1 month before they should be put on a charger and brought back to full charge).

    And, when the panel is larger than roughly 1% of battery AH rating, then you need to add a solar charge controller to reduce the chances of over charging the battery. And batteries are temperature sensitive--So that affects the required charging voltage too.

    So--we are back to the original question--what is the current/power requirement of the charge controller. A 10 watt panel would imply:
    • 10 watts * 0.61 eff * 1/14.5 volts * 4 hours of sun = 1.7 AH per day
    • 1.7 AH per day / 24 hours per day = 0.07 amps
    • 0.07 amps * 12.7 volts = 0.89 watt average load

    So, all of the "fudge factors" (3 hours of sun, 0.61 vs 0.50 charging eff, minimum 5% rate of charge, etc.) are all things we do here to have a very long battery life and to ensure that the power does not "go out" during bad weather, etc.

    A 10 watt panel is probably a bit on the small size (engineer speaking here). A 16 watt panel with a 17 AH AGM/GEL battery with a simple battery charger will probably keep you happy and last for 2-3 years--Then just replace the "cheap battery" and you are good good for another few years.

    wind-sun_2220_13994345SunGuard 4.5 Amp 12 Volt Solar Charge Controller

    wind-sun_2221_27935951Morningstar SunKeeper Panel Mount Controller 6 Amp



    -Bill "we over-think things here" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • MO-NH
    MO-NH Registered Users Posts: 9 ✭
    #6
    Re: Will this panel/battery setup work for DC fence charger?

    Thank you, Bill. That info. was awesome. :D
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