Solar Charging Rule of Thumb

Discussion in 'Power - Site Power/Batteries/Generators/Solar' started by Econ, Apr 2, 2021.

  1. Econ

    Econ Well-Known Member

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    Assuming average conditions in full sun what is the net amp output effect of a Renogy 100 watt solar panel on a battery? The bottom line after considering any efficiencies or everything else.

    Example. This panel puts out 6 (number used for example) net amps an hour so if your battery is down 24 amps it will take 4 hours to charge it.

    This is a planning number

    TX
     
  2. NMroamer

    NMroamer Well-Known Member

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    Nope there is no rule of thumb when using solar panels. Everything is variable unless they are mounted in such a way to follow the sun across the sky all day. Then there is always the occasional cloud.
     
  3. Anthony Hitchings

    Anthony Hitchings Well-Known Member Silver Supporting Member

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    Aside from the fact that we camp at different latitudes *sun angle) ad different times of year (changes net latitude) most of us do not have pivoting solar-tracking mounts, so we only briefly get full output, if ever. Which is why some folks buy Power Meters (Renogy makes one, ALI is a cheap Chinese version) that you can calibrate - and the meters will tell you how many amp hours are available from your battery bank. I do not yet have such a thing - we don't use enough electricity so I'm not bothered. One you set them up - they seem to work quite well. All energy in and out of the battery MUST go thru the meter's shunt - else your readings will go to heck.

    Where possible we will rotate our Aliner for maximum noon sun after we are at camp - that's where our double-wheeled tongue jack comes in handy. We also set out our portable 100W Renogy panel, - because - "why not?" (the sunshine is free).

    We run 2 Group 27 Flooded Lead Acid batteries. Our only big (relatively speaking) electricity draw is the Engel Frig. We don't use the heater (fan) much, nor the fantastic fan. LED lights are nothing at all. We do run an ipad or laptop for an hour or two each day.
     
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  4. generok

    generok Well-Known Member Gold Supporting Member

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    When I did my solar research, the only rule of thumb I could come up with, and it is very rough is each 100W panel should be able to charge one 12V battery in one day. There are many assumptions, and it's rough. So, when I wanted to run my system which has two 6V golf cart batteries, I went with three 100W Renogy panels. I can't go forever on solar alone, but I do get a pretty good charge each sunny day. My controller shows me how many amps I produce, but it is so variable based on sun, shade, etc, that I can't say I can give an Ah/day value with any reliability.
     
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  5. Anthony Hitchings

    Anthony Hitchings Well-Known Member Silver Supporting Member

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    And of course the real item of interest - is - annoyingly - the Specific Gravity of the battery electrolyte - which is a bit of a nuisance if its inside under the bed as our batteries are (in properly vented battery boxes). I just finished taking the S.G. readings with a real hygrometer, and got 1.29-1.3 S.G at 63.5 deg F. So that's a full charge (its continually on solar, and our panel points south-ish when the Aliner is parked. The battery was showing 13.5 volts (open circuit), likely because the surface charge was still present on the plates. I disconnected the solar system first and then isolated each battery for this testing.
     
  6. Anthony Hitchings

    Anthony Hitchings Well-Known Member Silver Supporting Member

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    The one-day notion seems to reflect getting only half the capacity out of a 100AH battery, which is the rule-of-thumb for draw-down with no battery damage.
     
  7. SteveP

    SteveP Well-Known Member Platinum Supporting Member

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    The charge rate is dependent on the chemistry and condition of the battery. LA batteries don't like to be charged. If you're charging at 6 amps you'll be lucky to add 6 amps capacity to the battery in the first hour. As the battery grudgingly accepts charge it's resistance to charge increases, further slowing down the charge. The last 10% to full charge typically takes more time than the rest of the charge, 50 to 90%.

    The only rule of thumb for solar applications that I have heard or used is that you should have enough battery bank capacity to keep you going for three days without charging, and a solar array big enough to recharge in a full day of GOOD sun. That's usually expressed as 1 watt of solar per amp hour of the battery bank for LAs. Going by this for a weekend camp you shouldn't need solar at all.
     
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  8. Econ

    Econ Well-Known Member

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    @SteveP
    @generok
    @Anthony Hitchings

    Thanks to everybody for their responses.

    Using solar in the Deep South is a nonstarter. The tree canopy is very dense. @jmkay1 is my expert witness as we both camp at a common place. In the SMNP you cant use solar in Elkmont, Smokemont, Cataloochee, cades Cove. Only at Balsam Mountain is the sky open.

    For a couple of years I had this idea on my project list. The only place where there is sunshine in the Deep South are parking lots, new subdivisions, Interstates,trailheads. Its 6 feet from the battery on the tongue to the cargo area of the SUV. Move the battery into SUV at sunrise. Park at the trailhead. Attach a Charge Controller to the battery. Throw 2- 100 watt renogy SPanels I already have on the SUV roof. Route wires thru the sunroof. Hydrogen gas escapes thru sunroof. If I could pick up 24 AH I'm happy.

    I have roof rails on the SUV. Considering hinging at the leading edge and designing a prop stick for angle. 2 side by side panels would fit between the roof rails. Bolt the stand to the roof rails at the beginning of the trip. Park vehicle pointing just west of the sun.
     
  9. SteveP

    SteveP Well-Known Member Platinum Supporting Member

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    Around here, if you want to get rid of something quickly you can leave it visible in a car parked at a trailhead. Otherwise I like your idea with a few changes.

    Work with 2 batteries, one in the back of the SUV and one on the trailer. Wire identical connections on each so that wiring can just be plugged/unplugged for either source. Figure a roadworthy way to hard mount the solar panels as part of the pre-trip prep so that they'd be less likely to walk off. Swap batteries between the SUV and the PUP as needed. This way you don't have to go through the rigamarole twice every day and the battery in the SUV receives the benefit of a full days charge, even if it is in the shade part of that time.

    You might even consider making a jumper to connect the two batteries in parallel at the campsite, so that you don't have to lug that heavy sob around.

    As I said before, the battery in the SUV needs to be secured to reduce risk of acid spill.
     
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  10. Eric Webber

    Eric Webber Well-Known Member Gold Supporting Member

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    recommendation #1: think in terms of watts. It will help with different voltage setups.

    I’d say a decent figure - if you aim for the sun reasonably well, is that you will see about 6 times the nominal capacity in an average day. Sometimes you will see 8 or more times, sometimes just a trickle


    So, you will be pulling about 600wh into your battery per day- about 45ah at 12.8v. Depending on a lot of factors of course.

    But I strongly recommend just thinking of the amps as part of how the power gets there (and affects things like wire gauge) and instead do the math for watt hours of capacity
     
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  11. rabird

    rabird Howdy!

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    I'd use solar tables that give average solar for various locations, time of year, direction.

    Here is what shows for Mont. Alabama.
    Mont.jpg
    Panels are rated based on 1kW of light per sq meter of panel, their output is ~proportional to the amount of light.
    The table gives values of 4-5+ kwh/m2/day average for spring & summer for a flat panel.
    I'd use 4 as factor multiplied by the the panels Isc (~5A for 100 watt)
    That yields ~20AH/day for an average spring/summer day in Montgomery A with one 100 watt flat or horizontal (not shaded/clean/...).
    A battery may or may not accept the full output of a panel at any given time
    Battery capacity would determine how many days without any sun!
     
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  12. jmkay1

    jmkay1 2004 Fleetwood/Coleman Utah Gold Supporting Member

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    I can see where that can work. I considered bringing everything to the visitor center and have a picnic lunch at the car while the battery charges. My biggest fear is something walking away. Umm your getting my brain a thinking though it is still before coffee to take it for what it’s worth. ...If you have a panel that is designed to attach to a roof, perhaps you can make a frame and attach the frame to the crossbars or something on your cars roof. That way it can be secured and able to go a short distance from camp to the trailhead or something. Like you said you can fish the cables in the sunroof of the car. I wouldn’t want to take it on the highway so something that can be removable fairly easy. As an added security measure to use perhaps a kayak lock around it as well. I only bought one panel myself as the one panel has been more than sufficient for me and my usage. When I go to the SMNP it’s usually in the summer so I don’t need the furnace so my battery can last me a long time. Though one of these days I am determined to visit in the fall, that will be my real test.
     
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  13. Econ

    Econ Well-Known Member

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    Happy Easter
    Hope you have a meaningful holiday. Tomorrow i will be attacked by the grand kids and the grand terrorists. Yes, i have grand twins aka terrorists.

    @rabird did it. I was wondering if there was enough return for the time and capital that could be spent hence the "planning number'. 40 AH did it.

    The park, SMNP, emailed us. Elkmont got flooded yesterday but they will reopen with porta potties. Our reservations not cancelled. Going up there with 2 solar panels and ideas to make observations. I have a basic design but not enough time to build it. It will be permanently mounted that can be removed in 5 minutes if you know the code. Hinged on the leading edge. will be rapid erection so you can use for 20 minutes while in the grocery store.

    @jmkay1 DW has a CPAP. Its a hog. See ya'll after the Honda Grand Prix

    Thanks to all
     
  14. Nelson

    Nelson New Member

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    In bright sun, I am happy to get 2 amps out of my 100 watt flexible Renogy panels rated at 5 amps. I am upgrading to 350 watts worth of panels this week. I will have 150 watts on the rear of my a-frame and 200 watts on the front. All day coverage is important. This also helps to counter the problem of shady camping spots. All panels are wired in parallel.

    Get a MPPT solar charge controller rather than the cheaper PWM. From my perspective, they double the amount you get out of your panels. Most MPPT charge controllers also display the current amp hour charge rate. They convert higher voltages coming off the solar panel into extra amps.

    The sad truth: The sun doesn't always shine. Two days in a row of overcast is a disaster unless you have a large battery bank. I got a small propane generator from Home Depot whose sole purpose is to get 10 amp hours into the trailer battery a couple times a year. Those ten amp hours make sure the propane furnace can run to keep my dear wife warm and happy on a cold night. I am not sufficient to keep her warm and happy for some reason.
    https://www.homedepot.com/p/Baja-900-Watt-Propane-Powered-Inverter-Generator-BAI911LP/306283803
     

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