Mammoth .015kW PV System <grin>

IntheswampIntheswamp Registered Users Posts: 16 ✭✭
edited January 2022 in Solar Beginners Corner #1
Ok, first post.  Name's Ed...I'm down in south Alabama.  Have some light experience with solar from years ago.  I like to tinker.  And, well, maybe I should've said 15 watt system...kinda of a miniature those little ponies. :wink:  

I've got a question regarding battery sulfation from not being discharged heavily.  I live in an area with a *really* high deer population in it.  I've also got a vegetable garden.  These two don't mix well...unless they're on the dinner table together, but I digress.  I have an electric fence around the garden powered by a Wallyworld group 27 deep cycle(ish) battery...~100aH.  The battery will power the fence good for probably 30 days, then so-so for another 15-20 days.  I contacted the manufacturer (Parmak) and was told that the fence charger "uses 1 amp a day".  I'm taking that as one amp hour daily which kinda makes sense from what I've seen in the lengths of the discharge cycles.  Then I got to thinking...  :D

I ain't gettin' any younger, the miles have been rough, and hauling that battery back and forth to the garage for charging was getting old.  I remembered that I had some small surplus pv panels I bought probably twenty years ago for a project that never happened.  They're small, probably around twelve or fourteen inches square.  They look like they were salvaged from old road-construction signs or something.  Hooking one to my jeep using a cheap chinese SCC it was putting out roughly 1 amp to the battery.  Open circuit voltage, as I recall, was around 18'ish volts.  So, I ordered a little Huine 20amp waterproof controller from Amazond and used some 12/2UF wire (planing on burying it later) to hook the battery to the controller.  The setup seems to be keeping the Group-27 battery topped off nicely.  I plant on mounting the panel to the top of a nearby wooden post, this is just temporary mount.

I've been watching the electrolyte level and there's been no change in the levels.  The battery shows 12.8v regularly when the sun isn't shining much...cloudy, in the evenings, early mornings, etc.,.  Using my high-tech 4-ball hydrometer it shows a 100% charge.  And, lastly the meter on the fence charger itself is staying pegged out...just right for Bambi to come sniffing!

My question it working *too good*???  The battery is not getting discharged much at all...during the day the pv panel is apparently more than enough to replace any power being used so the only discharging happens over night, which isn't much.  My concern is that without heavier discharging am I inviting sulfation to the battery?  Would I be better to put some type of load on the battery periodically to cause a deeper discharge?  The Wallyworld battery has worked great and will be going into it's third (I think) gardening year.  I just don't want to hasten it's demise by doing the wrong thing.   Thoughts?   Thanks!  Ed


  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Welcome to the forum Ed,

    In general, sulfation is the result of a partially discharged lead acid battery (typically 75% or less state of charge) sitting for days/weeks/months... It is the conversion of "fluffy gray lead sulfate" into a hard black crystalline lead sulfate. And once conversion to the hard black crystal, that sulfur+lead no longer is part of the charge/discharge cycle--And the crystal can (partially) insulate the lead battery plates and increase internal resistance.

    Keeping a FLA battery >~75% SoC will help ensure a longer life.

    FLA batteries "do like being cycled"--And for folks with off grid homes/battery backed systems... Some folks will "turn off" battery charger for a day to "force" a discharge to ~75% or less SoC, then recharge once a month.

    For off grid applications, we typically suggest 2 days of storage and 50% planned max discharge. Or 4x the daily load.

    Your system is a very small load and a large (cheap) FLA battery... If you want to (for example) disconnect the solar charger for a week every few months--It may increase the life of the battery--But it may be hard to see--Your battery is larger than needed, and you may need to lose 75% of capacity (sulfation, grid corrosion, etc.) before you experience any issues with your very small fence charger load.

    Regarding charging... For an off grid application, we suggest 5% minimum rate of charge for weekend/summer system (lightly used). And 10-13% rate of charge for the typical off grid cabin/home. Deep cycle FLA battery mfg. typically suggest 10% rate of charge for a healthy battery bank.

    Obviously, 10% rate of charge on your battery is expensive overkill--Your FLA battery will not even accept 10%+ rate of charge until it is something like ~80% or less SoC.

    For "floating" a FLA battery, typically 1% to 2% rate of charge is enough for long term "storage" to counteract the battery "self discharge". Add the to the self discharge 1 Amp*Hour per day charging for your load (I agree, 1 amp load * 24 hours per day = 24 AH or take your battery dead in 4 days or math wise: 100 AH capacity / 24 AH per day = ~4 days to dead).

    Some basic math... Charging the battery based on 2% rate of charge minimum:
    • 100 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.02 rate of charge = 38 Watt "minimum" float solar panel
    And then based on estimated "loads":
    • 100 AH * 12 volts * 0.02 rate of self discharge = 24 Watt*Hours per day self discharge (worst case old/dying FLA battery)
    • 1 AH per day * 12 volts = 12 WH per day load
    • 24+12=36WH per day "loads"
    Then based on your location and hours of sun per day for winter, fixed array, facing south, based on average winter hours of sun per day:

    Dothan Alabama
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 44° angle from vertical:
    (Optimal winter settings)

    • 36 WH per day * 1/0.61 off grid DC system eff * 1/3.89 hours per day (January average) = 15 Watt array "minimum" January "break even"
    Somewhere between 15 Watts and 38 Watt solar array is justifiable for your application (with solar, tending towards the 38 Watt panel is "better" for longer battery life--In general and better supports a week of "bad weather", etc.).

    Your thoughts/comments?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    I should add--With a DMM (digital multimeter), you should see around 13.6 volts (float, middle of sunny day) to 14.4-14.8 volts (charging middle of day)...

    Having to add distilled water every 1-6 months is typical for deep cycle FLA batteries. Adding water more often than once a month is usually "over charging" (too high of charging voltage)--And never having to add water can be "undercharging"...

    Part of the question--Is this a true deep cycle FLA battery (they "use water") vs an SLI (starting lighting ignition) "automotive battery" which have additives to the plates to greatly reduce water usage (most of us never need to add water to our car batteries during their 3+ years of life).

    Deep cycle batteries are designed to dischage down towards 50% or less state of charge... "Car batteries" are designed to cycle down towards 85% state of charge--"Deep cycling" a car battery, it will not last very long (100's of cycles vs 1,000's of cycles for Deep Cycle type).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    edited January 2022 #4
    Thanks for the welcome, Bill.  And for the great feedback!  My first comment, all that cipherin's got my head hurtin'!!!!!!!<grin>  My second comment is that I've got to study over all this a while!!!  I'm a bit "OCD about somethings and this seems to be suckering me in.<g>

    I have some background in batteries, not in AE, but in auto and light commercial.  But, I never got into the math and had no formal education.  We would do a hydrometer test, check open-circuit voltage, put a load on the meter and look for any cells deep-frying or smoking....sometimes even sniffing to see if we detected a shorted cell (sniffing that hydrogen gas was probably what gave me my third ear).  I started tinkering with batteries at my father's business back when commercial batteries still had tar sealing the cells in place and external straps, probably early late 60's /early70's.  But, never had any real education regarding batteries but apparently the knowledge my father brought to the table (schooled at "Cohens Electrical School" in Chicago and ended up teaching there for a while) helped us build a pretty good battery business along with tires and household goods...think DIY "Western Auto Store"...refrigerators, washers, Timex watches, batteries, tires, pocket knives, etc.,.  He started out with Exide and BFGoodrich brands and we ended with Douglas Batteries out of Newnam, GA.   A whole different life back then.  Anyhoo,...

    The battery I have is a Walmart part # "27DC", "Deep Cycle" 109aH.  The date code says it's two years old.  I'm not sure it's really a true  heavy-plated deep cycle but it seems to hold a charge well with the light, steady load of the fence charger.  I think probably that the next step up from these FLA "marine" batteries would be golf cart batteries...???   It does have removable caps.  I've only been running the panel on less than two weeks now, so not really enough time to judge electrolyte loss. 

    I just braved the cold light rain that's falling and checked the battery with my multi-meter....with the fence-charger disconnected the meter showed a reading of 12.7v...This was @ 45F on a slate gray, gloomy, overcast afternoon at 1:40pm.  

    I'm happy with the fence-charger pegging the needle all of the time so rather than letting it dip down over several days maybe do a more strenuous discharge over a shorter period of time rather than disconnecting the solar panel for a week?  That would get the battery discharged and charged back up quicker and would be a bit deeper discharge.  I've got an old low/high-beam headlight I pulled from an old Dodge old 6024.  Specs says it draws a tad over five amps on high-beam (both filaments).  I'm thinking of using it to discharge the 27DC.  What do you think of hooking the high-beam up and letting it run for four hours maybe once a month or every two months...worth the trouble?  Probably pick a time when the forecast is for clear skies the next few days. 
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    You are very welcome Ed.

    You probably should look at a better Hydrometer--If you want to get a better handle on the FLA batteries: (nice temperature compensated);sprefix=battery+hydrom,aps,523&amp;ref=nb_sb_ss_ts-doa-p_1_14 (lots of others--Typical glass tube and glass float models)

    There is monitoring cell specific gravity (typically around 1.265 SG units for "full charge"--Does vary between mfg. and battery type). And EQ charging (controlled over charging of battery to bring cell SG into balance--More or less less than 0.030 SG units between low and high cells in bank). EQ can be done "when needed"--Tall cased batteries tend to suggest EQ once a month to mix electrolyte.

    To be honest--Your loads are so small compared to battery capacity (daily loads are 1/100th of battery AH capacity), it is probably not worth doing much in battery operation (loading once a month, partial discharge, testing, etc.)... Just a solar charger that "keeps up" with battery charging and use the battery until if fails (increase panel wattage if you want to give you a bit more headroom in winter).

    The battery could probably be 90% loss of capacity / dead and still work for your application. With solar charging, a true deep cycle battery is probably not going to last better than a "cheap" SLI type battery given your shallow discharge.

    Some battery FAQs:

    The only other major issue I can think of is Lightning... Big issue in your area? Probably running a 6 AWG wire from battery negative to an eight foot ground rod is about all that is cost effective.

    There are some nice surge suppressors--But not cheap (vs you having to replace charge controller and fence charger for once in a lifetime nearby strike?).

    Might be nice to install a suppressor in your home's main circuit breaker panel (with ground rod/grounding to cold water pipe/etc.) (Alabama seems to be a pretty active region for lightning).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    edited January 2022 #6
    Thanks again for the feedback, Bill.  I've considered picking up a better hydrometer but for now and for what I'm doing the little 4-baller's aged out so no air-bubbles sticking to the floats. If I go for an upgrade it'll probably be the glass float type...I've used the pivoting needle ones in the past and a little "iffy" for me.  I think you're right about not needing to pay "ICU Room" attention to the battery and the little pv panel and SCC seems to be working well.  I just didn't want to be damaging the battery by keeping it peaked out charge-wise all the time.  I may hook the old headlight/lamp up every so often just to see how the solar panel setup actually works in charging a semi-discharged battery up...the kid tinkerer in me.<g>

    So far I've been fortunate with lightning, the fence has been up and running since 2017 and no strikes on it.  (No issues at the house in 30+ years.)  We *do* have a good bit of lightning at times, though, and the thunderstorms can get quiet interesting.  The fence charger is connected to an 8' ground rod so I may take your suggestion and run a wire from battery negative to the ground rod.  The garden is 82'x50' with five strands of polywire around roughly a 3.1 joule charger energizing  a quarter of a mile of fence....kinda concentrating a  conductor into a small area, eh?  And, on a hill, too. ;)  

    I think I'll let the tiny system run for a while and see how the pv setup works out.  I guess my big question was whether the battery would be harmed any by always being in a topped-off charged condition without at least a notable charge/discharge cycle occasionally to knock some of the sulfates that might form loose.  It appears that with the size of the battery, the very light load, and the small current of the panel that I've got a very light-duty trickle charger that borders on being a float charger.  We'll see what happens! :)

    I'm curious about something.  Do you know if ASW sold Xantrex/Trace power panels back around 2003?
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    I do not know about their selling of power panels back 20 years ago. I do not work for NAWS (Northern Arizona Wind & Sun). I am just a volunteer here.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    Thanks for the info, Bill.  I really appreciate your feedback on my fence charger setup.  I've got some larger projects possibly down the road and I'm sure I'll be hollering back *and* referencing your above calculations and info....very helpful!
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    I just thought I’d post a follow up on the electric fence PV system. It has worked great!  The battery stays fully charged, the electrolyte levels are good and steady, and most important of all...the fence charger’s meter needle stays pegged all the time...just right to turn a curious Bambi on her butt!!!!  :D
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Good to hear your fence charger system is working well... What size panel did you settle on?

    Take care
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    I'm still using the small surplus panel that I started out with.  :)  

    I really do need to check the open circuit voltage of the panel again.  I don't know much about solar, probably just enough to exhibit my ignorance of it.  ;) 

    The panel has 64 cells.  From what I've read 64 cells would make it a nominal 24-volt panel...???  I got two of these panels probably 20 years ago and it was surplus at that point...maybe age has "adjusted" (read that as "deteriorated") it to do good 12-volt charging?  Does that make sense or is it even possible? 

    The orange rubber gasket material around the edges makes me thing it came from a road construction sign or maybe some type of telemetry device.  Not sure if it's mono or poly, either.  And the lighter and darker cells? my uneducated eyes they almost look like a  mix of poly and mono cells...color-wise.

    Here's a closer look at the cells....

  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    I guess those could possibly be half-cells???
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Looks like a 32 cell panel for Vmp~18 volts to me (conductor down middle of cell, not "splitting" a cell in two).

    Should be perfect for you needs. (at least voltage wise)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    Thanks Bill!  I had racked my little pea-brain trying to figure this panel out!  Once you mentioned the conductor running down the middle of the cells the fog mystically lifted and I could see the circuit!!!   Makes sense now!  The next good sunny day I'll try to get a good open circuit volt reading.
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    You are very welcome Ed,

    You should see around 21-23 volts or so open circuit voltage (just meter connected and some direct sunlight).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    Well, I only saw between 17.95v and 18.07v at 1PM under mostly clear skies.  I say "mostly" because somebody's apparently burning some timber off somehwere and there is a barely detectable haze in the sky.  At the time, my Davis VP2 weather station was reporting around 890 W/m2.  The weather station will be nine years old in a couple of months so I'm sure how accurate the solar sensor is now.
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Generally, using a DMM (digital multi-meter), they draw very little current... Such that Voc (measured with DMM) will be pretty much the "factory" Voc reading (very little current--Something like 1 MOhm meter resitance), very loading/voltage drop on cells.

    HOWEVER, solar panel Voc and Vmp are temperature dependent... A crystalline solar cells will drop (roughly) -0.33% for every Degree C over 25C (77F). A panel in full sun, no breeze, hot day--You can see almost a 20% (typical) worst case voltage drop... Example:
    • -0.33% per C * (80C panel - 25C) = -18% drop
    • 22 Voc * -0.18 = 3.96 volt Voc drop
    • 22 Voc * 0.82 =18 volt Voc with very hot panel (full sun, no breeze, etc.).
    I would guess under whatever your ambient temperature is... Not quite as much drop -- But getting close to your ~18 Volt Voc (older panels, 20% output loss due to sun damage after 20 years, etc...) that could be expected.

    The "test conditions" is typically 1,000 Watts per sq meter "your 890 w/m^2 is not far off)--But in any case, as long as you have even weak sun on the panel--Voc is going to be (mostly) temperature (and possible age) dependent.

    Vmp (actual) will be much ore dependent on W/m^2 and cell temperatures.

    In the end, what is the charging current in the middle of the day to your battery. Is it keeping the battery >~13.6 volts (float) to >14.2 (charging)? If so--Good enough. If you do not see those numbers, then possibly the panel is not as helpful as a new panel would be.

    Isc is = W/m^2 ... You can measure Isc for the panel and see what you get (i.e., 0.89 kW/m^2 * x.x amps Isc = "measured" Isc in actual conditions).... And Imp-rated is something like 75% of Isc rating...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    Voltage reading at battery was 13.4v with panel connected.  Highest amperage that I could get at the positive wire going into the battery was 0.32a.  Temperature was very close to 65F.  I'll have to soak up some of the latter part of your post. ;)

    Thanks for the feedback, Bill!
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    • 0.32 Amp * 17.5 typical Vmp * 1/0.890 kW/m^2) =6.2 Watt rated panel (roughly, or a bit more)
    A current Vmp-17 volt @ 0.32 amp @ 5 Watt panel:

    • Dimensions: 9.84" x 8.74" x 0.71"
    • 9.84*8.74= 86 sq inches
    How big is your panel?

    Battery voltage/state of charge/charge controller settings and defaults can limit charging current from panel (i.e., panel can do "more", but battery and/or charge controller are limiting).

    You could connect your panel directly to battery and measure current (take charge controller out of the loop) during mid-day sun and see what the charging current is (closer to "actual" Imp without other confounding issues).

    Give you an idea of the "condition" of your present panel....

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IntheswampIntheswamp Registered Users Posts: 16 ✭✭
    I'll have to get some measurements on the panel, it's around 12"x12" maybe even 14"x14".  The cells are bonded to a plate of aluminum.  Before I connected the panel up to charge the fence battery I tested it by hooking it up to my jeep's battery, using a cheap, plastic ROC-manufactured controller that had a LCD reported roughly 1amp in good sun but I won't vouch for its accuracy.  I'll test the amps again going into the fence battery and leave the SCC out of the will be next week probably before I can being as rain/clouds have moved in for several days...gray, raining, and gloomy outside.  I'm thinking the 0.32 amp reading may have been due to the resistance of the battery being fully charged....I've gotten meter readings of ~0.70a in the past at the fence charger battery.

    I've got another of these panels that I could put in parallel with this one.  But, it seems to be doing the job so "if it ain't broke, don't fix it". :wink:

  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Add long as the battery is "happy" (specific gravity of electrolyte and or good battery voltage overnight), you should be good.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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