What do I need?

concretefireconcretefire Solar Expert Posts: 25
Attachment not found.Hello,

I just got one of these as a rain barrel for my garden. (overkill perhaps, but that's me :D) Anyway, the opening at the top is exactly 5 and 1/2 inches in diameter. I was thinking of putting a small pump in there and running it off of a solar cell and battery....(?????)

This would be my VERY FIRST attempt at anything solar. I wouldn't be watering at night of course. Just during the day with a regular hose.

Am I over thinking this? It has a regular hose end adapter on the bottom but I'm thinking I would have to raise it up higher than the garden to let gravity do the work. I HAVE A RAISED GARDEN!! So I'm thinking about going with a simple, small pump and having it be powered with a simple, small solar panel. Thoughts??? Please....
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Comments

  • ywhicywhic Solar Expert Posts: 621 ✭✭
    Re: What do I need?

    I have access to the same tanks.. so I'm all ears on this also.

    I was looking at a 2-3 GPH shurflo in 110v flavor (0.9 amp max).. and then there is the 2-3 GPH in 12v flavor (7 amps max) as well.. granted mine would be for a cabin.. a pressure switch and pre-pressurized accumulator tank for the pump will usually make the pump last longer..
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    Unfortunately, the answer is complicated...

    If you where pumping from a pond/well into the tank, then a solar panel + linear current booster (if needed) + DC water pump would be fine.

    However, if you are trying to pressurize a hose to use a nozzle for spraying, etc..., then a battery less system might not be the best solution.

    More or less, solar panels have a relatively fixed maximum voltage and a current that is directly proportional to the amount of sunlight hitting the panel.

    With "opportunity" pumping, you size the panels and pump to operate the pump near full power at noon, and when there is less sun, the pump turns slower and moves less water (using less electrical power). A linear current booster converts that (relatively) high voltage of the solar array and low current into the lower voltage/higher current needed by the motor to turn (a bit like a MPPT charge controller down converts high voltage/low current from a solar array to low(er) voltage/high(er) current for the battery bank (think multiple speed transmission from car engine to rear wheels).

    However, for sprinkler type systems (not sure about drip irrigation), you want a fairly constant water pressure so you water the entire area evenly rather than dribbling a lot of water near the sprinkler head.

    It is still possible if you only water around mid-day, that a large(r) solar panel plus a DC pump (might need the linear current booster) might work well enough for your needs. Also, depending on how much water you need--you could "replace" the battery with a pressure/bladder tank to store water when the sun is up and then have a reasonable amount of pressure/volume of water for your needs--But that is a lot of if's and how much water.

    A decent sized array, charge controller, and battery setup with a DC pump would probably be better in the end (and think of everything you are learning ;)).

    So--my two cents on the subject.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    yeah, I was just thinking a solar panel, a charge controller, and a battery setup with a dc pump honestly.......it would be 'neat' to go out late afternoon and just water knowing the power is coming from the battery. Also knowing there is a good chance it won't need watering again for 3 days or so should give the battery time to refill.

    And Bill, all my watering needs are less than 50 feet . 90% less than 10 feet. So I don't need some super soaker pump. So what pump would you go with? And do they make them 5 and 1/2 inches in diameter or smaller so I can fit it in the tank? Thanks.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: What do I need?

    Hey Concretefire, interesting project in an area with which I have some experience.
    Many years ago, when playing around with home built wind turbines, I had a couple of garden watering projects on the go. The first was a simple slow speed "windmill" that operated a crank, which in turn, through a connecting rod, stroked a piston type boat bilge pump that had been intended for hand operation. Pumped from the nearby lake to the garden. Worked great as long as the wind was blowing. The next was a larger electricity producing wind turbine that directly powered a 1/4 hp DC motor that ran a gear pump to fill a 45 gal barrel located high up in the barn loft. It switched on and off as necessary to keep the barrel full, and the garden was watered gravity feed from the barrel. The garden didn't really need to be watered, I was just playing around to see what I could do.
    This summer, with the extended and ongoing drought, my garden really did need watering. So I dug out an old piston pump like the one shown in my avatar photo except it was single cylinder (piston) instead of a twin cylinder, dusted it off and used it, powered through inverter from the solar system to pump water a couple hundred feet up over the hill from the brook to the garden. No sprayers were used as that would produce back pressure which would load down the 1/4 hp "110 volt" AC motor, thus requiring more electricity to run - - - and on those sunny HOT days, I wanted enough power left over to run the mini-split in AC mode at the same time, while also of course run two freezers and a fridge. All of which happened throughout the day.
    Using the Kill-A-Watt, I was shocked to find that with no back pressure, that old piston pump only sucked back about 165 watts! As soon as I held my thumb over the end of the hose of course, the wattage shot up. Since my garden is on a slight side hill, I just made a small trench an inch or so deep, down between two garden rows and let the hose pour it's water in the top of the trench half the day, then moved it down to water the lower half the rest of the day. Over several days, the garden had a thorough and complete deep watering. No back pressure required, so the pump just idled along using very little electricity in the process. Moral of the story - - don't use a sprayer if you don't need to. An open end hose with no back pressure is far more efficient at transferring water than having to build up pressure and spray that same water up in the air so it can fall back down on the garden.
    Good luck with you're project.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    I am probably not the best person to tell you what pump is best... But I can ask questions and make general suggestions.

    So, volume, pressure, and how many minutes/hours per day?

    As Wayne says--keeping pressure low will save a lot of energy.

    Pump choices... If you can get an external pump and feed it from below the water line, you will have the least amount of problems. In water pumps tend to be more expensive and prone to leaks. External pumps that "lift" water on the inlet pipe have problems with cavitation (drawing a vacuum causes small bubbles to form, interfering with pumping and causing internal damage to pump parts)--And it is difficult to put a particle filter on the inlet (more restriction on flow--some pumps will tolerate a bit of sand/debris, others will not at all).

    You can start by reading the specifications for the various models of DC powered Sureflow pumps and see what would meet your needs (probably less expensive).

    Also, a simple RV water pump may be good enough to experiment with too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    Lol, c'mon Bill, you're killin' me. Ok, I'm fine with it having no pressure. I do rather like the idea of having a battery and solar panel (obviously) that I could turn on and off. Something simple. You flip the switch, the pump starts..... you water your plants that are literally less than 20 feet from you and then when you're done you flip the switch off and the battery begins to charge again the next day, or that day. I DO NOT WANT to count down the minutes until it's high noon and sunny and run out and try to squeeze every last bit of DC energy from the solar cell. .......Bill, you know I'm struggling with this solar thing anyway, and I figure this would be my way of getting my feet wet in solar on a very low tech , low power project. I do like your idea of the pump being outside the container. Please respond. Based on the link you gave me, and someone held a gun to your head, which one would you pick and why? Then we'll worry about the solar and battery needs to accomplish this. The other solar stuff I've been talking about is way off in the future. THIS PROJECT I WANT DONE PRONTO!!.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: What do I need?
    THIS PROJECT I WANT DONE PRONTO!!.

    Then use an AC pump, or a DC pump with an AC/DC converter. In general solar is not amenable to "buy everything first, put it together then figure it all out later." Most of the work has to be done up front, and there's a fair amount of it.

    So get the pump put in, run it on AC power (or DC with a converter) and you'll be able to get it done pronto. SHURflo is a good brand for positive displacement pumps, and March makes some good centrifugal AC pumps. Then when you have the time get a kill-a-watt meter, figure out how much power you need and add the solar/battery system then.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    Just to add information--Here is a website with March Pumps (specs and pricing). Know nothing about the website/dealer.

    If you can draw water from the bottom of the tank, with a screen/filter and/or floating pickup (keep pickup out of the dirt).

    The basic Sureflo has a pressure switch--so a valve on the end of the hose will allow you/spouse/kids to water just as if it was from a pressurized water system.

    To make designing simpler... Start step by step.

    First, what pump will meet your needs. Say we pick this 12 volt Sureflo model that does 2.56 gallons per minute @ 7 amps.

    And you need 50 gallons per day to water.
    • 50 gallons / 2.56 GPM = 19.5 minutes ~ 20 minutes of runtime per day
    • 7 amps * 20 min / 60 min per hour = 2.33 AH per day

    Battery bank needed to support that for 3 days of no sun and 50% maximum battery discharge (for longer battery life):
    • 2.33 AH * 3 days no sun * 1/0.50 maximum discharge = 14 AH @ 12 volt battery bank

    However, I would recommend for a lead acid storage battery a maximum of C/8 discharge rate:
    • 7 amp load * 8 hour discharge rate = 56 AH @ 12 volt minimum battery bank (based on load)

    So, unless you choose an AGM battery, you should use a 56 AH @ 12 volt battery bank.
    • Next we need to size the solar array. There are two calculations. First is the solar array based on a 5% to 13% rate of charge. And second based on load power used and hours of sun per day.

    First, based on 5% to 13% rate of charge (note, larger batteries usually demand larger arrays for proper charging):
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 53 Watt minimum array
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 105 Watt nominal array
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 137 Watt "maximum cost effective" array

    And second based on amount of power used per day. Using PV Watts for Charlotte N.C. with fixed array tilted to latitude above horizontal:
    Month    Solar Radiation
    (kWh/m 2/day)
    1      4.04     
    2      4.46     
    3      5.30     
    4      5.89     
    5      5.79     
    6      5.77     
    7      5.55     
    8      5.59     
    9      5.26     
    10      5.23     
    11      4.25     
    12      3.74     
    Year      5.07
    

    Assuming that you will pump for 9 months a year, the lowest month is February with 4.46 hours of "noontime equivalent" sun:
    • 2.33 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 1/0.80 battery eff * 1/4.46 hours of sun per day = 12.3 Watt array minimum

    So, based on the larger battery bank to support the pump load, the minimum/nominal array size is larger than required for "simply" moving the water.

    As a hint, think of the Battery as the "heart" of your power system... Loads size the battery bank. Then the battery bank sizes the solar array (along with hours of sun per day). Always trying for a "balanced" system.

    In this case, the system will be much larger because of the "high" pump current--So you could power other loads too very easily. Say a ~135 watt panel:
    • 135 watts * 0.52 system efficiency (with AC inverter) * 4.46 hours of sun minimum = 313 Watt*Hours per day of AC power (February)
    • 135 watts * 1/12.5 volts battery * 0.77 panel+charger eff * 0.80 batt eff * 4.46 hours of sun minimum = 29.7 Amp*Hours per day of 12 VDC power (February)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tmarchtmarch Solar Expert Posts: 143 ✭✭
    Re: What do I need?

    Just getting into solar, I'd recommend forgetting the battery and just go with a simple solar pond pump. Lots less to get confused about and less expensive. Set your pump up with a valve that will run a hose when it's needed and a bypass when it's not needed so it just circulates the water back into the tank. You can find these pump kits with a small panel at greenhouse supply stores or on line. You can mount the panel on a pole so it can be rotated to face the sun at whatever time you want to water.
  • TheBackRoadsTheBackRoads Solar Expert Posts: 274 ✭✭✭✭✭✭
    Re: What do I need?

    It wouldn't be THAT difficult to wire up a KD140 panel to a single 55AH battery either.... but I'd rather be able to water when I want vs during the day (at work, cant just let run all day). just my $.02
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    Thank you Bill. That certainly cleared some things up. At least math wise. I believe what you were saying there at the end is that it wouldn't make much sense to get the bare minimum panel when the battery bank (even at the minimum just to pump water) is capable of holding so much more power. May as well get a larger panel and make more use out of the battery. Correct?
  • PhotowhitPhotowhit Solar Expert Posts: 5,365 ✭✭✭✭✭
    Re: What do I need?

    If your 'pushing water' let gravity be your friend and place the water tank as high as possible, 4x4's will hold a lot in compression, just be sure to brace them well, and give them firm footing. If your collecting from a roof 12' high if you get the tank up as high as 7-9 feet, that may well be all the pressure you need, depending on the lay of the land.

    Bilge pumps, are typically 'trash' tolerant to some degree, DC, submersible, often cheap and easy to find ratings since small boats often carry small batteries, doubt they would tolerate any back pressure...

    ...don't ever name your sailboat 'shark bait' it makes calling in for a tow interesting, "This is the sailing vessel sharkbait requesting a tow..."
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    Sort of--I really was just taking guesses about the amount of water that you will pump from that 150-200 gallon (??) water tank assuming it holds 3-4 days of water for you.

    I don't know your garden's water needs and 50 gallons may be way less than required...

    Anyway--Yes, such a system would be capable of quite a bit more power than my first pass estimate, which may be a lot more interesting to you as emergency/experimental solar power.

    There are other options--You could get a 14 AH AGM battery and run with the smaller panel (7 Amps/14 AH = C/2 discharge rate... Which is certainly possible with many AGM battery). But pushing an AGM at that discharge rate--probably will not last more than 1-3 years.

    If you like the single ~135 watt panel, then you could size the battery bank to (12 volts):
    • 135 watts * 1/14.5 volts charging * 0.77 panel+charger derating * 1/0.05 rate of charge = 143 AH @ 12 volts maximum battery recommended
    • 135 watts * 1/14.5 volts charging * 0.77 panel+charger derating * 1/0.10 rate of charge = 71 AH @ 12 volts nominal battery recommended
    • 135 watts * 1/14.5 volts charging * 0.77 panel+charger derating * 1/0.13 rate of charge = 55 AH @ 12 volts minimum battery recommended

    I would suggest you look at the MorningStar 300 Watts TSW 12 volt inverter. It is really hard to beat. It has a low power/standby "search mode" (checks for >6 watt AC load every second) and remote power on/off (just use a small switch to turn on/off the inverter).

    A ~143 AH @ 12 volt battery bank running a C/8 discharge rate would support:
    • 143 AH * 12.5 volt battery bus * 0.85 inverter efficiency * 1/8 rate of discharge = 190 watt average load

    The battery will support that for a bit less than 4 hours (to 50% discharge).

    And:
    • 135 watts * 0.52 system efficiency (with AC inverter) * 4.46 hours of sun minimum = 313 Watt*Hours per day of AC power (February)
    • ~313 WH per day (Feb) / 190 watt AC load = 1.65 hours per day (Feb)
    • 135 watts * 0.52 system efficiency (with AC inverter) * 5.55 hours of sun (July) = 390 Watt*Hours per day of AC power (July)
    • ~390 WH per day (July) / 190 watt AC load = 2.05 hours per day (July)

    If you "like" the system and want more energy per day, you can add a second 135 watt panel and ~double the daily production with the same battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    For ease of conversation, I will just thank 'everyone' for their ideas. Believe me, I have evaluated them. I am not comfortable putting this monster tank up on stilts or 4x4's simply because of the residential area I live in. If I was out in the country.....believe me, way ahead of ya'll on gravity fed water supply. I believe (will have to confirm) that for every 2 foot in drop of height, you gain 1 psi in water pressure. Or maybe it's the other way around but I know that's kind of on the right track. Anyway, since Bill seems to be going above and beyond in trying to help me...I just wanted to let you know Bill that I am digesting what you have said. Saturday, I will be moving the tank to it's final location and I will take pictures and upload them here as an update. I will also take some pictures of the garden to give you a better idea of how big it is and how much water I need and thus how long I would need the pump to run....etc. In other words, a picture says a lot. I think the first example you gave is probably going to be a little overkill. I think 2.5 gallons per minute is on the strong side for what I need. I also think 50 gallons is a LOT, but, on a very hot or dry period, I could see MAYBE using 50 gallon total for that day....but then not watering again for at least 2 days. My garden really is not that big and to be totally honest, I just think this particular project would be perfect for me to understand the basics, basics of solar. On the other hand, I've looked into those solar pond pump kits and they seem very not sturdy enough for my liking. Bill, I also have not ruled out your DC pump with the booster either. Once I have the pictures up, I think we can narrow it down further and YOU can see what I'm talking about. I sincerely appreciate your help and I hope you continue to aid me because I'm just a noob and I AM learning from you. (and others) I would love to be able to finish this project in this thread so that it may help others......all the way from "help" to "finished." I am determined to do this project and see it thru.
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    Here are the pictuers. Part 1 is complete. Now comes part 2......the solar pump. As you can see, my garden is very small and not far away from the rain barrel. So now that you can see just how little I have to water and how little I have to travel, perhaps it will be easier to understand what my needs are. I just want to be able to water the garden. High Pressure is NOT a requirement. I would like a good flow though...

    Lastly, YES, I understand that adding a solar pump and battery and controller and inverter (maybe all those) is highly NOT EFFICIENT for what I want to do. This isn't about $$. This is about practice and getting my feet wet with solar power. It's practice + just the sheer "neat" factor. So what do you guys think??? Attachment not found.Attachment not found.Attachment not found.Attachment not found.Attachment not found.Attachment not found.
  • Eric LEric L Solar Expert Posts: 262 ✭✭
    Re: What do I need?
    I think 2.5 gallons per minute is on the strong side for what I need. I also think 50 gallons is a LOT, but, on a very hot or dry period, I could see MAYBE using 50 gallon total for that day....but then not watering again for at least 2 days.

    O.k., so a very small pump is likely enough, unless you need a lot of head pressure for some reason.

    How about an inexpensive 12 volt DC submersible (like this, or similar)? Attach a few feet of vinyl tubing to the output with a little hose clamp, and lower it to the bottom of the tank. The water will come out of the top, but the pressure of the water in the tank will mean the only head pressure the pump is working against is from the top of the water level to the top lip of the tank and then down. Connect the end of the vinyl tubing to a small hose or whatever you plan to use for watering. If it isn't enough, you're only out about $25.

    At 12 volts DC, that pump would use about 6 watts to move around 1 gpm, counting a small bit of head. 50 gallons in a day = about one hour (slightly less) x 6 watts = 6 watt hours. If we double that to count for charging inefficiencies, you need about 12 watt hours per day.

    For power, keeping it cheap and cheerful: a small charge controller, a small panel in the 30 watt range, and a very small deep cycle battery (35 amp hours would be plenty). Wind-Sun sells all these things. You'd have to rig up a switch and wire and in-line fuse to connect the pump to the battery. The whole thing wouldn't be much more than $200.

    If you wanted to build for the future, you could buy a larger panel, controller, and battery; as long as it was a 12 volt system you should be able to use these little DC submersibles.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    Will the tank block the window as a fire exit?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tmarchtmarch Solar Expert Posts: 143 ✭✭
    Re: What do I need?

    Ok, a small solar pond pump doesn't impress you. Something to think about, what of the foreign matter (bird poop, sand etc. ) that gets into your rain barrell? I'd a lot rather try a pump designed for that than try to run a pure water transfer pump. Pond pumps will handle a small amount of sand grit etc. I've seen what asphalt shingle material will do to a pump, it's toast in a short while.
  • fix_it_guyfix_it_guy Solar Expert Posts: 26
    Re: What do I need?
    tmarch wrote: »
    Ok, a small solar pond pump doesn't impress you. Something to think about, what of the foreign matter (bird poop, sand etc. ) that gets into your rain barrell? I'd a lot rather try a pump designed for that than try to run a pure water transfer pump. Pond pumps will handle a small amount of sand grit etc. I've seen what asphalt shingle material will do to a pump, it's toast in a short while.

    He does have a "First Flush" hooked up. But the water will still have some debris in it.
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?
    BB. wrote: »
    Just to add information--Here is a website with March Pumps (specs and pricing). Know nothing about the website/dealer.

    If you can draw water from the bottom of the tank, with a screen/filter and/or floating pickup (keep pickup out of the dirt).

    The basic Sureflo has a pressure switch--so a valve on the end of the hose will allow you/spouse/kids to water just as if it was from a pressurized water system.

    To make designing simpler... Start step by step.

    First, what pump will meet your needs. Say we pick this 12 volt Sureflo model that does 2.56 gallons per minute @ 7 amps.

    And you need 50 gallons per day to water.
    • 50 gallons / 2.56 GPM = 19.5 minutes ~ 20 minutes of runtime per day
    • 7 amps * 20 min / 60 min per hour = 2.33 AH per day

    Battery bank needed to support that for 3 days of no sun and 50% maximum battery discharge (for longer battery life):
    • 2.33 AH * 3 days no sun * 1/0.50 maximum discharge = 14 AH @ 12 volt battery bank

    However, I would recommend for a lead acid storage battery a maximum of C/8 discharge rate:
    • 7 amp load * 8 hour discharge rate = 56 AH @ 12 volt minimum battery bank (based on load)

    So, unless you choose an AGM battery, you should use a 56 AH @ 12 volt battery bank.
    • Next we need to size the solar array. There are two calculations. First is the solar array based on a 5% to 13% rate of charge. And second based on load power used and hours of sun per day.

    First, based on 5% to 13% rate of charge (note, larger batteries usually demand larger arrays for proper charging):
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 53 Watt minimum array
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 105 Watt nominal array
    • 56 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 137 Watt "maximum cost effective" array

    And second based on amount of power used per day. Using PV Watts for Charlotte N.C. with fixed array tilted to latitude above horizontal:
    Month    Solar Radiation
    (kWh/m 2/day)
    1      4.04     
    2      4.46     
    3      5.30     
    4      5.89     
    5      5.79     
    6      5.77     
    7      5.55     
    8      5.59     
    9      5.26     
    10      5.23     
    11      4.25     
    12      3.74     
    Year      5.07
    

    Assuming that you will pump for 9 months a year, the lowest month is February with 4.46 hours of "noontime equivalent" sun:
    • 2.33 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 1/0.80 battery eff * 1/4.46 hours of sun per day = 12.3 Watt array minimum

    So, based on the larger battery bank to support the pump load, the minimum/nominal array size is larger than required for "simply" moving the water.

    As a hint, think of the Battery as the "heart" of your power system... Loads size the battery bank. Then the battery bank sizes the solar array (along with hours of sun per day). Always trying for a "balanced" system.

    In this case, the system will be much larger because of the "high" pump current--So you could power other loads too very easily. Say a ~135 watt panel:
    • 135 watts * 0.52 system efficiency (with AC inverter) * 4.46 hours of sun minimum = 313 Watt*Hours per day of AC power (February)
    • 135 watts * 1/12.5 volts battery * 0.77 panel+charger eff * 0.80 batt eff * 4.46 hours of sun minimum = 29.7 Amp*Hours per day of 12 VDC power (February)

    -Bill

    Bill, I have been studying this for several days and I have a few ?'s. I almost have it thanks to this explanation and chart. The hint part really helped. Ok, you stated "•2.33 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 1/0.80 battery eff * 1/4.46 hours of sun per day = 12.3 Watt array minimum"

    Where does the 14.5 volt battery charging come in? I've only seen reference to 12 volt. That 14.5 number is throwing me off. And you spoke of C8 batteries..."However, I would recommend for a lead acid storage battery a maximum of C/8 discharge rate:
    •7 amp load * 8 hour discharge rate = 56 AH @ 12 volt minimum battery bank (based on load)"

    I have read the differences in batteries. Flooded vs dry, AGM, etc...but that C/8 discharge rate is also throwing me off. I don't know what that means. The math, I get. All the math works and I get it. I just don't understand those two things. Thank you.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: What do I need?

    I have read the differences in batteries. Flooded vs dry, AGM, etc...but that C/8 discharge rate is also throwing me off. I don't know what that means. The math, I get. All the math works and I get it. I just don't understand those two things. Thank you.

    The Amp-Hour capacity of a battery at a discharge rate which will discharge it completely in 20 hours is called the 20 hour Amp-Hour rating, and is represented as C or sometimes C(20).

    So a particular FLA battery might have a capacity, C of 120 AH (at the 20 hour rate.)

    Now the amount of charging current you can put in to a battery will be proportional to the size of the battery in AH (the bigger the battery the more current it can accept), so charging rates are expressed as fractions of C. For FLA the minimum rate should be C divided by 20 (C/20) which for our hypothetical battery would be 120/20 = 6 amps.
    A better rate would be C/10, 12 amps. And the highest you should ever go for a deep discharge FLA battery would be C/8, or 15 amps in this case.

    For AGM batteries you can safely charge them at a higher rate (up to C/4, typically) as long as the Charge Controller very carefully monitors both the voltage and the temperature of the batteries to avoid overcharging. If you overcharge an FLA cell, you have to add water and may risk slow deterioration of the plates. If you overcharge an AGM battery you can cause the pressure relief valve to vent out your electrolyte, permanently damaging the battery.
    A Gel battery, on the the third hand, cannot be charged rapidly (more than C/20??) without risking gas bubbles in the gel, which will also permanently reduce the battery capacity.

    Similar things affect discharge, but with different details:

    An FLA cranking battery has thin spongy plates and can deliver lots of current and can recharge quickly. But we do not use those.
    A deep cycle FLA battery with its thick solid plates has a higher internal resistance, so you cannot draw as high a current from it without having too high a voltage drop to operate your load or heating the battery too much. A reasonable upper limit for FLA is C/8, just like for charging. You can draw higher peak current, but the voltage drop may cause an inverter to shut off.
    A reasonable value for AGM is C/4 or better (up to C in some cases, 120 amps in our example). But the charging current cannot go as high.
    A reasonable discharge value for a Gel battery is higher than the charging current limit, but I am not sure what typical numbers are.

    End of lecture. Questions? :-)
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?
    Ok, you stated "•2.33 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 1/0.80 battery eff * 1/4.46 hours of sun per day = 12.3 Watt array minimum"

    Where does the 14.5 volt battery charging come in? I've only seen reference to 12 volt. That 14.5 number is throwing me off.

    The battery "absorb" set point is usually around 14.5 volts for charging (charger in "bulk" stage takes the battery from whatever voltage up to 14.5 volts as the battery is ~80-90% full--supplying as much current as is available). At 14.5 volts (or whatever the set point is), the charger switches from "current limited" to voltage limited charging--here 14.5 volts. The charger will (typically) hold that voltage for ~2-6 hours (many higher end chargers are programmable for absorb time, or have other methods to "determine" when the battery is 100% charged). At that point, the charging current will be around 1-2% of the battery's AH rating (a 50 AH battery will be at ~0.5 to 1.0 amps). The charger then switches over to "float" state and holds the battery at ~13.6 volts or so (keep battery charged, but not boil it dry, and support other loads if they are "on" at the time to keep the battery from discharging).

    Also--Notice we have some "fudge" factors. 0.77 is a derating that takes into account that "hot panels" output less voltage (~0.81 of rated wattage) and ~0.95 efficiency for charge controllers. Note--These are for MPPT charge controllers (Maximum Power Point Tracking). Note that these controllers are "constant power" type controllers Power=V*I; or Vmp*Imp=Vbatt*Ibatt (ignoring losses). So the MPPT controller can take high voltage/low current from the solar array and "efficiently" down converter it to low voltage/high current needed to charge the battery bank.

    PWM controllers work differently--They are just On/Off switches (switching very fast)... Basically as long as Vmp>Vbattery-charging (plus 1-2 volts for wiring/controller drops), the current to the battery bank is Imp (based on how much sun light is hitting the panel). Hi or low Vmp does not really affect the current to the battery (again if Vmp>Vbatt-charging).

    So, the 0.77 derating (panels+charge controller) and using the "worst case" highest battery charging voltage (which is where most people charge at--batteries are typically>75% state of charge--The "simple" formula I use is close enough for a first cut at estimating power transfer from a solar array to the battery bank (assuming proper Vmp-array voltages, correctly configured controllers, warm climate, etc.).

    These "fudge factors" are close enough, and conservative, so that most people will easily meet or even possibly exceed by a few percentage points these numbers on their systems.

    More that happy to discuss in detail. Just ask.
    And you spoke of C8 batteries..."However, I would recommend for a lead acid storage battery a maximum of C/8 discharge rate:

    I am not sure I used "C8" batteries... The normal AH rating for batteries we use is the 20 Hour Rate. Basically, if you have a 100 AH battery and discharge at 5 amps, it will take ~20 hours for the battery to "go dead". This sort of represents the "typical" discharge profile for an off grid home... Roughly, 5 hour per night using 25% of the battery charge--4 nights 'till dead or 20 hours of use over the 4 days. Note that batteries appear to have more capacity if discharged slowly, and less capacity if discharge quickly.

    The actual formulas include fractional powers and log functions (see Peukert factor). It is usually "close enough" to just pretend that the battery has a capacity as defined by the 20 Hour Discharge Rate and wave our hands that if you discharge faster than C/20, the battery will not last "quite as long"--And slower than C/10, the battery will "last a bit longer". AGM's have less "internal resistance" and better chemical reactivity--And the spread between C/100 and C/5 is not as large as it is for flooded cell batteries.

    Anyway, back to C/8--That is the 8 Hour discharge or charge rate. For the typical flooded cell lead acid battery, that is the maximum current in or out from 100% to 0% (discharging) or 0-100% when recharging. If you pump in current faster, the battery bank may overheat and charge less efficiently (and other secondary issues begin to appear)--So, I use this as a recommended "Generic" maximum discharge/charge rate (that is the 13% of the 5-13% or 12.5% in the charging rule of thumb).

    AGM's because they are more electrically and chemically "efficient"--One AGM vendor recommends up to C*4 discharge/charge rates (yes, discharge to dead in 15 minutes). For our "off grid" solar systems, a battery pack that outputs a tremendous amount of current/energy in 15 minutes is not very useful--But it is very nice in UPS systems that just need to keep the computers running 15-30 minutes until they can shut down or the backup genset kicks in. Note that designing wiring to handle C*4 current in a large battery bank is not trivial.

    And that is why I use C/2.5 or C*0.40 as the maximum surge discharge/charge rate for a flooded cell battery (which can have voltage collapse if discharge faster than C/2.5) and for AGM. It is just less typing and confusion for 95% of the people out there. If there is a need for AGM surge capabilities, we can discuss when needed.
    •7 amp load * 8 hour discharge rate = 56 AH @ 12 volt minimum battery bank (based on load)"

    That is just saying, if you have a steady state current on a lead acid battery (charging or discharging), I would recommend limiting it to the C/8 rate (or the battery 20 Hour AH rating should be 8*Load Current) for long battery life and good performance over time/temperature/state of charge.
    I have read the differences in batteries. Flooded vs dry, AGM, etc...but that C/8 discharge rate is also throwing me off. I don't know what that means. The math, I get. All the math works and I get it. I just don't understand those two things. Thank you.

    Does this make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    Bill,

    I have taken your formula and come up with this math: Based on what you said and I have inserted my own numbers as I have thought about it very hard and I think these numbers are closer to what my needs will be:

    First, what pump will meet your needs. Say we pick this 12 volt Sureflo model that does 2.56 gallons per minute @ 7 amps.

    And you need 20 gallons per day to water.
    20 gallons / 2.56 GPM = 7.8 minutes ~ 8 minutes of runtime per day
    •7 amps * 8 min / 60 min per hour = .9333 ~ 1 AH per day

    Battery bank needed to support that for 3 days of no sun and 50% maximum battery discharge (for longer battery life):
    1 AH * 3 days no sun * 1/0.50 maximum discharge = 6 AH @ 12 volt battery bank

    However, I would recommend for a lead acid storage battery a maximum of C/8 discharge rate:
    •7 amp load * 8 hour discharge rate = 56 AH @ 12 volt minimum battery bank (based on load)

    Ok ^^^ That last part about the 56 AH battery and a larger array....I'm not following. I assume my math (with your formula) is correct until we get to the battery part. If I only need what amounts to a 10 AH battery....what's wrong with something like this battery here?

    Also you said unless I was using an AGM battery ........maybe like this one?

    Ok....let's just say for giggles, one of those batteries would work. Now on to the solar array. (your formula)

    First, based on 5% to 13% rate of charge (note, larger batteries usually demand larger arrays for proper charging):
    10 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 9.4 ~ 10 Watt minimum array
    •10 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 18.8 ~ 20 Watt nominal array
    •10 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 24.4 ~ 25 Watt "maximum cost effective" array

    Along with this charge controller

    Assuming that you will pump for 9 months a year, the lowest month is February with 4.46 hours of "noontime equivalent" sun:
    • 1 AH * 14.5 volt battery charging * 1/0.77 panel+charger derating * 1/0.80 battery eff * 1/4.46 hours of sun per day = 5.28 Watt array minimum


    What would be wrong with something like that setup????
    Yes, I know it would be very "tiny" on a solar scale but as I said, it's mostly just for messing around and getting a feel for the math, etc.
    Yes, I assume those batteries I pointed out would only last 1-3 years if I'm lucky but so what? That's pretty cheap prices for me vs 2 6v trojan batteries at 200$+ EACH....

    Anyway, I guess my question is, what immediately jumps off the page here as being "wrong." ?? Thanks. :D
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: What do I need?

    i'm letting bill address you primarilly as he's been following this with you the whole time, but from just reading of what you said in your post i'll give some general speak here.

    you are missing a point here. when the load run times are very low for a sizable load current, you still need a bigger battery to handle the load. even though 1ah of capacity used is small compared to say a 7ah battery, the actual draw is at 7a or 100%+ the capacity of a 7ah battery. the + is an understatement because we're talking of around a half hour rating for the battery or worse and would stress the battery right out of its usable lifespan. the 7ah rating is most likely at a 20hr rate, which is pretty much standard. to get the draw on the c/20 current for a 7ah battery is only 350ma. you might figure that 7a is 20x the draw so just divide the time by 20 for 1hr, but it doesn't work like that due to the peukert effect.

    that is why a bigger battery is used and at least the minimum of 5% of battery capacity delivered to the battery by pvs. if 5% doesn't cover the timeframe of a day to get the charge back into the battery, and that includes a 2hr absorb charge, then higher %s can and should be used. this is not the pv imp rating i'm stating and bill has already addressed stc ratings and efficiencies so i don't need to rehash that.

    if you actually get 4.46hrs of full sun in february don't forget about those pesky cloudy days and the fact that the absorb stage may need a couple hours to complete. one can very easily undersize a system for both batteries and pvs if you're not careful. it is also noteworthy that most have lower hours of insolation during december and january so be sure that february is your worst case scenario.
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    then no, I just don't get it. I even called and spoke to a "David" at Wind-Sun yesterday and he crunched all the numbers for me with the sureflo model in Bill's example at 7amps...... he's the one who suggested it would be a very tiny setup and I would only need a 10amp battery and he is also the one who recommended the Sunguard 4.5 amp controller.

    So, no, apparently I don't get it and neither do the people who work for wind-sun because even he said I wouldn't need but a 10-20 watt rated panel. So......?????????
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: What do I need?
    then no, I just don't get it. I even called and spoke to a "David" at Wind-Sun yesterday and he crunched all the numbers for me with the sureflo model in Bill's example at 7amps...... he's the one who suggested it would be a very tiny setup and I would only need a 10amp battery and he is also the one who recommended the Sunguard 4.5 amp controller.

    So, no, apparently I don't get it and neither do the people who work for wind-sun because even he said I wouldn't need but a 10-20 watt rated panel. So......?????????

    There are some ways that it might work:

    1. You would have to use an AGM battery to handle both the high discharge and high charging current.
    2. You would let the panels charge the battery until the insolation was near maximum output. Then turn on the pump and have 4.5 amps coming from the panel via the controller and the remaining 2.5 amps coming from the battery. That would be less of a load on the battery.
    3. As soon as the panel output dropped below some threshold, turn the pump off and let the remaining panel-hours recharge the battery.
    4. A 10-20 watt rated panel just would not cut it, since it could not deliver enough current to spare the battery during pump operation, and maybe not even handle the combination of bulk and absorb charging during the available hours.

    It would not be a just-hook-it-up system. There would have to be a custom programmed process controller (not CC) or a computer connection to make it work. You do not want to just let the pump run whenever the battery voltage is above some cutoff point.
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: What do I need?
    then no, I just don't get it. I even called and spoke to a "David" at Wind-Sun yesterday and he crunched all the numbers for me with the sureflo model in Bill's example at 7amps...... he's the one who suggested it would be a very tiny setup and I would only need a 10amp battery and he is also the one who recommended the Sunguard 4.5 amp controller.

    So, no, apparently I don't get it and neither do the people who work for wind-sun because even he said I wouldn't need but a 10-20 watt rated panel. So......?????????

    if david said so then he is the one putting it on the line if it doesn't work as you expect or getting a short battery life. i even have my doubts if the battery will deliver enough voltage with a steep load on it. if you go with it then keep us apprised of how it is all doing. if it fails in a year, or 2 or or 3 or whenever, then let us know. the results will be good info for us all and i'd love to be proved wrong here as it would be a feather in the cap for agms.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    In the end, either system will work. The AGM will allow a smaller system (smaller battery, smaller solar array). And if it meets your needs--Why not.

    The "larger system" was based on flooded cell batteries--and hopefully a longer battery life. And because the system is larger--it has more capabilities to supply other loads in your home (backup power for cell phone, lights, small laptop, etc.). You might even have a problem finding a good quality ~50-60 AmpHour 12 volt flooded cell battery.

    Either will probably meet your short term needs. The AGM will most likely cost less in the short term (less panels, less battery). But you may end up needing a new AGM every 2-3 years. Again, not the end of the world.

    Your choice--Won't hurt my feelings either way. And, who knows, you may end up with a third option based on what you learned here and your updated needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • concretefireconcretefire Solar Expert Posts: 25
    Re: What do I need?

    So, Bill, just to be clear, *MY MATH* Based on your formulas are in fact accurate. Correct? And yes, I am aware that my 'system' would be tiny and would ONLY power that water pump.....AND..I would have to replace the battery every 2yrs or so. I'm fine with that. I'm perfectly fine with that.

    However, it seems there are two posters above you, including a moderator who seem to think I'm missing something and make it sound like it won't work. Based on *MY* math. ?????? Math is math no? So am I truly missing something or am I understanding you to say my math is sound and my logic is sound and that a very small system (as described by my math) would WORK? Because I'm ready to order parts and I have conflicting arguments and both seem valid. So ???? Thank you.
  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin
    Re: What do I need?

    I think you are fine for a small system. And naws certainly knows their products better than I.

    But that is why I always try to do the math and let the reader make their own choices. I certainly do not know what is best.


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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