How much battery

WilisWilis Solar Expert Posts: 84 ✭✭✭✭
Hi,

I will be helping to install a solar system in Nicaragua in a couple of months. I know I am going at this backwards not knowing the load I will have but at this time the equipment is running on a 8k gen. 6 hours a day. Do not have any way to get amp values from equipment until we get down there. The gen is not being used at a very high amp rate. We are powering a .5k radio transmitter. The transmitter, a small AC, 3 lights and 2 computers. I read the posts and it makes sense but when I try to do the math I have question. We want to decrease the gen time because of fuel cost. I was asked what size AGM AH of battery we could charge with the PV’s we have?

30-Canadian Solar 230w PV. Pmax 230w, Vmp 29.8v, Imp 7.71a
48v system
3 FM 60 CC
XW 60-48-120/240
I would like to think we will have 4 sun hours a day. North/East side of country and not sure of sun.)
Would like to not discharge lower then 75% of battery.
I do not know the total load, but do have an 8k gen that will be used for backup.

What size of AGM battery bank would fit with this many PV’s?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: How much battery

    Ideally, you really want to be sure of your loads... If you assume the "wrong loads" or needs, you can end up with something very expensive, or something pretty expensive that does not meet your needs...

    Some rules of thumb... Size your battery bank for 3 days of "no sun/generator" and 50% maximum discharge--or 6x your daily use. It is not wrong to have more or less batteries--but you should have reasons for something different (something like short term use--year or two; or just don't have the funds up front; etc.).

    Generator sizing--if it is diesel genset, you should probably be operating at ~60-75% of rating for long life (lightly loaded engine carbons up, rings stick, cylinder walls glaze, etc.). Also, most gensets will use 50% fuel flow at 50% to 0% load--so lightly loaded gensets can "waste" fuel too.

    For solar panels, a good rule of thumb is roughly 5%-13% of the battery bank's 20 Hour Rating (in Amp*Hours). Less than 5%, undercharging the battery bank (and poor equalization--although with AGM, much less of an issue)--and over ~13% you can overheat the battery bank (again, with AGM, they can take more charging current). In the end, a good place to start.

    So, we know some "points" in your system (how many solar panels you want, how big a genset you have, and you are looking at AGM's.

    First, AGMs are a wonderful battery but are sensitive to over charging. And usually 2x or more the cost of equivalent flooded cell batteries. If you can keep the batteries well maintained (charge wise)--AGMs will be great. And if you have problems getting distilled/filtered rain water (for flooded cell battery electrolyte upkeep)--they may solve that issue too.

    So, using the Solar Array as your sizing tool (again, rough rule of thumb (5% and 13% rate of charge with a 77% derating factor for solar panel and charge controller losses on a 48 volt bank):
    • 30 panels * 230 watts * 1./0.05 battery capacity * 0.77 panel/charger losses * 1/48 volt bank =2,200 AH @ 48 volt maximum
    • 30 panels * 230 watts * 1./0.13 battery capacity * 0.77 panel/charger losses * 1/48 volt bank =850 AH @ 48 volt minimum
    Once you have the batteries as the "heart" of your system, you can look at how you supply the loads (DC and/or inverter); and how you recharge the batteries at the end of the day.

    For the Genset--assume 5% to 35% rate of charge (AGM's can take a higher charge rate):
    • 2,200 AH * 58 volts charging * 0.05 charge rate * 1/0.80% charger eff = 8,000 watts
    • 2,200 AH * 58 volts charging * 0.35 charge rate * 1/0.80% charger eff = 56,000 watts
    • 850 AH * 58 volts charging * 0.05 charge rate * 1/0.80% charger eff = 3,100 watts
    • 850 AH * 58 volts charging * 0.35 charge rate * 1/0.80% charger eff = 21,600 watts
    The above numbers give you a good idea of the range of charging choices/battery bank options. Yours will probably fall somewhere in between.

    You are running a balancing act... The generator has a minimum fuel efficient/maintenance efficient operating range and if you use the genset to just charge your battery bank, you will have battery, battery charger, and inverter losses.

    If you have known, "fixed loads", like the A/C and Radio--You might choose to run the Genset during those times (assuming 4+kW minimum load) and use the battery bank for off-peak loads (evening lights, tv/radio, computers.

    However, because you are looking at a whole bunch of solar panels -- you can probably supply much of your loads straight from the PV array.

    Depending on sun, you can roughly assume that your Array+charger+battery+inverter will be around 61% efficient (AGM batteries) or 52% efficient with flooded cell batteries. Assume 5 hours of "full sun (on average):
    • 30 panels * 230 watts * 5 hours of sun * 0.61 derating = 21,000 Watt*Hours per day
    Assume a 2k Watt A/C, a 1 kWatt radio, and 1,000 watts for computers+lighting+fans, etc.:
    • 21,000 Watt*Hours / (2,000 watt A/C + 1,000 watt radio + 1,000 Watt misc) = 5.25 hours per day
    Pretty close to your 6 hours per day you currently run the genset... And if your power usage is much less (I probably guessed way high) and/or you conserve (less A/C,, more insulation, laptop computers, florescent lights, etc.) -- you can probably cut way back on the power usage and use only solar for much of your needs (bad weather needing genset charging/AC Backup).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much battery

    Red flags:
    8kW generator. That's a lot of power even when "not being used at a very high amp".
    "small AC". No such thing. Even the 'small' ones are megapower consumers.
    2 computers? What type? desktop units suck up power alarmingly.

    On the whole, if the equipment is using 50% of that generator's capacity that's still a lot of power as far as solar is concerned.

    Basic math example (no losses included, nominal figures):

    4kW * 6hrs = 24kW/hrs per day. Or eight times what I use at the cabin.

    Calculating your battery bank 'backwards' is always a bad thing to do. But you have 6900 Watts of solar panel! You could possibly harvest 20 kW/hrs per day with that. Possibly not; lots of variables with losses due to temperature (higher temps = lower panel output) orientation and wiring.

    6900 Watts could provide 100 Amps @ 48 Volts (roughly speaking) which could charge a 1000 Amp/hr battery bank. Pardon the caveats and guesswork, but ...
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: How much battery

    in knowing your loads, a better idea would be to use a meter like the killawatt. this would account for variables in use and times used then everybody might be able to comment with better facts. try averaging it over a week to account for off days.
  • WilisWilis Solar Expert Posts: 84 ✭✭✭✭
    Re: How much battery

    Thanks again to everyone for the info. It is just what I was looking for. Like many projects I have entered this one in the middle. The equipment listed is already in a building in TN ready to be loaded into a container. Everything is there but the batteries. The people will be using the solar have just enough knowledge to push a button to make the radio station work. Things are set up once a quarter and run until someone comes from the States to change it. Computers are nice that way. No way to contact them unless someone goes down, very remote area, or they go 75 Km to town to use a computer. That is why I want AGM batteries. Flooded ones would be to much work to keep working. Once the solar system is working someone can go down every quarter to see how it is doing. Not a nice way, but hopefully will work. Gen is a diesel and I know it most likely is not run hard enough. Power and station is run only in day time. Small batteries and few lights are used at night. This system will save many gal. of diesel fuel. Hopefully the gen will not be started for many weeks. Thanks again to all.
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: How much battery

    One thing I would add is a Battery Monitor.

    To measure battery capacity you usually use a temperature corrected Hydrometer to measure specific gravity of the electrolyte--but you can't because of sealed AGM's. Or use an accurate digital voltmeter with temperature compensation chart (will work OK--but bank should sit ~3 hours with no load/charging currents to stabilize for accurate readings.

    A good quality Battery Monitor will allow you to program in the bank size (AH) and read the percentage state of charge (0-100%) from the digital meter. Some Battery Monitors have remote temperature sensors also.

    The instructions become pretty easy... Tell them to watch the meter... If the State of Charge drops below 50% and there is no sun (or unusual heavy loads)--then crank up the genset. Once the state of charge is above 80-90%, turn of the genset. Once a month or so, make sure the batteries get 100% charged (AGM's can take a "mild equalization" and the battery monitor will generally reset back to 100% when it sees the "batteries are full").

    The battery monitors are not ideal (because the batteries are not ideal)--but it will give you some very basic instructions that they local users can follow and you should have good battery life and lower fuel usage.

    One note, if you have big AC loads (air conditioning, water pumping, etc.), you might save some fuel by powering them directly from the generator instead of going through charging the battery and through the inverter to the heavy AC loads (no conversion losses)--If it looks like the solar panels cannot keep up with the daily loads (such as a hot summer season).

    By the way, get Remote Battery Temperature Sensors for the OutBack Charge controllers too (you need one RBTS + a mate and cables--I think--I am not the Outback guru here). AGMs are very sensitive to over voltage (too much charging) and hot batteries need their charging voltage reduced to prevent overcharging.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,244 ✭✭✭✭
    Re: How much battery

    I would be pretty scared to leave a system such as this for ~3 months unattended until you have a pretty good idea of whether or not it is up to the job! A fairly good chanc of having some very expensive lead weights at the end of a couple of weeks, much less a 1/4 of the year!

    I would tend to agree with Marc ('coot) on this. If you are running a Diesel ~4 kw*6=24kwh.

    A battery based system, in easy to calculate terms would have to be in the order of 12 kw to provide that same 24 kwh. 12,000*.53*4 hours=25,440 wh or 25.4 kwh. Even if you figure more than 4 hours, say 8 hours, you are still in the 6 kw range, (assuming no clouds/rain etc)

    As is so often said, (and you recognize) do the math, and calculate your loads before you buy any hardware.

    Good luck,

    Tony
  • myocardiamyocardia Solar Expert Posts: 118 ✭✭✭
    Re: How much battery
    Calculating your battery bank 'backwards' is always a bad thing to do. But you have 6900 Watts of solar panel! You could possibly harvest 20 kW/hrs per day with that. Possibly not; lots of variables with losses due to temperature (higher temps = lower panel output) orientation and wiring.

    Marc, you seem to have forgotten that Nicaragua is at least 500 times closer to the equator than British Columbia happens to be.:D As you can see from this South American insolation table, their yearly low in December of 5.05 kWh/m²/day has to be about as high your highest monthly average/year. They should be harvesting at least 25 kWh/day during the summer, if not more, I would think. That's assuming the panels are oriented properly, of course.
    DoD= depth of discharge= amount removed from that battery   SoC= state of charge= amount remaining in that battery
    So, 0% DoD= 100% SoC, 25% DoD= 75% SoC, 50% DoD= 50% SoC, 75% DoD= 25% SoC, 100% DoD= 0% SoC
    A/C= air conditioning AC= alternating current (what comes from the outlets in your home) DC= direct current (what batteries & solar panels use)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much battery
    myocardia wrote: »
    Marc, you seem to have forgotten that Nicaragua is at least 500 times closer to the equator than British Columbia happens to be.:D As you can see from this South American insolation table, their yearly low in December of 5.05 kWh/m²/day has to be about as high your highest monthly average/year. They should be harvesting at least 25 kWh/day during the summer, if not more, I would think. That's assuming the panels are oriented properly, of course.

    No, I just included the fact that Nicaragua is also a lot hotter than British Columbia; as the heat goes up, the panel efficiency goes down. Batteries aren't going to like those high temps either.

    Robbie Burns said it best: "The best laid plans of mice and men (and solar installers) are apt to go astray."
    :p
  • icarusicarus Solar Expert Posts: 5,244 ✭✭✭✭
    Re: How much battery

    Not to mention,that the humidity and therefore haze is is likely to have some detrimental effect on PV performance. Additionally, one must factor in (my perception) tropical rain forest cloud coverage.

    Tony
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