Question on battery siziing

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Mountain man
Mountain man Registered Users Posts: 8 ✭
in Off Grid Solar & Battery Systems #1
Hi guys i am new to this forum and have a question. I have acquired, used, a 5.7kw solar system and would like an opinion on sizing the batteries to go with it. Need someone to sanity check my math.
5700 Watts * 80% efficiency = 4560/48 V = 95 Amps. If i use two strings of 8 450 amp hour batteries that would give me 900 amp hours of storage. The 95 amps puts me just over the magic 10% rule. We average 8kwh/day with normal day to day use. Looking to see how many days i can go before having to fire up the generator or pray to the sun gods. We live in south central colorado mountains. We get 300+ sun days a year and average 5.5 sun hours a day. Is my math correct with the batteries or do i need to lay off the crack pipe?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Question on battery siziing

    Welcome to the forum.

    Let's check the numbers both ways. Starting with the usual efficiency of panels + controller being only 77%.
    5700 Watts * 0.77 = 4389 / 48 Volts nominal should yield about 91 Amps current. Maybe a bit more, maybe a bit less. Have you been able to look at any numbers and see what it really does deliver?

    5700 Watts * 4 hours good sun * 0.52 over-all efficiency = 11.8 kW hours per day, so that's good against your expect 8 kW hours usage.

    900 Amp hour 48 Volt battery bank @ 50% DOD is about 21.6 kW hours, at 25% DOD and allowing for inverter and adjusting to AC: 9.2 kW hours so again your good for 8 kW hours usage.

    By my usual estimations this system would fly. Two days before you'd need the gen, and probably better if you get any kind of sun and can control the usage a bit. :D
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #3
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    Re: Question on battery siziing

    Thanks the the info. The panels are only a couple years old so i would expect them to still be at full output but have not seen any data to support that. The family i am getting from is just looking to get rid of them cuz the person in the family that was into RE died. The rest of them are grid mongers. Curious why you put overall efficiency at 52%? Understand conversion losses etc. We have a small 940 watt system today that drives my computers and in the dead of winter it churns out 850 watts easily. Summer is much less with the heat but is still around 80%.
    So even at a lower efficiency you would estimate 2+ days at normal usage before external charging. Fully understand that during cloudy days we would cut back on energy usage like cooking or watching movies etc. Should probably be able to extend that an extra day.
    Same subject different question: What size generator do you think i would need to charge these as a fall back?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Question on battery siziing

    I think you have the math correct... One point of discussion, generally the last part of the charging voltage is done at ~58 volts or so... So, with an MPPT type charge controller (constant power), that reduces the output current (same power though)--More or less, this is how I do it (conservative side):
    • 5,700 watts * 0.77 panel+controller deratings * 1/58 volts charging = 75.7 Amps "Nominal" peak in non-freezing weather

    So, your battery bank rate of charge turns out to be:
    • 75.7 amps charging / 900 Amp*Hours capacity = 0.084 = 8.4% rate of charge

    Still not a bad number.

    How much AC energy per day assuming 5.5 hours a day of sun:
    • 5,700 Watts * 0.52 end to end system efficiency * 5.5 hours per day of sun = 16,320 WH = 16.3 kWH per day

    Assuming you charge during the day and discharge at night using flooded cell batteries (should be a "conservative" estimate of power generation.

    Your nightly power budget assuming 25% discharge (two days of storage to 50% maximum discharge):
    • 900 AH * 48 volt battery bank * 0.85 inverter efficiency * 0.25 discharge = 9,180 WH = 9.2 kWH per night (average) discharge

    Anyway--Some figures.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: Question on battery siziing

    The default over-all efficiency value for a battery-based system is 52% because of panel average output, controller and wire loss, battery efficiency loss (in and out), inverter consumption, and conversion loss. it adds up. But it can also be improved upon. For instance making use of the power available from panels once the batteries are fully charged makes a big difference (instead of just not harvesting hundreds of potential Watt hours).

    Couple of things about generator sizing. Exactly what is used for charging? Stand-along battery charger or built-in inverter-charger and its power factor, plus known loads being used while charging (detracts from net power into batteries). If you want to charge at around the 10% rate for that large a battery bank you are going to need at least a 5kW gen and some control over the loads.
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
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    Re: Question on battery siziing

    Ok that sounds reasonable. The average draw from the house shouldn't be anywhere near the panel capacity so the extra watts post charge should get used to run the house and not tap the batteries. Odviously if the well pump kicks on while the cooktop is running that is different story :)
    I plan on getting two magnum 4448 inverters and just using the built in charge controller with input from the generator to charge when needed. Charging efficiency is 85% at least according to the manual.
    Currently i have a 6kw/7kw surge generator which sound like it should work. Two mx80s would handle the charging from the panels and would never see the generator.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: Question on battery siziing

    Generator size--The rough range I would suggest is 5% to 13% rate of charge nominal, to 25% maximum rate of charge (with some sort of thermal management/monitoring--batteries can get hot at high charge rates, and their charging voltage falls as the batteries get hot--A charge controller can think the battery is under load and cause the battery to literally boil/melt down/fire).

    So, starting with basic math (note: for 48 volt battery bank @ 900 AH):
    • 900 AH * 0.05 rate of charge = 45 amp minimum
    • 900 AH * 0.10 rate of charge = 90 amp nominal
    • 900 AH * 0.13 rate of charge = 117 amp cost effective maximum
    • 900 AH * 0.25 rate of charge = 225 amp maximum with thermal management

    Now, come the issues about the battery charger... There are a wide variaties of chargers out there (efficiencies, Power Factor, and even the ability to run at full rated power for hours on end--or not). Very roughly, we want to look at the input "efficiency" of the charge controller--Which breaks into traditional "power efficiency" (around 80-90% efficient) and something else which is VA efficiency (around 0.65 to 0.95 Power Factor)...

    VA / Power Factor is a bit more complex... It has to do with the current wave form (spiky vs a nice smooth sine wave, and possible phase shift with respect to voltage).

    Anyway, more or less, Generator/AC Inverter/Wiring/Transformers are rated in VA ("volt amps") and the energy stored in the battery (gasoline motor output, etc.) is rated in Watts.

    So, just to keep the typing/reading to a minimum, say we have two battery chargers, one "standard" efficiency/PF and the second with high(er) efficiency. Comparing the input VA and assuming you run the genset at 75% of rated VA output (better/higher cost gensets can run closer to 100%, less expensive may have problems running at 50% loading). In any case, I would suggest trying to keep the generator >50% loading for best fuel economy (running a genset at 25% or less loading wastes a lot of fuel):

    So, taking 10% rated battery charge (healthy nominal, would recharge a 50% discharge battery in ~7-10 hours of runtime):
    • 400 AH * 58 volts charging * 1/0.80 charger eff * 1/0.65 PF * 0.10 rate of charge * 1/0.75 genset derating = 4,462 VA generator rating (inefficient charger)
    • 400 AH * 58 volts charging * 1/0.90 charger eff * 1/0.95 PF * 0.10 rate of charge * 1/0.75 genset derating = 3,618 VA generator rating (efficient charger)

    If you want to see all of the details/problems trying to find the "optimum charger", there is this thread:

    Question about battery charger selection with EU2000 generator.

    Any way--That is the basic rules of thumbs I would start with.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #8
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    Re: Question on battery siziing

    Checked the manual real quick and it doens't list VA output. Maybe tomorrow i can check the plate on the genny itself and see what it lists there. It does however indicate that 6kw is max recommended continous load which 100%. Kinda have hard time believing that but with any luck wont have to put it to that test either. It does say though that 50% load gives best economy with a 12 hour runtime on single tank. 7 hour runtime at 100% load. Not great but it is afterall a portable genny and not a fixed position one. Maybe someday on that one..
    Ok so nice to know i am not to far off on the numbers and this setup "should" work for our needs. Thanks for the time and info :)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: Question on battery siziing

    Note that most smaller/non-industrial generators are kWatt=kVA rated ouput.

    Larger gensets may use PF=0.80 or something else to have a kWatt and KVA rating:

    Watt=VA*PF
    VA=Watt/PF

    When charging batteries--They will take full battery charger current from 0 to ~80-90% state of charge. And from 80-90% to 100% state of charge, the current will taper down to ~1-2% of battery AH rating (a 100 AH battery will finish charge around 1-2 amps @ absorb voltage).

    So, ideally, you would use the genset to recharge the battery bank (for example, first think in the morning) from ~50% to 80% SOC and then let the solar panels finish the balance of the charge (save fuel).

    Here are a couple of Battery FAQs to read:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
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    Re: Question on battery siziing

    If I understand what I read here, you have an electric cooking range, and you have a generator. Have you considered switching your range to propane? The money you spend on propane may offset the gasoline and wear and tear on the generator, and reduce the size of your battery bank. If you do go for a propane range, make sure you get an offgrid range (no glowbar igniter).

    I'm sure you realize this, but it must be said again... it's usually cheaper to use less electric power than to generate and store more electric power. (emphasis on store).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #11
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    Re: Question on battery siziing

    Oh yes, completely understand the less is better approach. When i built our house a few years ago this was a conversation topic for sure. We decided it was easier to generate electricity than to generate propane. Induction cooktops work twice as fast as conventional coil tops and therefore burn less overall juice. Only time we really use the oven is during holidays when we have company or we just want to cook up a pizza for movie night. Solar oven fills the gaps. Our goal is to be completely self sufficient. We don't use the cooktop or oven a whole lot anyway. Now odviously if we were on battery power we wouldn't use at all. The utility says we are averaging 8kwh/day which i kinda find hard to believe. I went around with a kilowatt and monitered everything in the house with a plug. I could not come up with more than 4 however that does not take into account the cooktop, oven, cistern pump or well pump. The later two dont kick on very often at all. The cistern pump once or twice a day for 30 seconds at a time and the well pump kicks on once a week. Thinking about getting a whole house meter actually and see what the real story is. Think the utility is penalizing us for being at the end of the line somehow. Now all this being said the power here is unstable and unreliable which is why we want to move to purely off grid in the first place. I am willing to pay some extra up front as long as what we do meets our needs.
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #12
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    Re: Question on battery siziing

    So along the battery topic... I was looking at rolls S-600 batteries for this. Have seen many posts here about poor quality on rolls but nothing from the last 2 years. Any idea if the quality issues have been resolved or should stay away from them?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #13
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    Re: Question on battery siziing

    The Rolls question--It is a real mixed bag. Many folks are very happy, a few (?) have had real problems with failing cells (Rolls seems to be OK with warranty--but they may not make it "easy" all the time).

    Regarding a whole home system, they are out there an not to expensive... For example this company is used by a few people here (I have not used any--just a suggested starting point for research).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
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    Re: Question on battery siziing

    My opinion is to buy whatever equivalent Watt hours you get the deal on. I see no reason at all for paying a premium price for one brand of FLA over another as there does not seem to be any advantage to it. Batteries start to deteriorate from Day 1 so they will have to be replaced eventually anyway no matter how they are used. The important thing is to check and be sure you have recent batteries that are fully charged and have correct SG in all cells before you even take them home.

    Since warranties and customer service seem to be non-existent these days, save your money.
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #15
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    Re: Question on battery siziing

    Interesting. I was looking at the rolls because of their "designed lifetime" but wasn't sure if that was just marketing hype or for real which is why i asked. Is there particular brand that has better customer service than others these days? I have a couple trojan t105s and sun extenders, different systems, and they are doing well so far so no experience with customer service.
    I know you say they are all non-existant these days but someone has to have a better rep than others. If they all are truely bad then this is a sad day indeed.
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  • Mountain man
    Mountain man Registered Users Posts: 8 ✭
    #16
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    Re: Question on battery siziing

    The ted 500c was already on my short list but nice to hear someone else recommend it as well. Thanks for the input.
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