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  • Battery Capacity calculations

    Could you guys please check over my calculations for determining battery bank sizing.

    Watts * Efficiency of array&charger * (1/charge voltage) * (1/rate of charge) = Battery Amp Hours

    Which i believe is correct. The part i'm questioning is the charge voltages:

    for 12v i'm using 14.5v as the charge voltage
    for 24v i'm using 29.0V as the charge voltage
    for 48v i'm using 58V as the charge voltage

    Are these correct values?

    If you want to give it a test or have any suggested fixes/improvements let me know. Battery Bank Calculator

    Any corrections or suggestions is appreciated!
    Solar Power Tools and Calculators

  • #2

    Re: Battery Capacity calculations

    Re: Battery Capacity calculations

    doesn't look right to me.

    take a known pv like a us 64 that i have. now 77% of that is 49.28w. to translate this to current one needs to divide that by the vmp of the pv and the us 64 is 16.5v. others are usually higher than this so one size voltage does not fit all. anyway, 49.28w/16.5v=2.9867a. to figure the battery capacity now depends on the % charge rate employed.
    5%=59.73ah
    10%=29.87ah
    13%=22.97ah

    now that works for a pwm cc and it gets more complicated when using an mppt cc as the vmp then gets changed (usually downward) and the current output is higher and is made somewhat variable by the mppt action, but generally i'd say another 10% in current on average.
    NIEL

    Comment


    • #3

      Re: Battery Capacity calculations

      Re: Battery Capacity calculations

      Looks nice. You might want to have a little "FAQ" that lists why you used 14.5 volts instead of 12.0 volts, why a AGM (low self discharge/high efficiency) battery might work better at 5% rate of charge vs a flooded cell (or worse yet; forklift) battery that have higher self discharge, less charging efficiency, etc...

      You could also put a 1% rate of charge in for Floating/Storing batteries too (boats, etc.)--We get a fair number of those questions too.

      -Bill
      20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

      Comment


      • #4

        Re: Battery Capacity calculations

        Re: Battery Capacity calculations

        Neil.. So should i add in a vmp box to make it more fine tuned or make it more general for the masses. My calculator on the page lets you select a charge rate between 5-13%


        Bill - Good advice on the float charge. Ill add that in. Do you have a post or page with some info on the diff batteries vs charge rate (AGM/flooded ect) to gain some info for the faq?

        I'm slowly building calculators/tools to help figure out the stuff i initially struggled with! I figure it will help solve alot of the random questions for my self and other in the community!
        Solar Power Tools and Calculators

        Comment


        • #5

          Re: Battery Capacity calculations

          Re: Battery Capacity calculations

          Some good places to start:

          Deep Cycle Battery FAQ
          www.batteryfaq.org

          It does get very complicated very quickly... It it why here we try not to tell people to "search" for the answers on the forum, or cut and past a "newbie" post for everyone that asks how to buy/build an off grid system--But, instead try to answer the question in the way each new poster asks the questions (I some panels; I have some batteries; I want to run a freezer for a day; etc.).

          -Bill
          20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

          Comment


          • #6

            Re: Battery Capacity calculations

            Re: Battery Capacity calculations

            Neil - to answer your early question about why 14.5v - Float charges generally range from 13.02 to 13.20 volts and the absorption charging stage generally ranges from 14.2 to 15.5 volts. so 14.5 seemed like a happy medium. :)
            Solar Power Tools and Calculators

            Comment


            • #7

              Re: Battery Capacity calculations

              Re: Battery Capacity calculations

              I think what Niel was also trying to point out... Power=Voltage * Amperage (yes, I know you know that ). But when spec'ing solar panels/arrays and MPPT vs PWM controllers, the Vmp vs Vbatt does make for some differences...

              Say you have a 175 watt solar panels with Vmp=17.5 volts, then (using round numbers):
              • Imp = Pmp / Vmp = 175 watt panel / 17.5 vmp = 10 imp amps

              Now when looking at the charging characteristics of a PWM controller, as long as Vmp-array-operating-temp > Vbatt-charging, the current from the array is (roughly) Imp (at noon-time 1,000 w/m2. So, the "nominal power" charging a "12 volt battery" near 14.5 volts from a 175 watt panel would be (PWM controllers usually have very operational current requirements--so we will ignore it here):
              • Pbatt-charg = Vbatt-charg * Imp = 14.5 volts * 10 amps = 145 watts (noon time sun)
              • Eff = Pbatt-charg / Pmp-rated = 145 watts / 175 watts = 0.829 = 83% efficiency

              Note the efficiency here is purely based on the "mismatch" between Vmp-panel and Vbatt-charging:
              • 14.5 Vbatt-chrg / 17.5 Vmp = 0.829 eff

              And, technically, as the panel gets hot and Vmp-panel depresses, with a PWM controller, it does not change the Imp of the panel (Imp slightly rises with temperature--but we can ignore it here).

              Remember we used 0.77 derating for solar array (which, mostly, is 82% derating for Vmp-hot depression) and ~95% efficiency for a MPPT controller running somewhere near maximum rating (most efficient point).

              The same thing done with a MPPT controller:
              • 175 watts * 0.77 derating * 1/14.5 volts batt = 9.29 amps into battery

              So--we are assuming a hot panel, some dirt, controller, wiring losses... Using the "extra" 6% derating on a PWM controller (0.83 eff PWM - 0.77 eff MPPT):
              • 10a Imp * 0.94 = 9.4 amps for our "paper derating" of the PWM controller

              So the "calculated" difference between 83% derating for a PWM vs 77% derating for a MPPT controller--I chose to round down the ~6% to account for aging/voltage-wiring drop/dirt/etc. with the PWM controller and call it "close enough" for solar work.

              For very cold climates, the MPPT controller can add 10-15% or so to energy collection. But, the useful part of MPPT (in my humble opinion) is it allows you to use larger wattage panels that do not output 17.5 Vmp (large panels are cheaper). And it allows you to run Vmp-array to upwards of 100 VDC (depending on brand/model of MPPT controller)--Which makes it much easier and cheaper to send power longer distances (between Array and Battery Shed) using much smaller gauge wiring.

              All About Charge Controllers
              Read this page about power tracking controllers

              -Bill
              20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

              Comment


              • #8

                Re: Battery Capacity calculations

                Re: Battery Capacity calculations

                bill is correct and i'd say go ahead and put the vmp into the calculator with a note that mppt can see higher currents. also note that downconversions by mppt controllers can really muddle up someone from being able to figure out what the current actually is. you are getting the idea that it is hard to nail down what is going on with solar even from one moment to the next, but for the purpose of calculating battery bank capacity it may suffice as a rough minimum from a pwm standpoint.

                one has to watch what conclusions one can draw with the observed currents from calculations resulting from the use of the calculator as low vmps will show higher currents based on the formula power in watts = volts x amps. it is true you could see higher currents with low vmp for a given pv stc wattage rating, but during the warming from the sun and hot ambient temps this could drop too low to place a charge into the battery. on the other extreme one may put a pv with a vmp for example of 23.9v and on a pwm controller this does not up the current available to the battery, but wastes the extra power as the excess voltage is pretty much ignored from benefiting the battery as only an mppt controller will bring any excessive voltage into usable power through downconversion.

                i commend you on trying to simplify the figuring of a battery bank's capacity, but as you can see it's not so simple.
                NIEL

                Comment


                • #9

                  Re: Battery Capacity calculations

                  Re: Battery Capacity calculations

                  So if I add in the panel vmp voltage then i also need to add in PWM or MPPT to get a more accurate estimation... you are right! this is slowly getting more complicated! I'm still determined to figure it out (or atleast a very good educated guess of a calculator).. that being said if you guys know of any other common formulas you use let me know and Ill see if i can make some "simplified" calculators for the community.
                  Solar Power Tools and Calculators

                  Comment


                  • #10

                    Re: Battery Capacity calculations

                    Re: Battery Capacity calculations

                    I have seen some pretty sophisticated spread sheets that do a very good job of taking many issues into account (including voltage/Peukert factors/etc...). And--For Me--They really required a lot of knowledge to use correctly and interpret.

                    What would be neat is to have something that you can put in some information (amount of loads, XXX watt of panels, YYY AH @ ZZ volt battery bank, etc.)--Then the rest of the information that can be derived is then filled into the blank areas... For example:

                    Things people want:
                    • how many batteries do I need (5%/10%/13% rate of charge)
                    • how many watts of solar panels
                    • how much power will system produce
                    • how much does the system cost

                    Things people may start with:
                    • Minimum hours of sun (4 hours for 9 month system; 5 hours for 4-6 month "summer" system)
                    • AH @ battery volts of load per day
                    • WH of load per day
                    • I have YYY AH of ZZ volt batteries
                    • I have AAA Watts of panels
                    • I want 1/2/3 days of battery storage (default 50% maximum discharge)
                    • ~82% panel derating (from STC)
                    • ~95% efficiency for MPPT charge controller / GT Inverter
                    • ~85% efficiency for off grid inverter
                    • ~90% efficiency for AGM battery
                    • ~80% efficiency for Flooded Cell Battery
                    • ~C/8 maximum continuous load from flooded cell battery
                    • ~C/2.5 maximum surge load from flooded cell
                    • ~C*4 maximum load for AGM
                    • ~5% to 13% Rate of Charge for Battery Bank (flooded/AGM)
                    • ~C/20 = 5% rate of charge for GEL
                    • ~80% efficiency for AC Battery Charger (typical)
                    • ~0.67 PF for non-PFC (power factor corrected) AC battery charger (PF=1 for PFC charger)
                    • ~5% to 25% rate of charge for AC Battery Charger
                    • ~$5 to $8 per Watt (of solar panel) GT system
                    • ~$10 to $20 per Watt (of solar panel) for Off Grid system
                    • 0-1,200 Watt load -> 12+ volt battery bank
                    • 1,200 watt to 2,400 watt load -> 24+ volt battery bank
                    • over 2,400 watt load -> 48 volt battery bank

                    I can come up with equations to relate the major pieces to each other--But trying to figure out a useful interface for the above would be a bit of work too.

                    Do you setup different input screens for each major set of starting questions (battery, solar array, loads)? And then the output screen below has the "answers".

                    And where do you put the "tweaks"? For some, do you instead output a battery bank based three calculations like 5/10/13% so people can see range instead of having to put a different number and get a different result?

                    As you can probably see from my answers before--I tend to like to give the 100/200/260 AH battery bank answer (5/10/13%) so people can see the "range"... Obviously, there are many physical devices (like batteries/panels/inverters) that come in set sizes... I.e., 220 AH vs 280 AH battery--And not 260 AH. Sort of gives people an idea of what is a "big" vs "small" difference in relation to the overall system.

                    And, many times, stuff will have to be recalculated... They come up with a range of 200-260 AH battery bank, and pick 280 AH--So now they need to resize the Solar array for the 5-13% rate of charge...

                    Can become a very interesting human interface/design project in itself (iterative updating as people select exact values to finalize and print out for there design).

                    As you can see--that is why I choose, for example, to "collapse" the power differences between PWM and MPPT--And make their choice based on system size (less than 200 watts PWM, over 600 Watts probably MPPT; or MPPT because there is a long distance between Solar array and battery/charger shed) rather than worry about 5-15% better performance complexity in calculations for some folks that have sub freezing winters.

                    -Bill
                    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

                    Comment


                    • #11

                      Re: Battery Capacity calculations

                      Re: Battery Capacity calculations

                      shift-

                      Thanks very much for your contribution to the community. Sometimes folks question the accuracy of a contribution when there are typos in the wording. In that spirit, I offer the following:

                      "Finding the correct solar array power to battery bank Amp hour (AH) [ration] ratio will ensure your batteries stay happy and live a long time. If you have [two] too small of an array for your battery bank, it will not have enough current to properly charge; alternatively [to] too high of a charge rate may “cook” your batteries. In general, you want to stick with a 5-13% charge rate. If you just want to float charge your batteries, pick a charge rate of 1-2%. You can calculate this using the formula Watts * Efficiency of array&charger * (1/charge voltage) * (1/rate of charge) = Amp Hours… or just use the calculator below!"

                      I do not know enough about off-grid PV systems to offer any technical comments, but your calculator is of interest to someone like myself who might consider off-grid requirements, and be interested in sizing a system.
                      Grid-tie PV system: 3.15 kW, 14 @ SunPower 225 W panels, SunPower SPR-3000m inverter.
                      Solar Thermal system: 1 @ Viessmann Vitosol 200 F flat plate collector 25 sq. ft. absorber area, Heat Flo HP 50-gallon stainless steel tank.
                      Residential Energy Laboratory in a net-zero source energy modified production house.

                      Comment


                      • #12

                        Re: Battery Capacity calculations

                        Re: Battery Capacity calculations

                        Shift, I like your idea, good concept..

                        As to developing an all-in-one formula I think it can be done if there is a default value of 1 fro any variable, if a specific value is not entered on the input page. If you are familiar with Access there would be default values entered in the 'Mask" for the variable, other wise it is overridden with the entered value... make sense?

                        Once you get the formula page done , just protect (lock) the entire page, so the formulas are sacrosanct... with Values grabbed from an Input page.

                        HTH
                        100% Off Grid @ 51* 46' N lat 124* 44' W long

                        New House system: coming - 2 arrays @ 2240W
                        #1) CL150 & Epanel 12V PVs 4S @140 W , 24V C&D AT-15P AGM @ 950Ah, Linksys wet54g - WiFi bridge, Cotek ST1500W inverter, TBS Omni-charger 30a-24v, ,
                        #2) CL150 12V PVs 4S@140W , 24V C&D AT-15P AGM @ 950Ah
                        to ADD: E-Panel and inverter
                        Honda Eu3000is,Eu1000is

                        Guest cabin system: 3 - 70W panels to BS 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter

                        Comment


                        • #13

                          Re: Battery Capacity calculations

                          Re: Battery Capacity calculations

                          Originally posted by westbranch View Post
                          .... If you are familiar with Access....

                          NOOOOooooo Not ACCESS ! Just use a spreadsheet, not a database !!
                          http://tinyurl.com/LMR-Solar
                          http://tinyurl.com/LMR-BigLug
                          http://tinyurl.com/LMR-NiFe

                          Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
                          Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

                          Comment


                          • #14

                            Re: Battery Capacity calculations

                            Re: Battery Capacity calculations

                            I think his intent is to run this from a web server--not any sort of down-loadable spread sheet (other than maybe the results--my suggestion).

                            -Bill
                            20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

                            Comment


                            • #15

                              Re: Battery Capacity calculations

                              Re: Battery Capacity calculations

                              Mike,Shift, sorry, did not mean to use Access , just as an example...
                              100% Off Grid @ 51* 46' N lat 124* 44' W long

                              New House system: coming - 2 arrays @ 2240W
                              #1) CL150 & Epanel 12V PVs 4S @140 W , 24V C&D AT-15P AGM @ 950Ah, Linksys wet54g - WiFi bridge, Cotek ST1500W inverter, TBS Omni-charger 30a-24v, ,
                              #2) CL150 12V PVs 4S@140W , 24V C&D AT-15P AGM @ 950Ah
                              to ADD: E-Panel and inverter
                              Honda Eu3000is,Eu1000is

                              Guest cabin system: 3 - 70W panels to BS 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter

                              Comment

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