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Battery longevity question: Oversized bank AH vs. more optimally sized bank AH

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  • Battery longevity question: Oversized bank AH vs. more optimally sized bank AH

    Would a c/12 sized battery bank that is (ideally) never taken to more than 40% discharged appreciably outlast a c/10 sized battery bank that is (ideally) never taken to more than 50% discharged? I'm trying to determine if the added cost of extra AH can be warranted in terms of longevity. Or are there any serious downsides to oversizing the battery bank and intentionally limiting its discharge percentage?

    Based on 36 maximum amps of charging current calculated for my 4 x 260 Watt panels (the Midnight Classic sizing tool says 1,040 Watts/28.8V = 36.1 charging Amps) the ideal C/10 battery bank for my 24V system would be 360 AH in capacity, but I'm considering batteries rated at 420 AH, so that's why I'm asking this.

  • #2
    It's like asking how many gnats can dance on the head of a pin. With 60 amp hrs difference in over all bank size your never going to be able to measure the difference over time. The dod is hard to calculate daily and calendar life no one knows.

    It would be a nice project for someone to track on a daily basis with a data logger.

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    • #3
      Originally posted by Silver_Is_Money View Post
      Would a c/12 sized battery bank that is (ideally) never taken to more than 40% discharged appreciably outlast a c/10 sized battery bank that is (ideally) never taken to more than 50% discharged?
      That does not make any sense or have anything to do with how long a battery last. Charge rates have nothing to do with cycle life assuming you stay within acceptable charging limits.

      Cycle life depends on depth of discharge. 5 day autonomy (20% DOD per day) gives you the best bang for your dollar and keeps generator fuel cost in check.
      MSEE, PE

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      • #4
        And, since a battery bank will degrade just sitting there on float charge, at some point the extra battery capacity is just a waste of investment money.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          So is the consensus that a 360 AH battery bank should be my choice over a 420 AH bank? Or is something else even better than these choices? Given a 24V system with 4 x 260 Watt PV modules and a daily average insolation of 4.03 hours for my location, what AH capacity of battery bank would you choose?

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          • #6
            Originally posted by Silver_Is_Money View Post
            So is the consensus that a 360 AH battery bank should be my choice over a 420 AH bank? Or is something else even better than these choices? Given a 24V system with 4 x 260 Watt PV modules and a daily average insolation of 4.03 hours for my location, what AH capacity of battery bank would you choose?
            Again battery size is determined by how many watt hours you use in a day. That is the very first step in the design process. At that point you have no idea or clue what panel wattage, battery voltage, or battery capacity is needed. That comes later.

            So if you need 2 Kwh per day, the battery needs to have 4 to 5 day reserve capacity with 5 being the best long term economic choice. 5 x 2 Kwh = 10 Kwh. For a 24 volt battery to find the Amp Hour Capacity is 10,000 wh / 24 volts = 416 AH, just call it 400 AH is the correct size battery. Now you find the panel wattage and controller size.

            If you are using a FLA battery, your panel wattage needs to be withing a FLA charge window of C/12 on the low side, and C/8 on the high side. On a 400 AH battery the charge current window is 33 to 50 amps. Otherwise you have to make adjustments. For example if you live in an area with less than 2.5 Sun Hours in winter like Seattle is going to exceed a C/8 charge current of 33 amps. Which means you are going to have to use a Hybrid or AGM battery. Example in Seattle needing 2 Kwh/day require a panel wattage of [2000wh x 1.5] 1.2 Sun Hours = 2500 watt Soar panel. At 24 volts on a MPPT Charge Controller is 100 amps of charge current or C/4 on a 400 AH battery.

            On the flip sid eif you live in some place where you have excellent Sun Hours in winter above 4.1 Sun Hours, you wil have to over size panel wattage to meet C/12 or use a different battery that does not have a minimum charge rate requirement like AGM of Lithium. FWIW economics dictate use more panel wattage. Example same 2 Kwh/day requirement and 24 volt 400 AH battery. In Tiscon AZ winter Sun Hours = 5.5 So [2000 wh x 1.5] / 5.5 sh = 545 watts. At 24 volt battery is only 20 amps of charge current which is C/20 on a 400 AH battery and falls short of 33 amps or C/12. So in Tuscon you would need at least a 33 amps x 24 volts = 800 watt panel.

            So back to your question. you have 1040 watt panel operating into a 24 volt battery. Assuming you are using MPPT controller all I can say with certainty is that can support a 24 volt battery from 344 AH to 515 AH battery. However I am clueless if it will work because you have not defined how may watt hours in a day you require. You did not take the first step. It appears you leaped to the end and hope it works.
            MSEE, PE

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            • #7
              My area has only 2.18 hours of insolation in December for fixed solar panels mounted at latitude angle. That puts me in the Seattle class (or worse) in the winter. It appears that I do not have enough Watts of PV.

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              • #8
                Originally posted by Sunking View Post
                Again battery size is determined by how many watt hours you use in a day. That is the very first step in the design process. At that point you have no idea or clue what panel wattage, battery voltage, or battery capacity is needed. That comes later.....
                My primary intent for this small 1,040 PV Watts system is to be able to keep both our refrigerator and deep freezer running in the event of an extended grid power outage. Our newer and high efficiency refrigerator is rated at 345 KWH/year and our also newer and highly rated for efficiency deep freezer is rated at 375 KWH/year. Basically 2,000 watts per day (2 KWH) on average are needed to keep them both up and running if I'm looking at this correctly. Both appliances run on 120VAC. I'm not adverse to running one of them at a time on perhaps 8 hour shifts if need be in the winter when we get much less sunlight. A few lights (10 Watt LED, 60 Watt incandescent equivalent) on in the house would be nice also.

                After that this tiny system is being used purely as a learning experience.

                Per PVWatts (using TMY3 data) our regions average daily insolation for fixed position panels at 45 degrees is only 3.96 hours/day, and in December this slumps to about 2.13 hours. TMY2 data says 4.12 average daily hours, and 2.04 in December. We are at 41 degrees north latitude.

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                • #9
                  Originally posted by Silver_Is_Money View Post
                  My primary intent for this small 1,040 PV Watts system is to be able to keep both our refrigerator and deep freezer running in the event of an extended grid power outage. Our newer and high efficiency refrigerator is rated at 345 KWH/year and our also newer and highly rated for efficiency deep freezer is rated at 375 KWH/year. Basically 2,000 watts per day (2 KWH) on average are needed to keep them both up and running if I'm looking at this correctly. Both appliances run on 120VAC. I'm not adverse to running one of them at a time on perhaps 8 hour shifts if need be in the winder when we get much less sunlight. A few lights (10 Watt LED, 60 Watt incandescent equivalent) on in the house would be nice also.

                  After that this tiny system is being used purely as a learning experience.

                  Per PVWatts our average daily insolation for fixed position panels at latitude angle is only a tad greater than 4 hours/day, and in December this slumps to about 2.17 hours.
                  Did you ever think about just using a generator/charger/battery/Inverter?

                  aka Macgyver UPS with genny back up. You run the fridges off the Inverter full time. You plug in the AC charger to commercial AC, connect the batteries and Inverter. Under normal operation everything runs off Commercial power through your. Even better a salvaged commercial 24 or 48 volt telecom rectifier as they are cheap and better than any battery charger you can buy. They are built to last 50+ years and more.

                  Anyway when power fails, start the genny and plug the charger/rectifier into to recharge the batteries ones a day.

                  A couple of catches to make it work right.

                  You still have to use AGM batteries so you can charge fast to save fuel. However for Emergency standby use you only need to size them for 2 days vs 5 days so big cash savings.
                  Your Charger/Rectifier needs to be sized to fully power the loads, plus recharge batteries in 4 hours so you minimize generator run time to 4 hours each day after first day. Works like a hybrid car like a Chevy Volt. The batteries can handle a full day. If the outage is longer than 24 hours you run the genny 4 or 5 hours each day to recharge.

                  Best of all No Solar that doe snot work in inclement weather during power outages.
                  MSEE, PE

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                  • #10
                    I did not consider a system with no PV modules. I already have the 4 PV modules and the charge controller so I would like to utilize them.

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                    • #11
                      Originally posted by Silver_Is_Money View Post
                      I did not consider a system with no PV modules. I already have the 4 PV modules and the charge controller so I would like to utilize them.
                      Just asking. Don't get your feathers ruffled.
                      MSEE, PE

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                      • #12
                        Originally posted by Sunking View Post
                        Just asking. Don't get your feathers ruffled.
                        No worries. IMHO you are a great asset to this forum, and you have consistently attempted to guide me in the right direction (even when what I'm saying seems confusing or baffling or way off base to you) and for that I thank you.

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                        • #13
                          Originally posted by Silver_Is_Money View Post
                          No worries. IMHO you are a great asset to this forum, and you have consistently attempted to guide me in the right direction (even when what I'm saying seems confusing or baffling or way off base to you) and for that I thank you.
                          No problem here on my end, and you are welcome.

                          With 2 Sun Hour limitation just leaves a bad taste in your mouth for solar and an empty wallet. I just wanted to make sure you knew your options, and the consequences of the options. Sometimes you have no choice but to pick between two evils and figure out which is the less evil.

                          Despite my rhetoric, I hate seeing people get trapped into bad decisions and really try to show them another way. I leave it for you to chose and will help once you know the options open to you.

                          So are we OK here?
                          MSEE, PE

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                          • #14
                            Originally posted by Sunking View Post

                            With 2 Sun Hour limitation just leaves a bad taste in your mouth for solar and an empty wallet. I just wanted to make sure you knew your options, and the consequences of the options. Sometimes you have no choice but to pick between two evils and figure out which is the less evil.

                            Despite my rhetoric, I hate seeing people get trapped into bad decisions and really try to show them another way. I leave it for you to chose and will help once you know the options open to you.

                            So are we OK here?
                            But somehow... somehow I went off the rails with my own off-grid project with 2 hours of winter insolation. I couldn't find a decent site with power, and no other other idiot bought the place that sat there for 5 months. So I got a "deal" at 20% off asking price. Look at me )-: Whoo-hoo.

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                            • #15
                              Originally posted by Living Large View Post
                              But somehow... somehow I went off the rails with my own off-grid project with 2 hours of winter insolation. I couldn't find a decent site with power, and no other other idiot bought the place that sat there for 5 months. So I got a "deal" at 20% off asking price. Look at me )-: Whoo-hoo.
                              And what you saved in real estate costs will now have to be used for your power generation.

                              While I understand the benefits of finding low cost property, sometimes you get stuck trying to sell it after it loses it's glamor. I picked up 15 acres in 2005 for about $6k per acre while most other plots were being sold at $12k/acre.

                              Now that my retirement plans have change I might be able to sell my property for a little over $5k / acre. I never thought real estate would drop that much in value over tens years.

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