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What is the battery DOD % for a daytime only system?

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  • What is the battery DOD % for a daytime only system?

    Hello all, and thanks to all that contribute here. I have learned alot reading here the last 2 months. I have come up with a specific question that I had not seen and so this is my first post.

    In the example of a solar/electric powered boat that I have seen a few discuss here, I am trying to figure out or find out how to estimate battery life/longevity. I am pretty clear on most of the different aspects of charge/discharge rate, dod %, # of charge cycles and that they dictate how long your battery will last or NOT. The specific question is, in a daytime only system that is producing say 8 amps from the panels (avg) and the motor load adjusted to consume 10 - 20% less, some will say they are running the load off solar and still have a small amount going to charging. That seems to cover the basic reality of the moment, but from a longevity standpoint what are we really looking at? Is the load getting any of the current directly?, or is it all going through the battery and going to contribute to an early death if the battery bank is not sized properly? Is the battery staying charged and just passing it through? Anyway just trying to figure how this relates to what life span to expect from batteries in this situation if they are minimum sized.


    thanks for any insight.

  • #2
    Originally posted by LETitROLL View Post
    Hello all, and thanks to all that contribute here. I have learned alot reading here the last 2 months. I have come up with a specific question that I had not seen and so this is my first post.

    In the example of a solar/electric powered boat that I have seen a few discuss here, I am trying to figure out or find out how to estimate battery life/longevity. I am pretty clear on most of the different aspects of charge/discharge rate, dod %, # of charge cycles and that they dictate how long your battery will last or NOT. The specific question is, in a daytime only system that is producing say 8 amps from the panels (avg) and the motor load adjusted to consume 10 - 20% less, some will say they are running the load off solar and still have a small amount going to charging. That seems to cover the basic reality of the moment, but from a longevity standpoint what are we really looking at? Is the load getting any of the current directly?, or is it all going through the battery and going to contribute to an early death if the battery bank is not sized properly? Is the battery staying charged and just passing it through? Anyway just trying to figure how this relates to what life span to expect from batteries in this situation if they are minimum sized.


    thanks for any insight.
    Hello LETitROLL and Welcome to Solar Panel Talk!

    As long as the power going to the motor is less than what the panels are producing, you are not really cycling the battery and may get a 5 year life just from other aging effects. Longer if you care for the battery well, shorter if you neglect it between uses.

    But if you are actually sometimes taking more to the motor than the panels are producing at that moment, with the panels making up for it minutes or hours later, then you are "micro-cycling" the battery. What effect that has on the battery life is a topic of vigorous discussion, but the effective life will be at least slightly shorter that if the battery were sitting idle the whole time.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      OK, to run an inverter, you need about 100a of battery bank, for every 1,000 w of power you expect to draw. This capacity allows the inverter to react with the battery to properly produce AC power. You need to account for surge capacity in this calculation. If you have a 200w load, but it needs 600w to start up (even for a second) you need about a 60A battery (with good heavy cables) Solar panels themselves have no surge capacity, so this 120hz power pulse comes from the batteries, that's how AC is made.


      Take about 10% of the battery capacity, and [60a battery = 6 ] to have as a reserve for battery charging if you got a cloudy day.

      You need about 140% of your expected daytime load (in PV panel watts), to allow for system losses. The load calculation needs to have the battery charge reserve (6 in the above example), added to it. And about 1x a month, you should drain the batteries down about 50%, to keep fresh plate material active, or you will risk capacity fade.

      The controller provides voltage to the system, and both the load and the battery utilize it. So sizing the array to run a load, will not add "daily cycles" to the battery, it just goes through a morning absorb charge, and then drops back to float for the rest of the day.

      I have my big irrigation pump on a timer, to start about 11am, after the batteries are full from the night loads, and run the pump till 4pm, or when the tank float says it's full. That leaves me an hour to top off the batteries before sunset.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

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      • #4
        Thanks, that all sounds pretty reasonable. I appreciate the replies. How come I feel like I'm the only one on this thread NOT from Northern Ca. (I am native there though)

        thanks again

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