size of lithium battery bank - 3.2v cells

Discussion in 'Electrical Systems' started by jdory, Oct 4, 2018.

  1. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I have tested AGM vs Flooded Cell with increasing rates of charge/discharge. AGM batteries performed well and kept over 95% of the capacity after 50 cycles increasing to 10C. The Flooded Cell started to deteriorate at 1C cycles and most failed at 3C. That is testing to standard discharge to 10V, constant power charge and discharge. I used Bio-Logic battery cycling testers with high power boosters. I understand it is beyond most people to have access to testing facilities to get proper data. However, there is a lot of misunderstanding to what constitutes the useful capacity of a battery. In short, it is the useful power that can be discharged to do work with it. That is the reason to choose 10V as the discharge limit. Below that, lights, computers and other equipment won't work or start to function erratically.
     
  2. jangr
    Joined: Nov 2018
    Posts: 30
    Likes: 4, Points: 8
    Location: Chesapeake Bay

    jangr Junior Member

    Gonzo, I'm curious, what were you trying to figure out here? Destructive battery testing, well beyond reasonable or practical use, seems like a costly academic exercise. The batteries you tested would be good for nothing but scrap as a core exchange?!

    ANY lead acid battery run down to 1.66 Vpc is not long for this world. Anything below 1.70 Vpc is fatal, according to every reputable supplier. It has nothing to do with weather or not an appliance will still run, and everything to do with permanently degrading the battery. Anyone who owns a car knows that running a FLA battery flat, sometimes just 2 or 3 times, is enough to make cold cranking an unnecessary adventure.

    It's seems misleading to me to characterize batteries on the basis of discharge rates, which are only one of the limiting factors in design, and not usually the critical factor anyway. For most of us, capacity data published by the manufacturer is not only more readily available (C1, C5, C10, C20 rather than 1C, 3C, 10C) but provides sufficient insight for making a good decision.

    Consider a 100Ah FLA battery. If the rated capacity is C10, then it will reliably and safely discharge 10A per hour for 10 hours. But since we KNOW that running an FLA below 50% DoD will significantly degrade the number of cycles (lifetime), we also know that we should only design for a total capacity of 50Ah. So if our energy budget is 50Ah (or less), spread roughly evenly over a 10 hour duty cycle, we're in the clear. And our battery will likely last for the number of duty cycles the manufacturer publishes.

    If our energy budget is greater than than 50Ah, then we need a bigger/better battery. If our duty cycle is less than 10 hours, then we need to go back to the published data to see how many total Ah we get for C1, C5, etc. Because we KNOW that a shorter duty cycle will reduce the total capacity of ANY battery. We iterate through our energy budget again, and it will be obvious whether we need a bigger/better battery.

    The question of maximum continuous discharge only arises after iterating through the capacity process first. In our energy budget, we have a peak discharge current for all appliances running simultaneously. If our peak discharge is 10A, or less, we're still in the clear. If it's greater, then we also need a bigger battery, since we choose to design for the worst case in order to create a prudent safety margin. If the peak discharge is 20A, then we need a C10 rating of 200Ah, e.g. 20A per hour for 10 hours.

    This process is appropriate for any battery chemistry. Even though AGM can run down to 80% DoD, or 90% DoD for lithium, it is also true that shallower discharge will extend the life of ANY battery. For instance, the 5% degradation in capacity you note after 50 cycles for an AGM battery is not good. AGMs typically last 1000-1200 cycles at 50% DoD, but you lost 5% in just 50 cycles running them flat. At that rate you'd lose 20% of capacity at 25% of service life, which practically represents 40% of usable capacity (at 50% DoD). Not a happy battery, not a good investment of $350+.
     
    BlueBell and M&M Ovenden like this.
  3. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That was not academic but industrial research. AGM batteries can be discharged to 10V without immediate damage. All batteries degrade with each charge/discharge cycle. However, you should not put all lead/acid batteries in the same category.
    Further, your assumption that an 100 Ah battery will discharge 10A for 10 hours is totally wrong. The discharge rate for batteries is 20 hours. Using Peukert's equation you will see that a fast discharge will output less power. Also, once the voltage is below the minimum to run your equipment, the extra capacity is useless.
    Another wrong statement you are making is that discharging a 12V battery to 10.2 V is fatal. The term fatal means that the battery is damaged beyond recovery, which is false.
    Finally, you still have not quantified DoD. You simply make statements about it, but offer no way of measuring. Basically, your claims are nonsense unless you can offer some technique or standard defining DoD. Unless you can do that, all the claims about capacity and service life are meaningless. Also, you need to provide some references to the studies that back your claims.
     
  4. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    Gonzo, It seems your posts are just argumentative , and offer little else. You've just asked for some studies and references , can you post links to journal papers that you have written on the subject ?
    Most folks go on the standard ratings from manufactures, it seems the industry has come up with some good working definitions of DoD. If you disagree with these you should take it up with the industry, not some general post on a forum.
     
    rxcomposite, Ad Hoc and BlueBell like this.
  5. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    My posts point out that people refer to 50% DoD or 80% DoD, but are not quantifying it. It essence, the numbers are meaningless. The industry standard is discharge to 10V and then use Peukert's equation to compare batteries' capacity, or Ah discharge rate at 20 hours. The Ah is the usual rating from manufactures for marketing. However, the interesting capacity of a battery is to discharge to minimum useful voltage. The rest is meaningless regardless of how much capacity there is left.
    Also, jangr has some fundamental misunderstanding about what "C" means. He states that
    However, at C10 a battery would discharge completely in less than 5 minutes. At a fast rate there is less capacity output. Peukert's equation calculates that loss of capacity output.
    BU-503: How to Calculate Battery Runtime – Battery University https://batteryuniversity.com/learn/article/bu_503_how_to_calculate_battery_runtime
     
  6. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    50% DOD example: 100ah battery discharging 50 ah. It really doesn't need to be anymore complicated when designing a system. A boat with inconsistent loads and charging drives the battery bank size, not some minutia of the rating. Most boats charge and discharge at fractional C rates.

    *useful voltage* is not relevant with buck-boost converters. The life span of the battery is more important.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    What you describe is not 50% discharge. As the battery discharges, the voltage goes down. Therefore, the power output also goes down. 50% discharge is 50% of the total capacity. Further, the Ah rating is based on a 20 hour discharge. The capacity diminishes with higher discharge rates.
     
  8. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    Gonzo,
    We all get that we use normalized values for battery capacity that may not be precise for environmental conditions (loads / temps ) - but it's just a good way to size a nominal system. For example, when we need to size a starting battery, cranking amps are used - these are usually based on a single temperature point so again it's just a good way to compare battery to battery.

    So here is may question: What is your point to these posts - do you expect to change how the industry sizes battery capacity ?

    Cheers,
    Mark
     
  9. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The industry does not need to change the standards. I test batteries to those standards and they work well. The problem is the most people don't understand what the standards mean. A cranking battery has thin plates with a large surface area to provide large power for short times. A deep cycle has thick plates to provide low power for long times. You can't simply say you have x% of charge without somehow being able to quantify that. It makes the claim meaningless.
     
  10. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    BTW, this is an incorrect statement. Some loads will cause an increase in current when the voltage decreases.
     
  11. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Can you name a constant load that will cause an increase in current when the voltage decreases? It appears that violates Ohm's Law.
     
  12. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    Pretty much any electronics device these days will have a switching regulator on the input that will consume consistent power (assuming the device functions the same). A specific example of this would be a DC-AC inverter powering an electric heater. Nothing is breaking ohms law.
     
  13. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If you use a regulator to discharge at constant power, the load is variable and not constant. As the voltage decreases, the resistance must also decrease to increase current and maintain constant power.
     
  14. M&M Ovenden
    Joined: Jan 2006
    Posts: 355
    Likes: 75, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    You just said above in post #37 you can't supply constant power from a battery due to the voltage drop, and that's just not the case as I've shown in my post above. Now it seems you have changed your stand to that you can't have a purely resistive load draw constant power from a battery, which is true.

    (
    )
     

  15. gonzo
    Joined: Aug 2002
    Posts: 14,549
    Likes: 681, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    No I haven't changed anything. Either you have a constant load, which means the power draw will decrease as the voltage lowers, or you have a variable load. It doesn't matter if the load is purely resistive. However, I never specified what type of load, that is not a true statement you made. Further, if you use a switching regulator for regular loads, the power loss will be fairly large ( about 13%), so there is no advantage unless it is feeding sensitive electronics. Finally, you are still not quantifying DoD. Unless you can do that, the claims about percentages of charge are meaningless.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.