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Thread: Battery Capacity C Ratings

  1. #11

    Default Re: Battery Capacity C Ratings

    good description
    Most FLA manufactures seem to recommend 5% > 13% of 20 hr as a min/max charge current,
    Have a good one
    Tim

  2. #12
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    Default Re: Battery Capacity C Ratings

    it is the hour rate a battery has. c100 is 100hrs and what i said stands true that the battery will deliver more ah over 100hrs than at 20hrs and is what windsun was referring to as few of us will charge/discharge a battery at that slow of a rate. the problem with that is we'd need really big battery banks to reap this small extra benefit.
    as to the high end charging, i have said this many times that it varies by manufacturer. think of the 13% as a minimum maximum where you are not sure of its actual ability. many fla batteries can go into the 20+% range, but will also need more maintenance due to the high boiling of the electrolyte. most of us aren't feeding our batteries this high rate anyway due to the costs involved in buying the pv power to do so. fast charging isn't as good as the slower c/20 rate, but so many elect to do it because of the limited hours of sun available in a day allowing batteries to reach full charge (or at least closer to it) every day making sulfation a lesser problem.
    NIEL

  3. #13

    Default Re: Battery Capacity C Ratings

    Quote Originally Posted by niel View Post
    it is the hour rate a battery has. c100 is 100hrs and what i said stands true that the battery will deliver more ah over 100hrs than at 20hrs and is what windsun was referring to as few of us will charge/discharge a battery at that slow of a rate. the problem with that is we'd need really big battery banks to reap this small extra benefit.
    as to the high end charging, i have said this many times that it varies by manufacturer. think of the 13% as a minimum maximum where you are not sure of its actual ability. many fla batteries can go into the 20+% range, but will also need more maintenance due to the high boiling of the electrolyte. most of us aren't feeding our batteries this high rate anyway due to the costs involved in buying the pv power to do so. fast charging isn't as good as the slower c/20 rate, but so many elect to do it because of the limited hours of sun available in a day allowing batteries to reach full charge (or at least closer to it) every day making sulfation a lesser problem.
    I think we're all in agreement on the "C ratings". The only difference I see is that I do actually find the 100 hour rating to be helpful in a stand alone system. For those S-460's, we have two parallel strings which would amount to the following discharge rates and capacity:
    9.32amp load = 932AH capacity (C/100)
    12.3 amp load = 882 AH capacity (C/72)
    16.7amp load = 834AH capacity (C/50)
    35amp load = 700AH capacity (C/20)

    In our case a typical sustained load is somewhere between 8-15amps, meaning that we're typically discharging in the C/100 or C/72 rates and sometimes up to the C/50 rates. When designing a battery bank, if we were to go say 30% smaller or started to add on some additional loads (not that anyone ever does that), we can easily end up with less capacity than we think due to the fact that we're now in a comparetively higher discharge rate. I think that when sizing up a battery bank it's handy to also consider checking that the typical loads match up to the C/100 to C/50 rate for a battery bank with sufficient reserve capacity.
    Of course as I said in my post earlier, I think that you can compare any like ratings as long as you have an idea what that represents in terms of both charging and discharging currents, as well as the overall affect on capacity and lifecycle.

  4. #14

    Default Re: Battery Capacity C Ratings

    Hillbilly a good reply we need bases to found our calculations you have given me one opps all of us one to start from.
    Outback FlexpowerOne FP1-6 , with Mate 3, Solar 2400W 12 x 200wp Isophoton mono Fixed Array, Tracked Single axis 972wp from 6 x 162wp Sharp Poly. Midnite Classic 150 CC. The Battery bank 12 yrs old and counting ! 700ah Exide 24 x 2v Cells, Zepher Power Vent, Midnite PV Combiner and SPD. To fit Lakota Longbow 48v Windturbine. Gesan 4.5kw 1500rpm Deutz Engined Autostart Diesel Generator.

  5. #15

    Default Re: Battery Capacity C Ratings

    The C rating to use will depend on the loads, the number of autonomous days and the DoD. I've noticed that some Spanish installers recommend 5 day batteries discharged 70% (i.e. there's 30% left in the battery). So assuming ideal conditions and 0 losses:

    100Ah load per day, 5 days autonomy, 70% DoD:
    Battery size = (100 * 5) / 0.7 = 714Ah. So the battery would be 70% discharged after 5 days = 120 hours which means it will be 100% discharged after 170 hours. So C170.

    100Ah load per day, 3 days autonomy, 50% DoD:
    Battery size = (100 * 3) / 0.5 = 600Ah. After 3 days the battery should discharged to 50%, so after 6 days it will be 100% = C144.

    If you factor in the losses the C values should get closer to C100. So in _theory_ for typical deep cycle batteries sized for 3-5 days using < 70% DoD, C100 seems like a good number to use.

  6. Default Re: Battery Capacity C Ratings

    What is being considered is the Peukert effect. For estimating realized energy capacity, you want to match your loads (power draw) to the energy capacity of the battery at the current that will be pulled from it to supply that load.

    Using the C/xx approach is needed for battery banks because the wiring of the bank is going to determine the current drawn from a specific battery to provide an overall power from the bank.

    Note that C/xx is a rule of thumb and not a measure. It is a means to normalize current draw by battery size. As a contrast, the most common energy capacity ratings for batteries tend to involve time rather than current, i.e. the 20 hour rate being most common. For a 100 AH battery, the 20 hour rate implies a 5 amp 12v or 60 watt load.

    For the Peukert coefficient usually found in lead acid batteries (1.2 to 1.3 or so), a change in current draw means a change in battery capacity of about 15% over the typical useful range.

    For the topic here, the first thing to do is to figure out your use profile. What is the average power draw and how uniform is it? What is the peak draw and what is the median and well as the mean. These statistics will give you a target for your battery bank. You want it to supply peak loads without significant loss and you want it to be most efficient at the median load. From that, figure out the current you will be taking from a battery in various bank configurations and use that current to determine the effective capacity of the battery from the C/xx ratings that most closely match your loads.

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