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Thread: Question about lead acid Amp Hour rating

  1. #1
    FLAC
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    Exclamation Question about lead acid Amp Hour rating

    Hi.
    I was just curious. When battery manufactures give their lead acid batteries an amp hour rating, to what extent do they mean? I know that ordinary lead acid batteries should not be discharged past 50% capacity without risking permanent damage. Does the amp hour rating mean a load put on the battery untill full discharge? For example, my car has a battery with a 52 amp hour rating. If I were to put a 52 amp load on the battery, then the battery should be able to sustain that load for 1 hour (assuming the world is perfect). Once the load is off the battery after 1 hour, what state would my battery be in? Would it still be able to start my car? So I was wondering how to estimate how long it would be safe to put a specific load on a car battery, especially if we assume that the battery should be discharged so that there is still sufficient capacity for the battery to start a car.

  2. #2
    Constant Bitrate
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    Quote Originally Posted by nobb
    Hi.
    I was just curious. When battery manufactures give their lead acid batteries an amp hour rating, to what extent do they mean? I know that ordinary lead acid batteries should not be discharged past 50% capacity without risking permanent damage. Does the amp hour rating mean a load put on the battery untill full discharge? For example, my car has a battery with a 52 amp hour rating. If I were to put a 52 amp load on the battery, then the battery should be able to sustain that load for 1 hour (assuming the world is perfect). Once the load is off the battery after 1 hour, what state would my battery be in? Would it still be able to start my car? So I was wondering how to estimate how long it would be safe to put a specific load on a car battery, especially if we assume that the battery should be discharged so that there is still sufficient capacity for the battery to start a car.
    AmpHr rating is one of the those marketing terms.

    Take a fully charged lead acid battery. Discharged it over 20 hours until it reaches 10.5 volts. The AmpHr rating tells you how total energy it will give you over that 20 hours. Sometimes AmpHr rating are given over 100-hour discharge period as well. You get more AmpHrs if you discharge a battery over a longer period of time. Read the fine print.

    Cold crank capacity (CCA) and Reserve capacity (RC) are more useful measures when comparing battery, imho.

  3. #3
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    Battery Storage Capacity Ratings

    Two standard ratings are used to measure a battery's storage capacity.

    Amp Hours

    The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

    Reserve Minutes

    Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)
    Jan Bennett
    FS: VW MKIV Bezel for 8" Lilliput - 95% Finished

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