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meaning of "amp hour" battery rating

Discussion in 'Beginner Electronics' started by tom, Mar 16, 2005.

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  1. tom

    tom Guest

    If a 12 volt battery is rated at '14 amp hours', does that mean it is
    capable of powering a load that draws 14 amps, at 12 volts, for one hour
    before the voltage starts to sag below 12 volts? Or does it just mean that
    if the battery was completely exhausted using a custom load that continued
    to pull current out of it even as the voltage tapered off, and the total
    power added up and restated in terms of 'amp-hours', it would be the
    equivalent of 14, but the reality is that if you had a load that drew 14amps
    at 12 volts, long before the hour was up, the voltage would drop below 12.

    How long would a typical 14 'amp hour' gel-cell run a 12volt, 15amp load
    before the voltage dropped 10%?

    If you had a 12 volt gel-cell that was partially (say 1/3 )depleted, and you
    simply hooked it straight up to a 12 volt ac/dc power supply, would it
    recharge? What would happen when the battery was fully charged?
  2. Hi,

    The amp-hour rating of a battery (also called capacity) is an indication of
    how much charge a battery holds altogether. You can pull it out at a low
    rate for a longer time or at a higher rate for a shorter time. Capacity
    varies a bit with the discharge rate.

    What you are interested is called a battery's "discharge curve". Get one
    from the manufacturer's website. Discharge curves are different for
    different battery chemistries. For example, a lithium battery holds a
    pretty constant voltage until the end. An alkaline falls off slowly over
    the course of its discharge, then at the end drops off more quickly. The
    discharge curve (at or near your current of interest) will answer your
    questions about what a gel-cell will do.

    Every battery has an "internal resistance". If you hook a partially
    discharged battery up to a constant voltage power supply, it will draw a lot
    of current at first as it charges -- power supply voltage minus battery cell
    voltage divided by the internal resistance. Then as the battery cell
    voltage approaches the power supply voltage, the current will drop off. If
    the battery becomes fully charged and the cell voltage is still below the
    power supply voltage, another mechanism takes over and the battery starts
    heating up and other bad things happen, so better to use a battery charger
    (for the type of battery you are charging) than a power supply, unless you
    know what you are getting into.

    John Musselman
  3. tom

    tom Guest

    Thanks John --- discharge curve --- I'll look into that.
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