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measuring capacity.

Discussion in 'Electronic Design' started by [email protected], Jun 13, 2007.

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  1. Guest

    how do you measure the capacity of a cell? expressed in mAh. I suppose
    you can drain it at a constant rate until it's empty, but what if i
    want to know the remaining cap without draing the battery?
  2. Tom Bruhns

    Tom Bruhns Guest

    How do you measure the capacity of a cell? By draining it as you
    suggest. The capacity will depend somewhat on the rate at which you
    drain it. You can find lots of info with a Google search.

    If it's a rechargeable, you should fully charge it and conduct the
    capacity test, and then you'll have an idea of the capacity for future
    discharges, for a while, though it will "wear out" with each discharge/
    charge cycle.

    If it's not rechargeable, you can test similar cells, or rely on the
    manufacturer's data.

    With _some_ chemistries, you can get an idea of the state of charge
    from the voltage, and by monitoring the voltage versus the energy
    drawn (and likely the temperature, too, since voltage is generally
    dependent on temperature as well as state of charge), you can have an
    idea, at least, of the remaining capacity. However, with many
    chemistries, the voltage is pretty constant over the center of the
    amount-of-discharge curve, so you get limited info till the battery
    nears exhaustion.
  3. Phil Allison

    Phil Allison Guest

    ** That problem ( ie a " fuel gauge " ) has been the bane of all sealed,
    rechargeable cells since they were invented.

    There is NO electrical test that reveals a Ni-Cd or NiMH cells' state of
    charge - excepting if it is quite flat.

    The best effort is to have a uP monitor the cell or pack and compute the mAh
    consumption since the last full charge and give an estimate of what is
    therefore likely to be left.

    ........ Phil
  4. homermandoh

    homermandoh Guest

    If you go with Lithium, and some research, Maxim-IC (Dalas) has IC's
    that will monitor the usage of various cell configurations. They also
    provide free samples of many of their IC's.
  5. whit3rd

    whit3rd Guest

    Full discharge of an electrochemical cell happens when the
    oxidation/reduction reaction runs out of reagent. So, it isn't
    an electrical question at all, but a measure-chemical-components

    One way is to know the capacity and history of the cell and compute
    its remaining charge.

    Another is to observe changes in the anode or cathode or electrolyte.
    A auto battery can be evaluated in terms of the specific gravity of
    the battery acid; the density of the battery acid goes down as the
    battery discharges. And some rechargeable cells change their
    output impedance according to charge state - you can draw a little
    current and look at the voltage drop. Another variation, is to note
    that charge current causes charging of battery until it reaches full
    capacity, THEN the charge current just causes heating. A thermometer
    in the battery pack senses the temperature rise and stops the
    charge cycle.
  6. The "capacity" of a cell in mAh will vary with how and at what rate
    you discharge it, and to what voltage to choose to discharge it to.
    E.g. The advertised rated capacity of say a 2500mAh NiMH cell is only
    2500mAh under the discharge load conditions the manufacturer
    specifies. With any other load and rate it can vary.

    Charge gauge IC's are all the rage these days, they can measure the
    incoming charge (as you charge the cell from flat), and then measure
    the outgoing charge rate, so you'll have a pretty good idea of how
    much charge is left in the battery. Although it won't account for self
    discharge and other losses.

  7. neon


    Oct 21, 2006
    you answer your own question discharge at a constant rate into a load EMPTY that is not a good idea the battery if old may never recover again.
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