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Battery voltage as indicator of charge level

Discussion in 'Electronic Basics' started by [email protected], Dec 5, 2005.

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  1. Guest

    Is no-load voltage a reliable indicator of charge condition ? Are there
    universal voltage levels for a particular type of cell (e.g. NiCd,
    NiMH, Li-ion, lead-acid, etc.) that can be taken to indicate
    full-charge and full-discharge conditions ?

    There are gadzillion articles that discuss the general characteristics
    of various types of rechargeable cells, but I have not found any that
    specifically says that an X-type cell reaches Y Volts at full charge,
    and should not be discharged below Z Volts.

    Is it at all possible to specify voltage levels this way ? Temperature
    and internal resistance (and therefore charge and discharge currents)
    must play a part, but standard voltage levels under no-load,
    no-charging at standard room temperature would be very helpful.
     
  2. Bill Bowden

    Bill Bowden Guest

    For lead acid batteries, there are some no-load voltage vs capacity
    charts here:

    http://www.uuhome.de/william.darden/carfaq4.htm#load

    For li-ion, I believe full charge is 4.2 volts and should not be
    discharged below 3 volts. But don't know what represents 50% capacity.

    -Bill
     
  3. PeteS

    PeteS Guest

    Lead Acid battery noload terminal voltage can be used as an indicator
    of charge level (See other comment above). Other cells are not quite as
    easy.

    Li+. Fully charged : 4.2V per cell. Discharged 3.0V per cell (nominal).
    Nominal cell voltage: 3.7V. There are charts available, and maybe I'll
    dig out the links tomorrow at work. Fairly linear, but as with NiMH
    somewhat susceptible to ambient conditions.

    NiMH. 1.2V per cell. More difficult, but 1.1V is nominally discharged,
    1.3V (roughly) is fully charged. Fairly linear betwen these especially
    for the 20-8% range. Varies somewhat, see the manufacturers data
    sheets.

    NiCad. The noload voltage for a particular cell has zero bearing on the
    charge state of a NiCad. A discharged battery might read anywhere
    between 1.1V and 1.3V. The loaded voltage likewise has little bearing.
    I spent many years with these, and the only method I know that actually
    works is measuring the recovery time of the cell postload.

    Details: Measure the noload voltage. Load the cell at some arbitrary
    current (but well below the max load rating) for sufficient time for
    the loaded voltage to stabilise (perhaps 100millisec). Release the
    load. Measure the recovery curve (which is very close to a RC charge
    curve). The shorter the curve, the higher the charge. For the
    mathematical details, you'll have to wait until I dig out some books
    from 15 years ago (when I did those experiments and filed patent
    disclosure).

    So the general answer is --No--, there is no universal voltage for most
    cells. Indeed, we used to determine the charge of a lead acid battery
    by electrolyte specific gravity (that was quite a while ago).
    Not really very helpful - an indicator, nothing more. For anything
    approaching reasonable accuracy, noload voltage is not much use for
    most solid battery types.

    Cheers

    PeteS
     
  4. quietguy

    quietguy Guest

  5. not i

    not i Guest

    It depends what you are doing. Simply measuring no-load voltage is most
    likely not reliable on most batteries. However, you can probably get a
    crude state of charge by looking at the voltage. All battery chemistries
    will have their own unique voltage versus state of charge per cell. But
    again, in most cases the best you can hope for is just an estimate and if
    the battery isn't functioning properly the estimate could be *WAY* off.

    That said, if you are trying to charge a battery and want to know when to
    stop the correct algorithm varies wildly with battery chemistry, and you
    should never attempt to charge any battery without understanding what the
    correct behavior is for that algorithm. In particular, NiMH and Lithium
    based batteries require careful attention to proper charging or it can
    have... unfortunate... consequences. NiCd and lead acid are generally
    safer from that perspective.

    dan
     
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