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A Study of Lead-Acid Battery Efficiency Near Top-of-Charge

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  • A Study of Lead-Acid Battery Efficiency Near Top-of-Charge

    A Study of Lead-Acid Battery Efficiency Near Top-of-Charge and the Impact on PV System Design
    John W. Stevens and Garth P. Corey
    Sandia National Laboratories, Photovoltaic System Applications Department
    Sandia National Laboratories, Battery Analysis and Evaluation Department
    PO Box 5800, MS 0753
    Albuquerque, New Mexico 87185-0753

    Efficiency Near Top of Charge.pdf

    A test procedure has been developed to allow the
    examination of battery charge efficiency as a function of
    battery state of charge. Preliminary results agree well
    with established general understanding that the charge
    efficiency of flooded lead-antimony batteries declines with
    increasing state-of-charge, and that charge efficiency is a
    non-linear function of battery state-of-charge. These
    tests indicate that from zero SOC to 84% SOC the
    average overall battery charging efficiency is 91%, and
    that the incremental battery charging efficiency from 79%
    to 84% is only 55%. This is particularly significant in PV
    systems where the designer expects the batteries to
    normally operate at SOC above 80%, with deeper
    discharge only occurring during periods of extended bad
    weather. In such systems, the low charge efficiency at
    high SOC may result in a substantial reduction in actual
    available stored energy because nearly half the available
    energy is serving losses rather than charging the battery.
    Low charging efficiency can then result in the battery
    operating at an average SOC significantly lower than the
    system designer would anticipate without a detailed
    understanding of charge efficiency as a function of SOC.
    During normal weather, capacity degradation will not be
    evident, but it will manifest itself when the battery is
    called on to provide the full purchased capacity, which
    will be found to be unavailable. Extended operation in a
    low SOC environment can also result in permanent loss
    of capacity from sulfation if the battery is operated for
    long periods of time without a sufficient recovery or
    equalizing charge.

  • #2
    Pretty old news around here and has been posted and debated here many times. The article you linked too is a very old study and does not apply to today's batteries. When the article was written there was not many batteries made for RE systems. Today there are quite a few batteries made for RE systems. It also assumes using a 3-stage charging algorithm which is not compatible with Solar charging systems because there are not enough hours in a day to complete the cycle.

    Their test model was extremely flawed using a Trojan 30XHS 100 AH battery charged at a painfully slow C/30 rate, or 3.3 amps. No solar PV system would ever be designed to operate at C/30. IEEE-455 committee completely rejects all the test results because they are not real world test. That is what happens when academics try to do real things. They have no real world application or experience.

    All of the negative effects of using lead acid batteries can easily be overcome with proper design, battery/equipment selection, and commissioning of the system.
    MSEE, PE