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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.
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.
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