The Legendary Sandia National Laboratories ... Speaks

BillBlake
BillBlake Solar Expert Posts: 49 ✭
in Off Grid Solar & Battery Systems #1
"A Study of Lead-Acid Battery Efficiency Near Top-of-Charge

and the Impact on PV System Design"

<snip> 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.

<snip> The impact of low charge efficiency at high states of
charge has the greatest potential impact on systems
where high energy availability is needed.

Such systems usually utilize large batteries to ensure energy

availability during the longest stretches of bad weather.


This may not provide the energy required if the PV array is
insufficient to provide a recovery charge for batteries at
90% SOC and above, where charge efficiency is very low.


Charge efficiencies at 90% SOC and greater were
measured at less than 50% for the battery tested here,
requiring a PV array that supplies more than twice the
energy that the load consumes for a full recovery charge.


Many batteries in PV systems never reach a full state of
charge, resulting in a slow battery capacity loss from
stratification and sulfation over the life of the battery.

<snip> Notice also that the overall
efficiency shows high values, with full charge represented
by approximately 85% efficiency, a commonly used value
for battery charge efficiency.


More importantly, notice the dramatically lower efficiencies

for the increments above about 80% state of charge,

where most values are below 60% efficiency,

and full charge is represented by less than 50% efficiency.

(Actually, full charge, resulting in 100Ah output has not been

reached in the testing to date.
The greatest output was 96.5Ah, which resulted from
116Ah input. An attempt to achieve 100Ah output will be
made as part of the conclusion of this testing.)

<snip> It is generally understood that battery charge
efficiency is high (above 95%)


at low states of charge and that this efficiency drops off near

full charge.

However,actual battery charge efficiencies are often stated as
though efficiency is linear across all states of charge, with
general guidance that it drops off at higher states of
charge. Details concerning actual charge efficiency as a
function of state-of-charge (SOC) would be very useful to
PV system designers ......

<snip> INTERMEDIATE FULL CHARGE CYCLES

An observation early in the testing required a change
in the test procedure. The original intent had been to
perform several partial charge/discharge cycles in
sequence. For example, charge to 68Ah input, discharge,
then charge to 68Ah input and so on until the four
complete cycles at 68Ah input were complete.


Then fully charge and discharge the battery before proceeding

with the next level.

It was seen early in the testing that this was not going to work,

as the capacity resulting from 68Ah input dropped with each

succeeding cycle when no full charge cycles were performed between

partial charge cycles.

Therefore a full charge and discharge cycle was
added between each partial charge/discharge cycle.

This result has important implications to operational
PV systems. That is, if a battery is partially charged for
several consecutive cycles (for example, the array is
marginally sized and there is a series of less than full sun
days in the winter) the useable battery capacity decreases
each cycle, even though the same amount of energy has
been presented to the battery each day.

This is the result of battery inefficiencies,

electrolyte stratification,

and sulfate buildup

during these partial charges.

An associated full charge, with its attendant gassing,

is needed to destratify the electrolyte and remove the
residual sulfate.

This sulfate buildup can become a problem if this pattern

continues for several months.

In the short term it can be reversed by a full "equalizing"
type charge, which,

in most cases is not possible in small PV systems.

Battery equalization requires a PV charge controller that has been

specifically designed to include this function.

At low charge rates (for example, less than C/40)

equalization may not be possible because of charging time limitations.

In any case, this reduction in
useable capacity will impact availability and should be understood.

http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf

<snip from my esteemed research partner>


The benefit of "Finish Charging" Looms
in many corridors.

Tying up a Generator or Solar Panels to try and struggle a charge
into something that no longer has it's arms open
may be old news thinking one of these days.
Best to tie the Panels up at a HIGH acceptance Level.

Many people have different numbers but once in a while you see some
dramatic numbers. That Zapp Works Ni-Fe Spec. page shows self discharge
numbers that rival the finest high tech AGM Batteries.

These fellows talk of 20 - 40% per month?

Who is the closest?

http://www.mpoweruk.com/specifications/comparisons.pdf

Another important Cornerstone Report leading up to new conclusions.


Bill Blake
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