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SYSTEM DESIGN
Systems Design is the process of defining the architecture,
components, modules, interfaces, and load data for a system
to satisfy specified requirements. For a solar system these
components are the PV modules, inverter/charge controller
& batteries, as well as the different interfaces of those
components.
To properly size a battery/battery bank for a renewable energy
system the following parameters are required:
Load
– Amount of DC current (Amps, Ah) or power (Watts,
Wh) a battery is required to supply to a DC load, AC load or
both through an inverter.
Time
– expressed in hours the battery will be required to
provide the load.
System Voltage
– DC system operating voltage
Ambient Temperature
– Average temperature of battery
room or enclosure.
Depth of Discharge (DoD)
– The proportion of energy that
has been removed from a battery; typically in a 24hr period
Example:
100% DoD is removing all
of the energy from a battery.
Autonomy
– Length of time, typically in days the PV battery
bank can provide energy to load without energy from PV
array, generator, grid, etc…
Design Margin
- Factor (typically expressed as a percentage)
to allow for future load additions.
A Renewable Energy Worksheet is provided in
Appendix A
listing the above requirements along with additional information
requirements
BATTERY OPERATION
There are several factors that affect the operation of the battery
concerning its ability to deliver capacity and life expectancy.
Storage
Cells should be stored indoors in a clean, level, dry, cool
location. Recommended storage temperature is 0°F to 90°F
(–18°C to 32°C).
Consult specific battery Installation & Operating manuals for
time interval & boosting requirements.
Temperature
Many chemical reactions are effected by temperature, and
this is true of the reaction that occurs in a storage battery.
The chemical reaction of a lead-acid battery is slowed down
by a lowering of the electrolyte temperature that results in
less capacity. A battery that will deliver 100% of rated
capacity at 77° F will only deliver approximately 65% of
rated capacity at 32°F.
At temperatures below 32°F (0°C) a battery can freeze depend-
ent on the DoD (Depth of Discharge). The higher the DoD, the
closer to 32°F (0°C) before the battery will freeze. The graph in
Appendix C should be consulted to verify the DoD of the bat-
tery / battery bank at the end of the discharge will not be sus-
ceptible to freezing in a particular application. If the electrolyte
would freeze, the internal damage would be irreversible requir-
ing the battery to be replaced.
Depth of Discharge (DoD)
Depth of discharge is a function of design. The deeper the
discharge per cycle, the shorter the life of the battery. A cycle
is a discharge and its subsequent recharge regardless of
depth of discharge.
Systems should be designed for shallow discharges. The
result of shallower discharges is typically a larger capacity
battery at prolonged battery life.
A Cycle vs. DoD chart should be consulted to determine
the number of cycles at a specific DoD and the projected life
in years the battery / battery system will provide prior to
needing replacement.
Charging
Majority of battery capacity/life issues can be traced to
improper charging. Improper charging settings may lead to
an overcharging or undercharging condition.
Typical Inverters/Charge Controllers charging lead-acid
batteries use 3 stage charging: Bulk, Absorption and Float
with an optional equalize stage.
See Appendix B for an
example of a typical 3-stage charging curve.
Inverter/Charge Controller Settings
Proper Inverter/Charge Controller settings are necessary
to ensure peak battery performance and life. All bulk,
absorption, float and equalize settings should be verified they
are within the battery manufacturers settings. These settings
are included but not limited to; voltage, current and time.
Consult individual battery Installation & Operating manuals
for inverter/charge controller setting recommendations.
Default settings should not be presumed to be correct.
For battery systems located in an uncontrolled temperature
environment, temperature compensation must be used.
Bulk
Current is applied to the batteries at the maximum safe rate
they will accept until voltage rises to near (80-85%) full charge
level. The battery voltage rises because the charging current
that is provided by the battery charger is replenishing its
Renewable Energy applications that depend on battery power as part of the system operation must be at
maximum performance at all times. To ensure this high rate of performance is achieved, the battery charging
system must be set properly. A battery/battery bank that is undercharged or overcharged will affect the
battery system performance & life, as well as the performance of the entire system.
Key factors that affect a batteries ability to provide the capacity and long life that is expected are: System
Design, Storage, Temperature, Depth of Discharge (DoD), Charging and Maintenance.
