short. Electrolyte gassing at the battery terminals causes corrosion. Higher tem-
peratures accelerate corrosion. Corrosion shortens battery life.
Gassing happens when a battery is charging and is almost fully charged. Cell
voltage rises dramatically at this point. If the battery is sealed, the gas stays in the
cell until it reaches a predetermined pressure. If exceeded, the cell is vented, and
the gas and water are released. Gassing uses part of the charge current and reduces
the battery’s efficiency.
Gassing is necessary during equalization in flooded batteries. The equaliza-
tion charge allows gassing to agitate the electrolyte in the battery and prevents
stratification. It also knocks potentially loose lead sulfate crystals from the plates.
This keeps them from bridging the gap between positive and negative, therefore
discharging and shorting the battery. Battery performance declines when electro-
lytes are not mixed. Battery life decreases if over-gassing occurs.
The maximum voltage a charge controller allows a battery to achieve also
controls gassing. The risk for gassing increases with the voltage the batteries
reach. Charge controllers allow high rates of charging up to the gassing point
under normal circumstances. When the gassing point is reached, the charge con-
troller disconnects or dramatically reduces the PV current. Disconnecting pre-
The exception to this is during the planned process of providing an equaliza-
tion change. Equalization changes are for battery maintenance.
Gassing is caused by other factors as well. An increase in temperature can
cause an increase in gassing. Temperature and gassing effects determine which
controller should be chosen.
Battery System Design and Selection Criteria
There are many battery types to choose from when selecting the battery type for a
PV system. When choosing a battery, consider the PV system’s application.
Reviewing manufacturers’ literature and gaining field experience helps narrow
Use the following list of considerations to help narrow the choices of batteries:
System type and mode of operation
Characteristics of charging, such as charging profile
Autonomy issues (required hours or days of energy storage)
Amount of discharge current needed and its variability
Allowable maximum discharge depth
Daily depth of discharge requirements
Accessibility of location
Environmental conditions, including humidity, cold and hot temperatures
Cyclic life calculated in years (1,000 cycles is about 2.7 to 3 years, depending
CHAPTER 6 Energy-Storage Device (ESD) System Design 107