Installation Manual

31
BATTERY SELECTION
Choosing the appropriate battery for the application is key to long battery life
and performance. Once the load is determined a battery bank should be selected
to meet the system design. If a battery bank of a specific capacity is needed, it
is important to select a battery model which offers sufficient capacity, but also
minimizes the number of parallel strings required to accomplish the desired
voltage and capacity.
Systems with multiple parallel strings of batteries will often experience an
imbalance of charge. These systems will also require additional maintenance as
this increases the number of terminal connections requiring cleaning as well as
the number of cells to water. When charge imbalance is not addressed through
adjustments in charge time and periodic equalizations this will eventually lead to
premature failure of the battery bank.
Determining the capacity requirement for off-grid systems is done by completing
an audit of energy consumption based on actual load requirements. The load is
calculated by the total amount of power necessary to support the load for a 1 day
period and then factors in how many days it may be required before recharging.
In a typical Renewable Energy system the 20 hr AH rate (C20) is used when sizing
systems to cover three days of autonomy or less. The 100 hr AH rate (C100) is
used when designing systems to cover more than 3 days of autonomy.
Most systems are designed for a 1-2 day rate, due to the cost of batteries verses
the cost of adding a generator and/or additional renewable energy sources. This
is also most typical for solar applications as these systems allow the battery bank
to be charged each day.
Deep cycle batteries will perform best when they are maintained at a full charge.
By holding at a full state of charge this will extend the overall life of the battery
bank. For float applications, such as backup systems, it is also important to use
the battery bank on occasion as this will prevent stratification and sulfation.
It is important to be aware that as load profiles change this too will affect how
quickly battery capacity will be reduced. Often a backup system is sized to meet
the load requirement at the time of install, but this demand will increase with
every small addition to the load.