BATTERY SELECTION FOR OFF-GRID SYSTEMS
Choosing the appropriate battery for the application is key to long battery life and performance. Once the load is calculated, a battery bank should be selected to meet the system design as well as installation requirements. Flooded lead-acid models using lead-antimony plates require frequent cycling to maintain capacity and prevent sulfation buildup.
Off-grid Renewable Energy systems are typically sized to operate between 20% and 50% DOD. Deep cycle batteries should be used to a maximum 50% DOD as this offers a balance between capacity vs. cycle life, also taking into consideration the cost of replacement.
Lead-Calcium plates used in VRLA AGM and GEL models are more tolerant and better suited to float applications where the battery may not be cycled as often. If a battery bank of a specific capacity is needed, it is important to select a battery model which offers sufficient capacity, without oversizing, and also minimizes the number of parallel strings required to accomplish the desired voltage and capacity.
Systems with multiple parallel strings will often experience an imbalance of charge. These banks will also require additional maintenance as this increases the number of terminal connections requiring cleaning as well as the number of Flooded cells to inspect, test and water. When a charge imbalance is not addressed through adjustments in charge time and corrective Equalizations this will lead to capacity loss and premature battery failure.
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 Renewable Energy System, the 20 hr AH rate (C/20) 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 1-2 days of autonomy due to the cost of batteries versus 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 maintained at a full charge. By holding at a 100% SOC this will extend the overall life of the battery bank. For float applications such as backup systems, it is important to discharge and charge the battery bank periodically to exercise the battery bank as this will prevent stratification and sulfation buildup.
Backup systems are often sized to meet the load requirement at the time of install. It is important to note that as load profiles change this will also affect how quickly battery capacity will be reduced.