While the advantages of limiting the number of series strings required to meet a system’s design capacity are many, some integrators are resistant to designs where the entire battery bank is composed of a single series string. Their concern? The lack of redundancy in the event of a cell failure. For large battery systems, some designers prefer to use two seriesconnected strings in parallel to provide redundancy. Specifying two strings is a good idea for sites where redundancy is absolutely critical, such as a telecommunications facility. Two paralleled strings allow one string to be disconnected for maintenance or testing while the system continues to operate on the other half of the battery bank.

However, this concept is somewhat flawed when used in systems where redundancy is not as critical, such as most residential applications. While two strings do provide redundancy, this approach also doubles the number of connections that could potentially corrode, cells to water and check regularly (if flooded batteries are specified) and cells that might potentially fail. The system design is compromised to eliminate the possibility of a failure event that may not occur—and if it does occur, that the backup generator could mitigate until the battery is serviced. In addition, two-string banks are typically designed to require the full capacity of both strings of batteries to limit the DOD. With only one string operational, the system would limp along with poor system performance. This would significantly impact battery life if it were allowed to persist. True redundancy would require specifying each of the two battery strings to meet the full required capacity, which would double the cost of the battery bank and is typically not a practical option.

For most applications, a better option to limit the impact of a cell failure is to use a single string of 2 V cells. If a cell failure does occur, one course of action is to replace that cell. This may not be a sound option, depending on the age and condition of the battery pack. Replacing a cell in a battery bank that has been in service for several years may cause severe imbalances to develop later. A second option is to operate the system on fewer cells (such as 23, 22 or 21 cells) by adjusting the setpoints of the system. The most recent SMA Sunny Island inverters allow for the easy adjustment of a single nominal voltage setpoint from 48 to 46, 44 or 42 Vdc, that automatically changes the other required setpoints accordingly. This feature saves a significant amount of labor compared to reprogramming every required setpoint manually and also ensures that no setpoint is accidentally missed.

Another failure mitigation approach to consider is that some inverters have voltage setpoints that can be adjusted high enough to allow installation of an additional 25th 2 V cell in series at the time of installation. If a failure occurs, a cell can be removed without having the battery bank’s capacity drop below its initial design capacity. In many cases, this approach is an option only when using sealed AGM or Gel batteries because they have lower charge-voltage parameters than flooded cells.

While the advantages of limiting the number of series strings required to meet a system’s design capacity are many, some integrators are resistant to designs where the entire battery bank is composed of a single series string. Their concern? The lack of redundancy in the event of a cell failure. For large battery systems, some designers prefer to use two seriesconnected strings in parallel to provide redundancy. Specifying two strings is a good idea for sites where redundancy is absolutely critical, such as a telecommunications facility. Two paralleled strings allow one string to be disconnected for maintenance or testing while the system continues to operate on the other half of the battery bank.

However, this concept is somewhat flawed when used in systems where redundancy is not as critical, such as most residential applications. While two strings do provide redundancy, this approach also doubles the number of connections that could potentially corrode, cells to water and check regularly (if flooded batteries are specified) and cells that might potentially fail. The system design is compromised to eliminate the possibility of a failure event that may not occur—and if it does occur, that the backup generator could mitigate until the battery is serviced. In addition, two-string banks are typically designed to require the full capacity of both strings of batteries to limit the DOD. With only one string operational, the system would limp along with poor system performance. This would significantly impact battery life if it were allowed to persist. True redundancy would require specifying each of the two battery strings to meet the full required capacity, which would double the cost of the battery bank and is typically not a practical option.

For most applications, a better option to limit the impact of a cell failure is to use a single string of 2 V cells. If a cell failure does occur, one course of action is to replace that cell. This may not be a sound option, depending on the age and condition of the battery pack. Replacing a cell in a battery bank that has been in service for several years may cause severe imbalances to develop later. A second option is to operate the system on fewer cells (such as 23, 22 or 21 cells) by adjusting the setpoints of the system. The most recent SMA Sunny Island inverters allow for the easy adjustment of a single nominal voltage setpoint from 48 to 46, 44 or 42 Vdc, that automatically changes the other required setpoints accordingly. This feature saves a significant amount of labor compared to reprogramming every required setpoint manually and also ensures that no setpoint is accidentally missed.

Another failure mitigation approach to consider is that some inverters have voltage setpoints that can be adjusted high enough to allow installation of an additional 25th 2 V cell in series at the time of installation. If a failure occurs, a cell can be removed without having the battery bank’s capacity drop below its initial design capacity. In many cases, this approach is an option only when using sealed AGM or Gel batteries because they have lower charge-voltage parameters than flooded cells.

While the advantages of limiting the number of series strings required to meet a system’s design capacity are many, some integrators are resistant to designs where the entire battery bank is composed of a single series string. Their concern? The lack of redundancy in the event of a cell failure. For large battery systems, some designers prefer to use two seriesconnected strings in parallel to provide redundancy. Specifying two strings is a good idea for sites where redundancy is absolutely critical, such as a telecommunications facility. Two paralleled strings allow one string to be disconnected for maintenance or testing while the system continues to operate on the other half of the battery bank.

However, this concept is somewhat flawed when used in systems where redundancy is not as critical, such as most residential applications. While two strings do provide redundancy, this approach also doubles the number of connections that could potentially corrode, cells to water and check regularly (if flooded batteries are specified) and cells that might potentially fail. The system design is compromised to eliminate the possibility of a failure event that may not occur—and if it does occur, that the backup generator could mitigate until the battery is serviced. In addition, two-string banks are typically designed to require the full capacity of both strings of batteries to limit the DOD. With only one string operational, the system would limp along with poor system performance. This would significantly impact battery life if it were allowed to persist. True redundancy would require specifying each of the two battery strings to meet the full required capacity, which would double the cost of the battery bank and is typically not a practical option.

For most applications, a better option to limit the impact of a cell failure is to use a single string of 2 V cells. If a cell failure does occur, one course of action is to replace that cell. This may not be a sound option, depending on the age and condition of the battery pack. Replacing a cell in a battery bank that has been in service for several years may cause severe imbalances to develop later. A second option is to operate the system on fewer cells (such as 23, 22 or 21 cells) by adjusting the setpoints of the system. The most recent SMA Sunny Island inverters allow for the easy adjustment of a single nominal voltage setpoint from 48 to 46, 44 or 42 Vdc, that automatically changes the other required setpoints accordingly. This feature saves a significant amount of labor compared to reprogramming every required setpoint manually and also ensures that no setpoint is accidentally missed.

Another failure mitigation approach to consider is that some inverters have voltage setpoints that can be adjusted high enough to allow installation of an additional 25th 2 V cell in series at the time of installation. If a failure occurs, a cell can be removed without having the battery bank’s capacity drop below its initial design capacity. In many cases, this approach is an option only when using sealed AGM or Gel batteries because they have lower charge-voltage parameters than flooded cells.