ABB CPS6000 Product Manual page 180

Battery Voltage Imbalance Detection
This feature requires the use of the ES771A Remote Voltage Monitoring Module. Note that this feature
is automatically enabled if the controller detects the presence of the ES771A module. This module is to
be placed in the electronics cabinet and utilizes the QS873A VT- Probes to measure the voltage of the
battery string being monitored. The VT-Probe is to be placed on a battery terminal in the middle of the
battery string. The CPS6000 controller has data on the plant voltage; the half-string voltage measured
from each monitored battery string is compared to the plant voltage minus the measured half-string
voltage. If the comparison results in a difference of greater than 1.7V for longer than 24 hours, the alarm
is asserted. The alarm may be retired by initiating the Clear Events command from the front panel.
After this feature has been enabled, the system waits for 12 hours to ensure the battery strings are
stable. A battery string is considered to be stable if the charging current is less than 3A. If a stable
battery string yields a difference measurement greater than 1.7V for over 12 continuous hours, a Battery
Voltage Imbalance alarm is generated.
When the alarm is issued, the Float Set-Point (fsp) and the Battery on Discharge (BoD) threshold values
are reduced by 1/24th. Once the alarm has been cleared by the CLE command, the plant reverts to its
normal fsp; however, the BoD threshold is maintained at the new threshold for up to four minutes
before reverting back to the old threshold. This is done to ensure the batteries have had enough time to
charge up to the nominal fsp and to prevent any spurious BoD alarm conditions.
This feature may be used by customers as part of their overall battery maintenance program in
determining the health of battery strings. A voltage imbalance of 1.7V between half-string voltages may
mean a shorted-cell somewhere in the string. A service person should be sent out to the site and
determine if the string should be replaced.
Battery Recharge Current Limit
This feature must be enabled from the front panel. The battery recharge current limit feature enables
the QS840A controller to limit the recharge current flowing into a battery section during the charge
cycle. The recharge current flowing into the battery section can be limited to any value between 5A and
100A.
Note that this feature will not have any impact on the current being delivered to the load. Further, there
will be no effect on the discharge current flowing from the battery strings to the load during an ac fail
condition. The controller will maintain the recharge current within 10% of the set level.
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