NUREG/CR-7148, BNL-NUREG-98563-2012, Confirmatory Battery Testing: The Use of Float Current Monitoring to Determine Battery State-of-Charge.
To ensure that a battery has the capability to execute its safety function, it is necessary to confirm its fully charged condition and operational readiness. For the past three decades the typical nuclear power plant Technical Specifications required the measurement of specific gravity to determine the state-of-charge of the batteries. This requirement was based on RG 1.129 Rev.1, “Maintenance, Testing, and Replacement of Large Lead Storage Batteries for Nuclear Power Plants,” which endorsed IEEE Std. 450-1975, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Large Lead Storage Batteries for Generating Stations and Substations." The more recent version of this standard, IEEE Std. 450-2002, that was endorsed by the NRC, suggested that either float current or specific gravity could be used for determining a vented lead-calcium battery’s state-of-charge. This report describes the project to validate this approach on batteries that are used in the nuclear industry.
In conducting this study, three sets of nuclear qualified batteries were procured from three different battery vendors. Each battery set consisted of 12 battery cells. These cells are the same models that are typically used in a Class IE dc system application. Two suitably sized battery chargers and a load bank were also obtained; the second battery charger was used to maintain the batteries not being tested on a continuous float charge. The test setup was as close as reasonably practicable (not including seismic battery racks) to a typical nuclear power station’s Class 1E battery design. Once the battery was fully charged and stabilized, a 4-hour discharge test was performed based on the battery vendor’s specifications. After the discharge test, the float current was continuously recorded while the battery was recharged. During the recharge, periodic specific gravity measurements were taken at the vendor-specified location (about 1/3 down the length of the cell) for all of the cells, and at the top and bottom as well for two of the cells. The three measurements taken on two of the cells allowed us to obtain a vertical profile representation of the electrolyte’s distribution within the cell during the entire discharge-recharge cycle. Discharge test current and specific gravity readings were compensated for temperature as discussed in IEEE Std. 450-2002.
The major findings that were derived from more than thirty cycles of deep discharge testing are:
1) Both float current and specific gravity provide adequate means to determine battery state-of-charge. Float current has an advantage in that it provides an indicator of the entire battery string, while specific gravity is measured on a cell by cell basis.
2) Both float current and specific gravity have similar response times when the battery is recharged. Generally speaking, 100% of the ampere-hours discharged are returned to the battery within 24 hours of the start of the recharge cycle.
3) The amount of electrolyte stratification is significant following a performance test and it takes months before equilibrium is reached within the cells. Therefore, it is critical to measure specific gravity at the correct point as indicated by the battery vendor’s manual and supported by IEEE Std. 450-2002.
4) The use of pilot cells to ascertain specific gravity is supported by the consistent response observed among all cells during both discharge and recharge.
5) Measuring float current through the use of a simple shunt connected to a data acquisition system provides accurate and repeatable measurements. We used a 200 amp (A), 50 millivolt (mV) shunt placed in series with the output of the battery charger. We found that a more sophisticated device based on the principles of the Hall Effect (similar to a clamp-on ammeter) is also effective, but it is less accurate at the low ends of the float current range (< 2 amps).
Using a standard length of tubing to draw the electrolyte from the same point resulted in consistent “trendable” specific gravity data.
• Temperature compensation for capacity testing, specific gravity readings and conductance readings is important. If not performed properly, the data will be skewed.
• Float current response will vary based on the recharge voltage applied to the battery. However, regardless of the voltage applied during recharge, the float current of a nearly fully charged battery becomes stable at less than two amps.
• Calculations of the ampere-hours returned to the battery during recharge can be used to verify the battery’s state-of-charge. The majority (>60%) of the ampere-hours returned to the battery occurs while the battery charger is still in a current limit mode.
IEEE Std. 450-2002 contains the following criterion related to return to service for a battery:
“When the charging current has stabilized at the charging voltage for three consecutive hourly measurements, the battery is near full charge.” Our test program also verified the point where the battery can be safely returned to service. In a series of six additional tests (two tests per battery string), the battery strings were able to meet their capacity and capability requirements at the point where the float current was stable for three hours. Thus the criterion used in IEEE Std. 450-2002 was found to be an acceptable practice for ensuring the capacity and capability requirements of the battery were met before returning it to service.
Similarly, three cycles of tests were performed in which each battery was returned to service when the float current reached the value equivalent to three time constants on the recharge/float current curve. This occurred within about twelve hours and at a higher current than the previously described return to service tests. In each case, the battery was also able to meet its capacity and capability requirements. This calculated float current value obtained from the battery-specific recharge/float current curve may be a more practical method for returning the battery to service at the point where it is capable of meeting its capacity and capability requirements.