Although batteries are now made of different materials and use different electrolytes, one thing hasn't changed — batteries need regular maintenance to function properly. At the Bureau of Reclamation's Morrow Point hydroelectric power plant, we used to spend about 104 hours or $5,200 annually on battery maintenance. This included weekly and monthly checks of cell voltages and their specific gravity readings. Once a year, we removed, cleaned, lubricated, and reinstalled all of the intercell connections. It took the installation of a battery monitoring system to make us realize that our approach toward battery cell management was less than optimal.
The battery at Morrow Point is a 58-cell, 420Ah unit that operates on float-only charge at 130.5VDC. The Bureau of Reclamation's research group recommended the battery monitoring system, which was developed by MCM Enterprise Ltd. (now sold by Serveron Corp.) through an EPRI research project. Plant engineers chose the system because it:
- Provides long-term trends on individual battery parameters.
- Doesn't intrude on the battery case.
- Includes hardware and software that is compatible with Windows NT operating systems.
- Provides cell voltage, cell-specific gravity, and bank voltage readings.
- Tests the integrity of cell-to-cell connections.
- Monitors the amount of cell float current necessary to maintain a uniform charge on each cell.
- Gives the direction and magnitude of the bank current.
- Automatically monitors the bank during discharge.
The monitoring system includes two sensing modules attached to the side of each battery jar with self-adhering tape (see Photo 1). In our case, a wire gutter was attached to the jars to contain the cell-to-cell data links and other wires. The photo also shows the measuring leads attached to the bus bars and the chokes attached to the main battery lead.
Photo 2, on page 24, shows the communications interface (with a built-in modem) mounted on the wall, along with the bank monitor/system control box. It also shows the sensor mounted at Cell No. 1 and the rest of the hardware. The overall system is shown schematically in Fig. 1, also on page 24.
When the monitoring system was set up, connections were installed between the battery posts of each bus. These connections measure cell voltage and voltage drop across each cell when a 1 ms bank load is applied for a current path integrity test.
Current path integrity tests employ the 4-wire resistance technique. To prepare for the test measurements, the bank monitor simultaneously instructs all cell monitors to measure their voltages and store the results in memory. The bank monitor then passes an electronic load across the bank, and the load draws current for 1 ms. At the peak of the bank current, which is arranged to be roughly equal to the amp-hour capacity of the bank, the bank monitor instructs the cell monitors to remeasure the cell voltages and subtract the results from the zero-current voltage values already stored in memory. The voltage difference for each cell, reported as “delta V,” is proportional to the resistance of the current path and includes both the cell and strap/post connections. Fig. 2 shows the relationship of the voltages and currents during this test.
Around each main battery lead is a choke (isolation inductor). The data links between the cell monitoring modules are optically isolated to eliminate the possibility for a current path to exist between modules. The chokes provide test isolation for approximately 1 ms during current path integrity tests.
After the installation was complete, engineers took a complete set of readings on the battery, and the data (entered into the software) became the starting point for future monitoring.
On the Alert
Much to our surprise, the monitoring system started to report problems within days of putting it into service. The first deficiency it found was high resistance at several intercell-link and battery-post connections. We had previously taken resistance readings on the connections without finding any problems. The monitoring system, however, takes voltage-drop connection readings while a 400A load is applied for 1 ms during current path integrity tests. This method found poor connections that we had missed.
The monitoring system's software allows the user to view the battery unit cell by cell or by the average, high, or low cell in each category.
If you look at Fig. 3, on page 25, you can see the trends of Cell 1 over a period of time. Interestingly, there is a gradual rise in the voltage drop across the cell's connection. With every voltage drop alarm, our electricians found that the connecting bolts tightened before the torque wrench reached the desired value.
It's also interesting to note that, just prior to developing a high-resistance connection, the battery underwent a load cycle, as indicated by the downward spike in the readings.
After several months of operation, MCM personnel reviewed the data taken up to that point. They noticed that the battery's water consumption was fairly high. The float current bypass cell readings indicated that all of the cells were bypassing 100% of the float current. In simpler terms, the battery was overcharging, making it use more water. Once engineers lowered the float voltage until most of the cells read in the 80% bypass range, water consumption decreased (see Fig. 4, on page 25).
Fig. 4 also shows the high, low, and average readings for all of the Morrow Point cells. In this figure, notice that the fluid levels decrease. The straight line shows where water was added. You'll also find that the slope of the water-use curve changes at the place where the bank voltage is lowered and the cell-bypass currents drop.
The monitoring system performs a test on the battery however often the user desires. At Morrow Point, we've set it up to automatically do a test and collect data every 24 hours. This gives us good data without building a huge database. However, during load tests on the battery or actual emergency events, the monitoring system steps up its data measurement rate to capture the battery performance during the event. Once a week, we download the data to a computer, review it, and take any necessary actions.
The monitoring system cost nearly the same as the 58-cell battery unit. To justify this cost, we had to take a close look at our battery unit and consider its function and how important it was to the overall power system.
Not all battery systems are used for the same purpose. However, most power plants and substations employ them as part of their control and protection schemes for large rotating equipment and breakers.
For the most part, the DC control and protection circuits in Reclamation Bureau facilities run on battery chargers most of the time. We need the battery unit only under charger failure or loss of station AC power.
Oftentimes, as Murphy's Law dictates, station AC disturbances will occur during times of system or equipment problems. These are the times when controls, protection devices, and breakers must operate properly to prevent damage. If battery systems fail during these periods, the consequences can cost facility owners millions of dollars. Therefore, it makes sense to base the justification for a monitoring system on the cost of these consequences, rather than comparing the cost of the battery to the cost of the monitoring system.
At Morrow Point, the battery monitoring system has drastically reduced maintenance man-hours. Since the monitor's installation, engineers have corrected several bad connections, and the battery bank has had water added only twice. Manual readings taken one year after the installation were identical to the system's readings.
In addition, we've concluded that some of the maintenance done previously actually caused some problems. For example, redoing the jumper bar connections most likely ended up loosening more connections instead of repairing possible corrosion and tightness problems.
With the battery monitoring system, we know our battery unit will operate properly when it's needed most. That, along with the reduced maintenance time, makes the cost of the system easily justifiable.
This article is based on a paper that was presented at the EPRI Substation Equipment Diagnostics Conference in Feb. 2002.
Gerald W. McDermott is a power plant supervisor at Morrow Point in Montrose, Colo. You can reach him at firstname.lastname@example.org. For information on the Battery and Cell Management System, contact Serveron Corp. at email@example.com or 1-800-880-2552, or visit www.serveron.com.