Acceptance Inspection and Testing of New Battery Installations
To ensure the reliability of and protect the investment in a battery system, it is important that the battery system receive a proper acceptance inspection and test prior to being commissioned into service. A properly specified and performed acceptance inspection and test on a new, installed battery system by trained and qualified personnel will optimize battery life and performance, thereby increasing the reliability of the dc power system and the equipment it is servicing.
Acceptance inspection and testing of new battery installations is one of the most important steps of any new battery installation for the following reasons:
Provides verification that the battery purchased can perform to the battery manufacturer’s specification.
Provides verification the battery system meets all procurement specification requirements, including the design requirements for which it was sized.
Provides verification of proper installation.
Provides baseline data of the vital battery parameters for future reference, trending, and evaluation of battery performance.
Provides the necessary documentation for any possible warranty claims against the battery manufacturer.
Acceptance Inspection and Test Requirements
This section does not include all the requirements in an acceptance inspection and test program. It does, however, address key issues regarding acceptance testing.
I. Receipt Inspection
The acceptance inspection and testing of a new battery generally begins with the receipt inspection of the battery once it arrives at the site and prior to installation. At this point it is important to verify that the number of cells, battery type, and model specified is what was received. A detailed inspection of each cell should be made to confirm that the cells did not incur any damage during shipping. An inventory of all battery accessories that are identified in the procurement specification such as required inter-cell connections, inter-tier and inter-rack cable connections, terminal plates, terminal post connection hardware, and flame arresters (for vented cells only) should be made. One should also account for all specified battery rack or cabinet materials. This will also aid the battery installation process. All battery documentation (records and instruction manuals) provided by the battery manufacturer should be reviewed at this time.
II. Following Completion of the Battery Installation and Prior to Performance of the Initial Charge
Verify Proper Polarity Connection of Cells
The battery open-circuit terminal voltage should be measured to ensure that the individual cells are connected correctly. The total voltage should be approximately equal to the number of cells in the string multiplied by the open circuit voltage of one cell. If the battery open circuit voltage is less than expected, then each cell should be checked for correct polarity connection. A good rule-of-thumb for determining expected open circuit voltage of a single lead-acid cell is to add 0.845 to the nameplate nominal specific gravity of the cell. For example, a cell that has a nameplate nominal specific gravity of 1.215 should have the following open circuit voltage:
Verify Inter-cell Connection Integrity
The resistance of all inter-cell connections should be measured and recorded using a digital low-resistance ohmmeter (DLRO). The battery manufacturer should be consulted to determine acceptable values. IEEE Std. 484-1996 recommends that each connection should be no more than 10 percent or five microhms which ever is greater, over the average for each type of connection (i.e., inter-cell, inter-tier, inter-rack). IEEE Std. 484-1996 also provides excellent guidance in Annex A for performing inter-cell connection resistance measurements. Performance of inter-cell connection resistance measurements as part of the acceptance test verifies adequacy of the installation and provides baseline data for future maintenance requirements. A visual inspection should be made of all terminal posts and connection contact surfaces to verify that a thin film of manufacturer’s approved corrosion-inhibiting grease has been correctly applied. Correct application of the corrosion-inhibiting grease will help reduce the possibility of corrosion at the terminal posts over the service life of the battery.
Verify Electrolyte Levels (Vented Cells Only)
A visual inspection should be made of all cells to verify that electrolyte levels are acceptable and that there are no cell container leaks or container-to-cover leaks. Approved water should be used to adjust electrolyte to acceptable levels. Verify Battery Charger/Rectifier Float and Equalize Voltage Settings The float and equalize voltage outputs of the battery charger/rectifier should be set in accordance with the battery manufacturer’s recommended values.
III. Following Completion of the Initial Charge
The following measurements should be performed on each cell as part of the acceptance test. After completion of the battery manufacturer’s recommended initial charge, one should wait a minimum of 72 hours for lead-acid and 24 hours for nickel-cadmium batteries before performing the measurements.. The battery should be on float charge during performance of these tests:
Battery Float Current
Battery string float current should be measured and recorded. In the past, electrolyte specific gravity had been the accepted method to verify lead acid battery state of charge. However, float current has now become a more accurate and accepted measurement of state of charge. In addition, float current can be measured on a VRLA where specific gravity can not. Baseline float currents on a new, installed lead-acid battery can provide very useful information because as a battery ages and degrades, float current will in-crease. Table 1 provides float current demands of fully charged lead-acid cells.
IV. Acceptance Load Test
The acceptance load test is a capacity test performed to determine if the battery is capable of supplying the manufacturer’s rated discharge current for a given duration under a specific set of conditions. The battery is discharged at a constant current or power to a pre-defined end of discharge voltage. The actual discharge time is then compared to the manufacturer’s rated discharge time to determine actual capacity.
Acceptance Load Test Criteria
The acceptance load test should be performed a minimum of 72 hours (for lead-acid) and 24 hours (for nickel-cadmium) after the completion of the initial charge on the battery and after the electrical parameters of the battery (i.e., cell float voltage, specific gravity, internal ohmic) have been verified. The battery must be isolated from the battery charger/rectifier and any external load prior to the start of the test. A constant current or constant power dc load bank is connected to the main battery terminals to simulate the load. Voltage monitoring through a data acquisition system is connected across the main battery terminals and each cell to continuously monitor battery voltage and individual cell voltage during discharge. Recording of individual cell voltage during discharge allows for evaluation of the performance of each cell in the battery string and can identify any poor performing cells. The duration of the acceptance load test should correspond to the expected duty cycle or load profile duration of the battery to which the battery was sized. For example, a flooded lead-acid battery installed in switchgear and control application may be sized to carry the expected load for an eight-hour duration. In this case, the acceptance load test should be per-formed at the manufacturer’s published eight-hour rate to a specified end voltage. Battery performance ratings are given to specific end-of discharge (EOD) voltages. The EOD voltage selected for the acceptance load test should match the minimum allowable volt-age to which the expected load can operate and the EOD voltage to which the battery was sized. Using the switchgear and control battery example, a typical EOD voltage for this application is 105 Vdc for a 125 volt nominal battery. The battery would be sized to an EOD of 105 Vdc. If the battery string contains 60 cells, then the EOD voltage per cell would be 105 Vdc/60 cells = 1.75 Vdc. The resultant discharge current for the acceptance load test of this example would be the published eight-hour rate to an end voltage of 1.75 volts per cell. The acceptance criteria for the acceptance load test should be clearly stated in the project specification. Unless otherwise specified, many lead acid batteries will have less than rated capacity when first placed into service. Unless 100 percent capacity upon de-livery is required by the procurement specification, the battery capacity might be as low as 90 percent of the manufacturer’s rating. This initially low capacity will improve during the first couple of years of float service operation and should eventually rise at least to rated capacity. The reason a new installed battery might not have 100 percent capacity is because the plates within each cell are not yet fully formed. Over time, the plates gradually become fully formed. Typically, design margins and aging margins are included in the battery sizing calculations to insure that the battery can meet its design function at less than 100 percent capacity. In order to establish an acceptance criteria for the acceptance load test it is important to know what margins were used in the sizing process. For example, if a battery was sized with zero percent design or aging margin, then a 100 percent capacity battery is required to meet its design function. In this case the acceptance criteria for an acceptance load test would be á100 percent of the manufacturer’s rating. If an aging margin was included to allow the battery to meet its design function at 80 percent capacity, then one could argue that the minimum allowable capacity during the acceptance load test would be 80 percent. However, 80 percent capacity is considered end of battery useful life. When a battery has been sized to allow for operation down to 80 percent capacity, it is generally accepted that the acceptance criteria for the acceptance load test should be at 90 percent of the manufacturer’s rating.
It is highly recommended to consult both the battery manufacturer and IEEE Std. 450-2002 (for vented lead-acid batteries), IEEE Std. 1106-1995 (for nickel-cadmium batteries), and IEEE Std. 1188-1996 (for VRLA batteries) when performing an acceptance load test.
Long-Duration Vs. High-Rate (Short-Duration) Load Test
Changes have recently been incorporated into IEEE Std. 450-2002 to address high-rate discharge tests (durations of one hour or less) versus the more typical long-duration discharge tests of a lead-acid battery. Battery discharge tests have historically been performed using the time-adjusted test method. This method involves discharging the battery at 100 per-cent of its published rate to a specified EOD voltage. Battery capacity is then determined by the measure of time it takes to reach EOD voltage. For example, if a battery is tested at its published eight-hour (480 minute) rate and it reaches EOD voltage in seven hours and 12 minutes (432 minutes) then battery capacity is calculated as follows:
% Capacity = (Ta/Ts)x 100
This method of determining battery capacity has long been accepted and is accurate for long-duration discharges such as those found in switchgear and control applications because it reflects performance in ampere-hours which is a time-based measurement. The situation is not so simple when performing high-rate (short-duration) discharge tests of a lead-acid battery (typically one hour or less) due to the inherent changes in efficiency that occur at the higher discharge rates. These changes are partly due to sulfation that builds up rapidly at the surface of the plates during a high-rate discharge, thus limiting electrolyte penetration. Since there is limited time available for electrolyte diffusion, the loss of voltage is primarily due to the internal resistance of the cell. In this case, changes in the discharge current can have a substantial effect on the time to EOD voltage. This is why lead-acid batteries designed for high-rate discharge applications such as UPS are rated in amperes or watts for a specific time instead of ampere-hours or watt-hours and why discharge time is not the most accurate measure of battery performance. In these cases, a more accurate method for determining bat-tery performance is to compare the test rate mployed to the battery manufacturer’s published rate for the actual or measured discharge time to EOD voltage of the test. This is otherwise referred to as the rate-adjusted method. When performing a discharge test using the rate-adjusted method it is important to understand the application in which the battery is being used and how the battery was sized. For example, in an UPS application where a battery reserve time of 15 minutes is required for a 100 ampere dc load to a specific EOD voltage, a battery that is rated to deliver 125 amperes for 15 minutes to the EOD voltage is selected (assuming that the battery was sized using IEEE Std. 485-1997 where an aging margin of 1.25 is used ). When the acceptance load test is performed, it would be appropriate to discharge the battery at 100 amperes to the specified EOD voltage. The percent of rated performance of the battery would be determined by comparing the actual rate used in the discharge test (100 amperes) to the manufacturer’s published rate for the actual time to EOD voltage of the test. In this example, the battery reached EOD voltage during the acceptance load test in 20 minutes. The battery manufacturer’s published 20 minute rate to the speci-fied EOD voltage is 90 amperes. The battery capacity would be determined as follows:
% Capacity = (Ia/It)x 100
By using this test method for an acceptance load test in the above example, it clearly demonstrates that the battery could meet its intended function of delivering 100 amperes for 15 minutes. Temperature correction was omitted from this discussion for both the time-adjusted and rate-adjusted methods. However, it is important to measure the electrolyte temperature of a vented lead-acid battery and the negative terminal temperature of a VRLA battery before the start of the discharge test so that the appropriated temperature correction factors can be used when calculating battery capacity to a standard temperature of 77 F. It is very important to understand the battery technology, the application in which the battery is being used, and to consult with the battery manufacturer prior to performing any type of capacity discharge test in order to determine the appropriate test method to employ.
The acceptance inspection and testing of a new battery installation performed by trained and qualified personnel will ensure that the battery is in compliance with all specifications related to the battery installation project and that the battery will perform its design function. Adherence to a proper acceptance inspection and test will optimize battery life and performance, thereby increasing the reliability of the dc power system and the equipment it is servicing.