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Battery Capacity Testing – Field Experiences

Introduction

In case of emergencies, it is essential that the battery systems perform as designed, or the substation may be left unprotected. Although many test methods can be performed to estimate the condition of the battery, the capacity test at specific time intervals and discharge rates is presented as the only method that can measure the real capacity of the battery. Nowadays the capacity test is not performed because of the belief that this type of test is too expensive, and damages the battery. Most battery users today apply the internal resistance measurements in order to determine the battery state of health and save time. This approach is not wrong. However, the real capacity cannot be decided from this test.

Before the test starts, it is important to visually inspect the cell connections are in proper condition as well as the charger has been disconnected. The load connected to the battery needs to be adequately backed up, or the current taken by the load needs to be included in the discharge rate. It is important to emphasize the test preparation and conditions before its start can affect the test results. Having that in mind, it is important to check the key parameters such as the cell voltage, float voltage, and cell temperature before the test starts. Once the initial conditions are met, the load may be connected.

In battery capacity tests, there are some facts that are misunderstood. Mistakes are made as a result of myths, misunderstandings, and lack of training and experience.

Myth 1 – Capacity Testing Ruins the Battery

This myth creates confusion among test professionals. Excessive charging and discharging, with too many cycles, will reduce the service life. However, when conducted in accordance with standards and manufacturers’ recommendations, the capacity testing will not reduce the battery life.

The effect of the performance test on a battery was analyzed on the Powerfit S312/7S battery. The test was performed with a higher discharge rate and reduced test time. The graph in Figure 1 illustrates the test results collected from the battery with the manufacturer declared five years of service. After being subjected to five discharge tests, the voltage values and the capacity levels at the last test were unchanged compared to the results of the first test, even the voltage in the last test was a bit higher than the voltage in the first test. This confirms that five discharge cycles (as recommended by standards) will not ruin the battery.

Battery Voltage Graph
Figure 1 – Measured battery voltage as a function of time in five discharge tests on Powerfit S312/7S

Myth 2 – Battery Needs to be Equalized Before the Test

This statement depends on whether the battery system is new or it is in an existing system that is about to undergo a capacity test. When a battery is new and the acceptance test is to be performed, a commissioning charge including an equalize charge should be performed. After that, a battery should be placed on the float for at least three days. However, in everyday practice batteries are tested from the float state.

Myth 3 – Stop the Test When the First Cell Reaches Cell-End Voltage

During the capacity test, it is not uncommon for a cell or a few of them to fail before the end of the test. Terminating a test as soon as the first cell fails may result in incomplete or wrong test results. The total string voltage should be used as terminating criteria as shown in Figure 2.

String Voltage Graph
Figure 2 – Total string voltage on the battery that needs to be replaced

However, a test person should bypass a weak cell except the test has run 90-95% of its course. Terminating the test when one cell reaches end voltage will not allow us to find other damaged cells. The detection of the two weak cells is presented in Figure 3.

Cells Voltages During Battery Capacity Test
Figure 3 – Voltages of individual cells during the battery capacity testing

The bypass can be done by pausing the test, then bypassing the cell with appropriate connectors. The test needs to be continued with the new end-voltage value taking into account once cell less. The maximum allowed time to finish the bypassing process is around six minutes, and it should not be counted in the total test time.

Effect of Temperature on Discharge Test

The temperature has a big influence on cell performance. As the temperature of the cell increases, a chemical reaction increases what improves the capacity of the battery. However, keeping the battery warmer than recommended, will cause a higher level of active material shedding, and grid corrosion that in turn will reduce the service life. This includes
ambient and cell temperature. Battery manufacturers publish recommended operating temperatures for their batteries. If the operating temperature of the battery system is above or below recommended, a correction factor needs to be applied after the test. The table with temperature factors can be found in standards.

Test Process

Once the initial conditions are met, the load and other necessary test equipment need to be connected and setup. The first step is to define the test current, end voltage, and the test duration. These parameters in our test are found in the manufacturer’s published recommendations. The battery installation used for testing had 126 cells and the string voltage of 252 V.

Battery Ratings
Table 1 – Manufacturer published ratings for tested battery at 20°C

It is important to emphasize that current values were not adjusted for temperature. After completing the test it is necessary to perform temperature compensation and recalculate the capacity.

The load unit (BLU200A) along with extra load unit (BXL-A) was used to increase loading capability. The test current is chosen from Table 1.

The terminal voltage was measured during the entire test as a function of time, and the battery should be discharged until it reaches the minimum terminal voltage, or once the total test time has been reached.

String voltage during battery capacity testing
Figure 4 – String voltage as a function of time

During the discharge test, it is important to monitor cell voltages at least three times during the test. One measurement should be taken at the beginning of the test and the others at specified intervals. The sample rate should be more frequent when the voltage starts to decrease faster.

Individual cell voltages can be measured manually or using an automated monitoring system. Voltages in this particular test were measured using the Battery Voltage Recorder (BVR device) and the voltage value of each cell is illustrated in Figure 6.

Individual Cells Voltage Measurement
Figure 5 – Voltage measurement of each cell

Capacity Calculation Method

The battery capacity is expressed in ampere-hours (Ah). However, the capacity at the end of the test is always expressed in a percentage of the manufacturer’s published capacity. Standards define the two methods for capacity determination: time adjusted method and rate adjusted method. In this paper time, the adjusted method is described since it has been used in our tests.

In the time adjusted method, the test current is kept constant as defined in the manufacturer’s published table as a function of the selected test time duration. The battery capacity is calculated after the completion of the test by using the published performance data at 20 ̊ C.

The capacity in % is calculated using following formula.

C%=(Ta x 1 000)/(Tm x Kt)

In our test it was:

C%=(8 x 1 000)/(8 x 1.045)

Total capacity in % after the test was 95%.

C – Capacity at 20 ̊ C.
Ta – Actual time duration of the test.
Tm – Manufacturer’s rated time to reach the string end voltage
Kt – Temperature correction factor (in our case 1.045)

Replacement Criteria

The recommended practice is to replace the battery if its capacity is below 80% of the manufacturer rating. Following the test, it is necessary to review the battery sizing to conclude if the remaining capacity is sufficient for the battery to perform the intended function. A capacity of 80% indicates the battery (cell or string) rate of deterioration is increasing even there is still a capability to support the load. Additional characteristics such as abnormality of the cell temperature and the cell voltage are often determinants for complete battery or cell replacements. The cell voltage is a good indicator for further investigation and cell replacement. In case we use replacement cells, it is important to ensure they have electrical characteristics compatible with the existing cells and they should be tested before installation.

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August 6, 2020

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