Introduction

Battery systems are entitle_d to routine and periodic maintenance. The measurement of several battery specifications and characteristics, such as the temperature, current, voltage, internal resistance, and connection resistances, should be done at specified manufacturer intervals. This is something that battery users should not avoid at all costs. The above measurements cannot provide the quantitative value related to the battery capacity. Therefore, the battery users should carry out a capacity test, which can be a good informer on the battery's performance in terms of capacity. This article is a good example of how you can carry out capacity testing of a given battery. Let us jump into the deeper content of this article.

Defining Capacity Testing

Capacity testing is also referred to as discharge testing or load testing. This a dynamic method of battery testing where a load that is simulated is connected to the battery for a specified period, and the discharge is allowed to go on until it ends at a point known as end-of-discharge while doing the referencing of the temperature of the battery that was recorded at the start of the discharging process. The process is suitable for determining the actual battery capacity, and it has the benefit of allowing a comparison of the test results' rated capacity.

Suppose the process is done at specified discharge intervals. In that case, a noticeable trend has been established that can indicate how the battery capacity is lost as the battery gets old. If the process is run, subjecting all the battery cells to the same procedures, then there is a possibility of identifying the failed cell in real time that can be replaced.

Testing Intervals

Testing for new batteries should be done as soon as possible, and to be precise, it should be done after installation and commissioning. Only nickel-cadmium batteries are excluded from such immediate testing. Such initial installation and commissioning testing is known as acceptance testing. Acceptance tests set the datum to which future battery test results are compared. The discussion of acceptance tests for the nickel-cadmium battery will be introduced later in this article.

Batteries can be subjected to acceptance tests during manufacturing before they leave the facility, after the installation, or after both activities. Remember that the acceptance tests done in manufacturing factories have cost implications based on the quantity of the cells subjected to the tests and the duration of each test.

Vented lead-acid batteries should be subjected to a follow-up test, which should be suitably carried out two years later after doing the acceptance test. All the tests carried out after the acceptance tests are known as performance tests. The performance tests should be carried out at a predetermined interval that is not beyond 25% of the expected battery life. This means that for a battery that is likely to serve for 20 years, the performance test should be done every 5 years until when it starts to undergo degradation.

We can define battery degradation in the following three ways:

• When the battery capacity falls below 90% of the rated capacity
• When the battery under test has hit the 85% mark of the design life
• When the battery capacity has decreased by 10% as compared to the previously undertaken test.

The acceptance test for the nickel-cadmium batteries is different. The test is not carried out immediately. After installation, you have to wait for the battery to float for about twelve weeks while not discharging it. This action causes the battery to lose the float voltage effect, which can affect the battery capacity test outcome. Their first performance test was carried out approximately two years after the battery was commissioned. Other additional tests are carried out at an interval of five years until when the excessive capacity loss is achieved.

Carrying Out Capacity Testing

This capacity testing process can be carried out in two ways: using a testing company or conducting in-house testing.

It is advisable that if the users are not well informed about these methods of performing such tests, they should consult those who are knowledgeable with such experience. Acquiring equipment for such tests is costly, and you have to be well-trained to use them. Modern-day capacity testing involves the use of microcontroller and microprocessor-based testing equipment, which includes necessary software and hardware that is suitable for test performance and conforms to industry standards.

Some of the test systems integrate a control system into the test hardware, while others run the test using a Windows PC system. The choice of the model is determined by the size of the battery to be tested. Using manually operated banks and data collection is impractical, so the automated process is advised. The latest equipment comes with advanced wireless sensors that report real-time data to the data acquisition hardware.

The testing equipment hardware consists of a load bank made of high-power precision resistors that help draw current from the connected battery. Parallel load banks will be incorporated into large battery systems. A single hardware product configuration exists that is portable but has a very reduced capacity for power handling.

The data collection facility or system can be good for all batteries. The facility can handle analog signals like the overall voltage of the battery, the voltage of the cell, the discharge current, and the cable voltage drop. Then, depending on the option given by the product, all this data is sent to the computer system or the embedded microprocessor for proper conversion and data logging in the testing procedures.

The Capacity Test Specifications

The battery capacity test should be valid and conform to the manufacturer’s guide.

In my example, let us make use of the electricity utility substation backup battery to establish a simple test specification. This backup battery has an overall capacity rating of 200AH, consisting of 60 cells in its structure. The battery is of type VAR, has 1.215 nominal specific gravity, and serves the station for 8 hours non-stop. The DC system has an end of discharge at 105V. This means that each cell end of discharge is at 1.75V. The battery performance temperature is placed at 770F or 250C. All we need next is the battery discharge current. Note that even if the system battery is rated for 8 hours, our test is not necessarily done for eight hours.

Test Mistake Avoidance

• Never test the battery unless you know its general conditions. Never leave the battery in a worse condition than you found it. Avoid leaving the DC test systems unprotected if no battery is connected.
• Never terminate the discharge or test process when a single cell reaches the average end of discharge voltage. This mistake invalidates the results.

Methods of Calculating the Battery Capacity

After completing the capacity test, we have to compute the demonstrated capacity. Two methods, time adjustment and rate adjustment, are used to calculate the capacity.

The Rate Adjustment Method

This method is suitable if the duration of testing is less than an hour. If the test goes beyond one hour, either a time adjustment or rate adjustment method is considered.

The Time Adjustment Method

This is the most preferred method when it comes to testing durations exceeding one hour. This method is very easy to calculate and gives very accurate outcomes. In this article, we are going to explain how we can carry out time adjustment test calculations.

Percentage Capacity Calculation Using Time Adjustment Method

The process involves dividing the actual battery discharge time by the rated discharge time and multiplying the results by 100%. The whole process assumes that the battery temperature is at 250C. See the formula below.

Summary

• Capacity testing is a method for determining whether a battery meets the manufacturer’s specified battery capacity rating.
• The process is made successful through testing intervals that are done on the battery after installation and commissioning.
• The battery is said to be degraded if its capacity falls below 90% of the rated capacity if it has hit 85% of the design life, and if the capacity has reduced by 10% of the previous capacity.
• Capacity testing process can be carried out either through the testing company or the in-house testing methods.
• The battery manufacturer sets capacity test specifications, which are on the datasheet attached to the batteries.
• Never test a battery in an unknown general condition, and never terminate the discharge process because a single cell reached EOD.
• The battery capacity can be calculated using two methods, namely, the time adjustment method and the rate adjustment method.
• The rate adjustment method is suitable for batteries undergoing test under one hour. Both the rate adjustment and the time adjustment methods can be used for discharge times exceeding one hour.