Introduction
This is a question that is often asked, and the answer is relatively simple. A discharge test measures the capacity of the battery and a battery monitoring system is a maintenance tool. Let me explain further.
Capacity Testing
A lead acid battery is a collection of lead-based electrochemical cells, each with a nominal voltage of 2 Volts, assembled in series to achieve the operating voltage and in parallel to obtain the capacity required for the load they are intended to power.
Depending on the application, the battery will have a predicted life of between five and twenty years. For example, a VRLA high-rate battery with a fifteen minute runtime in a UPS application will be marketed as a five-year battery, and a VLA battery in a communications application with a required runtime of four to eight hours can be expected to last twenty years.
This estimated life is based on the volume of lead in the plates and the rate of corrosion that occurs as the battery ages under ideal conditions. This corrosion will cause the battery to lose capacity, and a lead acid battery will be considered at end of life when it reaches 80% of its original capacity. The reason this is considered end of life is that up until that point the loss of capacity will have been relatively slow, and if the battery was correctly sized to cover that loss of capacity, the load should be protected. When it reaches 80%, the rate at which capacity is lost will increase and the protected load may be at risk.
A discharge test is designed to measure this loss of capacity. In the IEEE recommended practices for battery maintenance, it is recommended that a discharge test should be carried out at intervals of between two and five years depending on the type and age of battery installed.
Battery Maintenance
Now that we have established that a discharge test is intended to measure remaining capacity, can that be considered sufficient to ensure that the battery is fully operational between tests? The answer is NO. In both IEEE 450, the IEEE Recommended Practice for Maintenance, Testing and Replacement of Vented Lead-Acid Batteries for Stationary Applications, and IEEE 1188, the IEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications, discharge testing is only a component of the maintenance plan required to ensure battery reliability.
In both the recommended practices there is a detailed list of tasks that should be undertaken during monthly inspections. The monthly tasks are intended to ensure that the battery is being charged correctly and the environmental conditions are satisfactory – both factors that can reduce the intended life of the battery if not maintained within the battery manufacturer’s specified limits. Additional tasks to inspect and record the operational parameters of the individual units within the battery are required at three- and twelve-month intervals.
These additional tasks include the measurement of the ohmic value of the individual cells or units within the battery. This is measured by applying a load across the battery for a short period and measuring the resultant volts drop. The load can either be AC or DC, with the readings being referred to as impedance or resistance, respectfully. One manufacturer of the test equipment used converts the reading obtained to Siemens, the measure of conductance in order to differentiate their product from the rest. Irrespective of the method used, the objective is to identify changes in the electrochemical reactions when the individual cells or units are placed under a load over time, which will provide an indication of the level of corrosion that has taken place due to aging.
Battery Measurement
The measurement of ohmic value was originally intended to be used on VRLA batteries when they were introduced in the 1970’s, as they could no longer be inspected visually due to their opaque cases. However, it is now widely used for both VLA and VRLA batteries.
After the battery data is collected manually during scheduled maintenance visits, it must be downloaded and reviewed using the monitor manufacturer’s software. The software will typically use both limit and trend-based analysis on the collected data to alert the user of changes that could impact the battery’s ability to support the load. The static values, such as voltage and temperature, are compared against the battery manufacturers high and low limits, ensuring that the battery is being operated in the correct operational environment. To identify the rate at which the battery is aging, the current ohmic values are compared to a set of baseline measurements typically taken when the monitor is installed. This will provide an indication as to the rate at which the battery is ageing. An overall rise of 30% for a battery assembled with multi-cell units and 50% on a battery with single-cell units is generally accepted as an indication that the battery is reaching end of life. This is the point at which a discharge test can be carried out to establish the actual capacity loss.
So, what if after a maintenance visit, the data for a cell or unit shows a change in ohmic value when compared with the other units in the string? Theoretically, all the cells within a string should age at the same rate so any divergence in the ohmic value is indicating a change in the electrochemical response of the cell and represents a risk that the battery will not operate as required.
That risk will be dependent on the battery type and application. If it is a long duration, the VLA battery risk will be in the reduction in runtime, as loss of capacity is the most common impact of a unit failure. For a VRLA battery in a High Rate application such as a UPS, the risk is much greater, as a failing VRLA cell can go open circuit when placed under load. To identify the level of risk associated with a premature increase in ohmic value, it is the rate of rise that is the indicator, not that it has reached a specific limit value.
Conclusion
It is generally accepted that the value of any analysis is based on the volume of data available, and this is particularly true with the rate of change in the ohmic value. The ohmic value of a cell in VRLA batteries can, under certain failure conditions, rise to a level at which the cell is virtually open circuit in a period of less than three months. Therefore, a calendar-based maintenance schedule, or a discharge test every two years, does not ensure the continuity of service for which the battery was installed.
To achieve this requires that the key battery parameters be monitored to identify any changes that can identify the inception of a potential point of failure within the battery. It also requires that the data collected is at a frequency and consistency that can only be achieved with a permanently installed monitor. With the increasing use of artificial intelligence software in failure analysis, the volume of battery data collected by these monitors will play a key part in achieving greater battery reliability.
