Predicting whether a battery system will perform its intended operating role is a major problem for battery users. There are a number of different tests, which are recommended by IEEE, NERC and other standards for diagnostics of battery banks conditions. Two of the most commonly used methods to determine how a battery is going to perform when required, are the internal resistance test and the capacity test.
Batteries are designed and sized to produce a certain amount of current for a given amount of time. The only method to determine whether the battery will support the load for the needed duration is the capacity test. That is why among all the tests, the discharge test, (also known as a load test or a capacity test) is the only test that can accurately measure the true remaining capacity of a battery system and consequently get an insight in an operational condition of a battery.
A discharge test needs to be performed with automated test equipment that can log data of all individual cells during the discharge process. At the end, it helps to locate the weak cells and faulty inter-cell connectors. The figure below illustrates the weak cell detected during the discharge test performed with the BLU and BVR series of instruments using DV-B Win software.
Internal Resistance Test
Since capacity testing is time-consuming and expensive, there needs to be an additional method to estimate the battery operational condition in a faster and a cheaper way. Internal ohmic measurements are used to learn about a battery condition by monitoring the internal resistance of its individual cells. A number of factors can affect the internal resistance and capacity of a cell simultaneously. However, not all factors affect a cell capacity to the same degree as they affect the internal resistance, or vice-versa.
There is a general correlation among the most frequent factors that increase the internal resistance and, at the same time, tend to decrease the capacity. To compare how various factors influence cell internal resistance, please log in and download the full document “Comparison Between Internal Resistance and Capacity Test“.
The table from the document indicates that the internal resistance test may be used instead of the capacity test. However, the internal resistance tests cannot tell us everything regarding battery capability or condition. Low capacity cells can be identified, but absolute predictions regarding the cell capacity are more difficult to make. Some points to consider are:
- Ohmic measurements are not a substitute for capacity testing and cannot predict real capacity values.
- Ohmic measurements can be a trending tool to identify cells requiring further evaluation. When significant deviations from the baseline occur, the capacity testing should be used to verify whether identified cells are defective and need replacement.
- For the team members that are involved in battery testing, the capacity testing procedure is very highly recommended. It is the only test that can accurately measure the true capacity capabilities, and provide an accurate insight into the battery operational status.
July 31, 2019