The baseline for battery testing

1 July 2016

Electrical Tester

Andy Sagl - Product manager

When the testing of stationary battery installations is discussed, the concepts of baselines and baseline testing are often mentioned. But, in this context, what exactly is a baseline, why is it needed, and how can it be established?

To start with the first question, a baseline is simply a reference value used in impedance testing of stationary battery systems. Measured values are compared with the baseline value to determine how much the batteries have changed chemically over time.

Baseline values are needed because the results obtained from impedance testing are relative, not absolute. If a 100-ohm resistor is measured, it either measures 100 ohms or it does not – this is an absolute test. But with battery testing, things are different.

As a battery ages, chemical reactions affect its electrical characteristics and, as these characteristics change, so does the internal impedance of the battery. For this reason, no impedance test can indicate whether a battery is good or bad; it can only indicate whether there is a problem that needs to be addressed.

When the impedance value measured for a battery increases, compared with the baseline, by approximately 30% for a VRLA battery, or approximately 50% for a VLA battery, this indicates that there is a problem that requires investigation. It is worth mentioning that these figures are illustrative only – the actual values that indicate a need for action vary depending on battery type and application.

An increase in impedance of the magnitude mentioned does not, however, necessarily mean that the battery is defective. The string may simply have poor strap connections that are partially sulphating the cells, or it may need an equalisation charge to balance it. Remember that batteries can act as a source or as a load within the string so one weak cell can drain current from others.

Standards state that, in order to make effective use of battery impedance measurements, they must be compared against accurate baseline values. And by far the best way of ensuring that baseline values are accurate is for users to establish their own. The use of baseline data provided by the battery or test equipment manufacturer is not recommended. This is simply because manufacturers’ data can be unreliable, as there are too many variables and unknowns.

The battery manufacturer’s baseline may, for example, relate to green batteries – that is, batteries that have not completed formation. Such batteries will not have reached their full capacity and their impedance value will certainly change as formation progresses. Another complication is that there is usually no way of knowing the type of test equipment that was used for the measurement. Different test sets operate at different frequencies, and this will affect the measured values. Finally, when manufacturers test batteries, they will not be in your configuration, and they are unlikely to be at your temperature.

When it comes to baseline data supplied by a test equipment manufacturer, many of the same comments apply. The cells used to establish the baseline may be green or they may be old. Once again, the batteries won’t be in your configuration or at your temperature. In summary, using baseline values that haven’t been established by the end user is always a gamble.

But how should baselines be established? The standards recommend that baseline testing should be carried out on new batteries six months after installation, and that they should be fully charged and in service. This is the ideal approach, but what is the procedure for older strings? In such cases, the first step is to verify that the string is good, which is best done with a discharge test.

After this, the baseline can be established by taking an average of the impedance of the string, being careful to exclude any cells that have values that might skew the results. The baseline should be established separately for each string. It is not satisfactory to assume that similar strings will have similar baseline values because batteries from different production runs are likely to have different impedance values.

When working with baseline values, it is important to always bear in mind that although, in general terms, as the impedance of a battery increases, its capacity decreases, this is not a direct correlation. This means that an impedance measurement cannot be used to determine the capacity of a battery. Nevertheless, impedance measurements provide a useful indication of battery condition. This is particularly true with lead-acid batteries because, as these approach the end of their lives, their impedance increases at a much greater rate.

Impedance testing is an invaluable technique for monitoring the condition of stationary battery installations, not least because the tests can be performed quickly and easily without the inconvenience of having to discharge and recharge the batteries. Impedance tests are, however, only meaningful if a reliable and accurate baseline has been established for the battery under test, using the techniques that have been discussed in this article.