The unexpected failure of a battery when it is called upon to supply power can have far reaching and very expensive consequences. However, as Megger explain, testing and servicing these batteries can be far more cost effective than out and out replacement.
From tripping batteries in substations to a power source for UPS systems in data centres and in industry, storage batteries play a key role in many types of installation. Their unexpected failure in these types of application can be so costly that the consequential costs of a failure in the electricity transmission network or in a nuclear power station can run into millions of pounds.
Regular testing is obviously the best plan of attack to avoid this scenario occurring, and there are a number of ways in which to do this. Discharge testing is the method that provides the most accurate and reliable determination of battery condition, although impedance testing, which can be carried out quickly and easily, is a valuable method of routinely monitoring battery status between discharge tests.
While discharge testing is usually seen as the best way of deciding whether or not a battery is in need of replacement, it does have one important limitation - it looks at the health of the complete battery and provides no information about individual cells.
The consequences of this is that on discovery of a defective battery in a critical application, the most likely reaction is to completely replace it. While this is without doubt an effective solution, the problem with the existing battery was most likely limited to one or two faulty cells, so it’s also a very inefficient approach in cost terms, and also in terms of environmental impact, as the materials making up storage batteries are not noted for their environmental friendliness.
What’s needed in these situations is a way of pinpointing the defective cells. Unsurprisingly, test equipment manufacturers offer products that are designed to do exactly this. They work by simply monitoring the voltage of the individual cells in a battery while a discharge test is being performed. Any cell where the output voltage falls faster than it should, based on a comparison with the manufacturer’s data, is quickly identified as faulty and in need of replacement.
While the principle of operation of these cell monitoring devices is simple, the implementation varies greatly from manufacturer to manufacturer. Many cell monitoring products are, for example, designed to be installed permanently on a battery, which is an expensive option if multiple batteries need to be tested and monitored. Other cell monitoring devices are cumbersome to connect and use, or are limited in the number of cells per battery they can accommodate.
The ideal solution is a modular system that uses one voltage monitor module per cell, with the modules linked by simple plug-in connections. The modules report back to a software package that analyses the results and flags-up failing cells. The modules must be easy to connect to the cells using, for example, some form of spring loaded connector, with provision for other options to cater for ‘difficult’ applications.
Megger’s new BVM battery voltage monitoring system adopts this arrangement. This allows up to 120 cells to be monitored simultaneously, making it suitable for use with a wide range of battery banks, particularly those used in substations and similar applications.
In this system, each module connects to the next with a single cable daisy-chain connection, and the results are logged and analysed using a standard PC and the software supplied. The testing is performed in line with the IEC test method, and complies fully with NERC (North American Electrical Reliability Corporation) and FERC (US Federal Energy Regulatory Commission) requirements, confirming that it is suitable for use in the most critical applications.
Using a cell monitoring system of this type adds little to the cost of testing a battery and has no significant impact on the time needed to carry out the test. It does, however, provide additional information that positively identifies defective cells, allowing these to be replaced to restore the battery to health.
This approach cannot, of course, be continued indefinitely. Ultimately the battery will age to the point where a complete replacement is essential. Nevertheless, cell monitoring and the replacement of individual defective cells frequently allow the life of a battery to be very substantially extended, yielding big cost savings and minimising the potential environmental impact of end of life battery disposals.
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