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Why replace serviceable batteries?

Why replace serviceable batteries? A new method of testing batteries has been developed - a modular cell monitoring system that identifies the defective cell, eliminating the need to replace working batteries.

Storage batteries play a key role in many types of installation - as tripping batteries in substations, for example, and as a power source for UPS systems in data centres and in industry. In almost all of these applications, the unexpected failure of a battery when it is called upon to supply power can have far-reaching and very expensive consequences. In fact, it is by no means inconceivable for the consequential costs of a battery failure in the electricity transmission network or in a nuclear power station to run into millions of pounds or dollars.

The best strategy for avoiding such incidents is, without doubt, regular battery testing, and a number of techniques have been developed to make this as convenient as possible. Of these, discharge testing is the one 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. This means that the most likely reaction to discovering a defective battery in a critical application is to completely replace it. This is undoubtedly an effective remedy but, as the problem with the existing battery was most likely limited to one or two faulty cells, it's also a very inefficient approach in cost terms, and also in terms of environmental impact.

What's needed is a way of pinpointing the defective cells and, unsurprisingly, test equipment manufacturers offer products that are designed to do exactly this. The way in which these products operate is very straightforward - they simply monitor 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.

Modular cell monitoring solution
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 flag 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.

This is, in fact, the arrangement adopted for the new Megger BVM battery voltage monitoring system. 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 even 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 invaluable 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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