Feature Batteries & Chargers
Why replace when you can service? F
Unexpected battery failure can have far reaching and very expensive consequences. However, as Megger explain, servicing these batteries can be far more cost effective than replacement
rom tripping batteries in substations to a power source for UPS systems, storage batteries play a key role in many types of installation. Their unexpected failure in these applications can be so costly that the consequential costs of a failure in the electricity trans- mission network or in a nuclear power station can run into millions of pounds. Regular testing is the best plan of attack to avoid this scenario, and there are a number of methods available. Discharge testing provides the most
also a very inefficient approach in cost terms, and also in terms of environmental impact, as the materials making up stor- age batteries are not noted for their envi- ronmental friendliness.
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 bat- tery status between discharge tests.
Drawbacks
While discharge testing is usually seen as the best way of deciding whether or not a battery is in need of replacement, it looks at the health of the complete battery and provides no information about individual cells.
Therefore, on discovery of a defective
battery, the most likely reaction is to com- pletely 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
Pinpointing faults What’s needed in these situations is a way of pinpointing the defective cells. Unsurprisingly, test equipment manu- facturers 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 manu- facturer’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 mod- ules report back to a software package that analyses the results and flags-up fail- ing 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 BVM battery voltage moni- toring system adopts this arrangement. This allows up to 120 cells to be monitored simultaneously, making it suitable for use with a range of bat- tery 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) require- ments, confirming that it is suitable for use in the most critical applications.
Summary
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 infor- mation that positively identifies defec- tive cells, allowing these to be replaced to restore the battery to health. This approach cannot, of course, be continued indefinitely. Ultimately a complete replacement will be essential. Nevertheless, cell monitoring and the replacement of individual defective cells allow the life of a battery to be sub- stantially extended, yielding cost sav- ings and minimising the environmental impact of end of life battery disposals.
Megger
www.megger.com T: 01304 502 101
Below: Megger’s new BVM battery voltage monitoring system
Left: Megger’s new BVM battery
voltage monitoring system allows up to 120 cells to be monitored
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simultaneously, making it suitable for use with a wide range of battery banks, particularly those used in substations and similar applications
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