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April 2010 Maintenance, Test & Measurement, Calibration

Loic Moreau, LEM Geneva Switzerland

Improved measurement and analysis yields cost-effective standby battery monitoring.

Abstract

The infrastructure that underpins contemporary society must operate with very high reliability. In contrast to the advanced technology deployed in data storage centres, Internet server farms and communications central offices, most of them rely for near-100% up-time, or system-availability, on a very mature technology – the lead-acid battery. It is standard practice to provide such critical nodes, and many other essential services, with back-up power, and the first layer of that back-up is usually an inverter that draws on banks of valve-regulated lead-acid (VRLA) cells, or the closely-related sealed gel-cell types.

This venerable technology continues in widespread use for a variety of reasons, not least the fact that the cells are economical, and offer excellent reliability. Excellent but not perfect. VRLA cells do have a finite life (a design life of 12 years is common), and in normal use in critical-system standby power they are routinely replaced on timed basis. Failures can, and do, still occur. In a typical standby supply, the cells do exactly as the name implies – they are held at full charge awaiting failure of the primary supply. This is achieved by maintaining a small, continuous ‘float’ charge. If the float charge current is below a certain limit, the gases produced by electrolysis within the cell will recombine. In this condition, cells can be vulnerable to damage if the float-charge voltage is even a little higher than the standard value of 2,27 V per cell. A small over-voltage will cause the electrolyte to produce more gas than the recombination chemistry can cope with, and this will escape through the valve. Excessive cell temperatures, even with correct charging voltage, will also lead to loss of electrolyte.

Other failure modes include early sulphating, poor connection between the posts and the grids, poor connection between the grids and the plates, stratification of electrolyte and accelerated grid corrosion. An infrequent, but catastrophic, mode of failure is thermal runaway, which is peculiar to VRLA and Gel batteries and can result in fire or explosion. The only way to detect its onset is to monitor the internal cell temperature.

Readers wanting more information can read the full paper at http://instrumentation.co.za/+C13634

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Maintenance, Test & Measurement, Calibration

LEM's battery monitoring solution

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