COMMERCIAL FEATURE Avoiding catastrophic failures of DC battery back-up systems S tandby batteries are needed to protect critical systems in the event of mains power failure. Typical applications are in electrical substations, telephone systems, hospitals, banking, data centres, industrial applications and processes. Stationary battery systems utilize lead acid, nickel cadmium or lithium cells and remain charged but are not used unless needed. These batteries will stay in a float mode for prolonged periods. Different types of batteries in use have slightly different properties that make them better suited to specific applications, and suffer from different types of failure. By performing regular battery maintenance, it is possible to identify and avoid many of the various failure modes before they can escalate into a costly failure. Visual inspection: Inspection will locate such things as cracks, leaks, bulging batteries and corrosion. Simple problems can be found before they become catastrophic failures. The operational status of the ventilation system should be verified as well as the room’s environmental conditions. Check each individual battery for cracks, leaks, deformation and strap corrosion. DC float voltage measurement: Low float voltages can indicate that the battery charger is not set properly and the batteries are not fully charging and can therefore cause sulfation. High float voltages can indicate that the batteries are over-charging and can accelerate grid corrosion and high temperatures in the battery. Float current measurement: Float current will vary with battery size. The larger the battery the more float current it will take to keep it fully charged. High float current could indicate that there is a ground fault present in the system or there is a short in one of the cells and this could cause thermal runaway. Temperature measurement: Ambient and cell temperature will have a direct effect on a battery’s life span and capacity. High temperatures will decrease a battery’s life span. Low temperatures will decrease a battery’s capacity. Ripple current measurement: AC ripple current is a by-product of the charging system, and if high it can cause internal heating of battery, which in turn reduces the life span. Specific gravity measurement: A specific gravity measurement will provide the state of charge of the battery. Strap resistance measurements: These need to be taken periodically as well. If the torque is not sufficient or corrosion is present there will be a higher resistance in the connections. This in turn will cause a greater voltage drop that will cause heat. Poor strap connections are the major cause of string failure but can be quickly identified using the Megger BITE or DLRO products. Discharge testing: This is the only test that will determine the capacity of the cells in the string. A load bank such as the Megger Torkel is used to discharge testing. Tests can be conducted at constant current, constant power, constant resistance or in accordance with a pre-selected load profile. It is also 96 possible to use Megger’s battery voltage monitoring system (BVM) to monitor individual cell voltage during the test in order to identify individual weak cells. Ohmic testing: Impedance testing using Megger’s BITE products will inject an AC signal into the battery cell and measure the voltage drop across the cell or intercell to determine the impedance of the cell using Ohm’s Law. Aging effects cause a chemical change in the battery, which in turn causes a change in the battery’s internal impedance/ resistance. It is not a direct measurement of capacity as with discharge testing, but rather an indication of the state of health of the batteries. When performing an ohmic test each cell comprising the string is compared to the string’s average ohmic value. This is useful in identifying weak cells within the bank. Comparing the string average to a baseline helps establish the overall health of the string. Impedance testing will be able to detect certain problems that resistive measurements can miss, such as negative lug rot as well as negative plate corrosion, which show themselves as changes in inductance and capacitance, not in resistance. In addition, many chemical changes in a battery will be seen as impedance changes before they are seen as resistive changes. Conclusion In order to achieve a high reliability battery backup system it is not only essential to select the correct types of batteries for the application at hand but regular battery maintenance is essential for the safe and reliable operation of the DC system. The cost of proper battery maintenance is minimal compared to the cost of a failed battery backup system. ESI ABOUT THE AUTHOR: Marius Pitzer is the regional sales and applications manager at Megger Limited. Copies of Megger’s 30-page Guide to Battery Testing publication are available on request. SAenquiries@megger.com ESI AFRICA ISSUE 3 2014