Battery Power - May/June 2013 - (Page 10)

Feature A New Method for Maintaining the Charge of VRLA Batteries in Telecommunications Standby Systems David P. Boden Encell Technology, Inc. Standby battery systems have been an essential part of modern life for many years. Initially used to support the telephone system by the Bell Telephone Company they have now become necessary in virtually every walk of life. Batteries are used in series strings to provide a matching voltage with the load and they can be connected in parallel to provide the necessary capacity for the required backup time. Because they provide emergency power to critical equipment it must be assured that their batteries are maintained at full charge. This is done by continuous charging at a constant voltage, a process called float charging. In order for the user to know the state of health of his batteries they must be monitored continuously to determine that they are healthy. The benefits of standby system monitoring are well recognized and a variety of equipment for this is now available from several suppliers. Such variables as voltage, temperature, current, AC impedance and DC resistant are measured, which can be used to assess the state-ofhealth of the batteries. This data can be stored and transmitted to the user to provide trends that allow an estimate of remaining battery life to be made. This allows actions to be taken Figure 1. Photograph of an outside plant cabisuch as inspectnet that was destroyed as a result of a battery ing, testing or fire caused by thermal runaway. Courtesy replacing batter- Andrea Quezada/Light Reading. ies before failure takes place. The cost of a battery failure can be very dramatic; for example, failure of a battery costing as little as $250 can result in hundreds of thousands of dollars in consequential costs. Approximately 62 percent of cell tower failures are power related and 80 percent of those are due to battery problems. A photograph of a cell tower cabinet that was burned down by a battery fire is shown in Figure 1. At the fundamental level some form of remote monitoring is essential. It is simply too expensive to send maintenance technicians into the field to examine and test batteries. Additionally, on-site battery capacity testing actually reduces the life of batteries and results in system down time while the battery is being discharged and recharged. The optimum level of testing and 10 Battery Power • May/June 2013 monitoring is still under debate but the need for some form of monitoring is widely recognized. However, the data that battery monitors collect are worthless unless they are analyzed regularly and trended over time to develop predictive analysis of which batteries need to be replaced and when. Because battery failures can sometimes be unpredictable the data must be reviewed daily requiring skilled personnel. Battery monitors are widely used and have been successful in identifying deteriorated batteries but they do nothing to solve the principal causes of short battery life. A new concept is needed that controls how batteries are operated so that the inherent failure mechanisms that occur when they are operated in a state of continuous charge are eliminated. A solution is needed that automatically takes action to detect and isolate defective batteries from service before they can degrade other batteries in the string and cause the backup system to fail. This article describes a new innovative solution that manages how standby batteries can be charged to guarantee maximum life. It also tests and isolates defective batteries from service, monitors essential data and transmits it to the system owner. Negative Effects of Float Charging The universally used process for maintaining standby batteries in a fully charged state is by continuous constant voltage or float charging. The charging voltage is set at a value where sufficient current is flowing through the batteries to overcome parasitic self-discharge arising from the inherent thermodynamic instability of lead-acid batteries, the effects of impurities that enhance gassing, and cell-to-cell chemical or manufacturing variation. Although effective, floating also introduces some problems, especially in valve regulated lead-acid (VRLA) batteries. For example, it increases the battery temperature as a result of i2r heating and oxygen recombination[1]. It also maintains the electrode potential of the positive plates in the region where grid corrosion takes place[2]. The combined effects of high electrode potential and temperature significantly increase the rate of grid corrosion. Temperature has a significant effect on battery life and has become a major issue because there has been a tremendous increase over the last twenty years of batteries deployed in uncontrolled outdoor installations. Temperature affects battery life because of the acceleration of electrochemical processes as temperature is increased. For every 10°C increase in temperature the life is halved. The float current, water loss and rate of corrosion of the positive grid alloy all increase. Most significant for the life of a VRLA battery on float is the combined effect of temperature on grid corrosion and water loss. www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - May/June 2013

Polaris Battery Labs and Carestream Tollcoating Form Alliance
Intelligent Enumerating Battery Charger Provides Safe and Fast Charge for Mobile Equipment
Shippers of Lithium Batteries and Equipment Experience Delays as New Air Transport Requirements Take Effect
A New Method for Maintaining the Charge of VRLA Batteries in Telecommunications Standby Systems
Battery Power 2013 Conference Preview
Batteries
ICs & Semiconductors
Charging & Testing
Components & Materials
Industry News
Research & Development
Marketplace

Battery Power - May/June 2013