Battery Power - September/October 2011 - (Page 14)

Feature While accurate, the test current is so large the test can take its toll, and if over-used, can shorten the life of the lead acid battery. Therefore advocates of this method proposed relatively infrequent testing to monthly or even less. Why was such a big load used? When these tests were first attempted it was very difficult for the existing technology to determine the difference between the background UPS inverter and charger electrical noise, and the ‘signal’ produced by a battery monitoring system (BMS). It was like trying to have a conversation in a high school lunchroom. Everyone has to speak loudly just to conduct their own conversation, thus the noise grows louder and louder. With all of the background noise in a typical UPS environment it seemed obvious that the best approach was to create a test signal that would stand out against the background noise. So early battery monitoring systems hit the battery with a large current load so the instrument could see the test signal amongst all the noise. You hear the expression “signal to noise ratio”, this is what it means. The net effect was that the first inventors used large loads, non-reactive measurements and infrequent testing based on the technology they had and therefore the expression internal resistance defined their comfort zone with that technology. After a few years, companies came along with improved electronic technology. These inventors realized that if they used an AC load at a very specific frequency they could discriminate the load signal by frequency from all the other random AC noise on the battery. Much like the effect of a dog whistle in the same lunchroom cafeteria. Even with all of the noise in a crowded room the dog can still hear the whistle because their ears are capable of distinguishing the frequencies since they stand out against the background noise. With this testing method each time the load is switched on, there is a slight drop in voltage and in the off half of the cycle, there is slight rise in voltage. By measuring the AC ripple induced on the battery at that specific frequency, the system can get a measure of the internal ohmic value of the battery. Using an AC load means the test equipment may get effects from the internal inductance and capacitance of the battery which affects their signal quality. Because the test signal used on the battery does not need to be so strong, or as large in terms of current, more frequent tests may be used, perhaps monthly or weekly. Users of this AC measurement technique found that the value they measured was a good indicator of battery capacity, performance and life of the battery. They became very keen on calling it internal impedance and fought hard against the internal resistance lobby promoting its merits. It is clear through this evolution of battery monitoring that the periodicity of testing was determined by the test load used and that was determined by the electronic sophistication used in the designs. A stimulus was to occur that triggered the rapid development of the next step in technology. VRLA Technology In the 1990’s VRLA batteries quickly became the norm in standby applications and they started to replace the more reliable, better built (and much more expensive and code-restricted) flooded or wet cells in the battery room. While flooded cells take weeks and months to fail, VRLA batteries fail rapidly and the ‘holy grail’ became the development of a battery monitoring system that gave sufficient time resolution such that it could detect the imminent future failure of a VRLA jar before it actually failed. Testing each month or every two weeks would simply not do that. The next challenge to be addressed was test current level. There are a number of negative effects on a string of batteries caused by applying large load currents. Many system application designers think of batteries as large, highly fault tolerant entities. They are not. A string of jars, connected in series has a different fault tolerance and behavior than the sum of the individual jars. Taking large currents from sections of the string can cause effects in the string that can impact the life of the entire battery and even create risks to the assets. Therefore the problem with test currents is not limited to just taking the current down but how it is done and the timing. In the last twenty years, the problem has been approached by electronic design engineers rather than battery technologists. Using the latest modern solid-state technologies involving lock-in amplifiers (3) and synchronous demodulators or digital signal processors, these engineers were able to generate signals using switched DC circuits to stimulate the load on the battery. With this advanced technology they were able to understand the internal characteristics of the battery with tiny loads thought impossible until recently. The new designs were able to use loads as low as one amp applied over a series of pulses measured in thousandths of a second and then take the resulting data and produce a clean signature which defines the ohmic value of the battery using Ohm’s law. This would be analogous to hearing the sound of a butterfly’s wings flapping in that same high school cafeteria full of noisy students. Figure 1 shows an actual battery reading where the ohmic value testing has captured a battery failure. Note the rapid decline in the battery performance. This particular jar went from fully functional to complete failure in just four days. With an older measurement technique the failure may have gone undetected Figure 1. Ohmic Value curve showing VRLA jar failure over a matter of two to three days. 14 Battery Power • September/October www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - September/October 2011

Battery Power - September/October 2011
Contents
Portable Battery Market to Reach $30.5 Billion Worldwide by 2015, Forecasts Pike Research
Panasonic Introduces High Rate Li-Ion Cell
A Comprehensive Management Approach to Maximizing UPS Availability
The Evolution of Battery Monitoring: Impedance, Resistance, Conductance or Ohmic Value
The World of Alkaline Batteries
Nickel Zinc’s Powerful Future in Stationary Storage
Batteries
Components
ICs and Semiconductors
Charging & Testing
Power Supplies
Industry News
Marketplace
Calendar of Events
Research & Development

Battery Power - September/October 2011