Can the Uninterruptible Power Supply (UPS) be enabled at all times?

　　Figure 1: Test setup used to evaluate monitoring devices

Aging Issues

　　Most of these systems use lead-acid battery technology, which is a well-known technical drawback of aging, which causes capacity loss and increased internal resistance. However, because the technology is so mature and aging conditions are so well understood, several conditions can be detected to determine aging conditions.

　　One particularly common effect is capacity reduction, which is largely the result of the battery's usage pattern. Inside the UPS, the battery is discharged at high currents, which causes large crystals to form on the electrodes. This condition can be partially controlled by properly conditioning the battery, but it has proven to be irreversible in severe cases. This condition also forms small crystals, called "dendrites," which can link together and short the battery if not detected.

　　Internal corrosion, where thin pieces of the terminal fall onto the electrodes, can also cause a short circuit. Important factors that cause corrosion include temperature, voltage, and local acid concentration, usually affecting the positive terminal. These aging effects all cause a loss of battery capacity or charge, so any diagnostic must be able to identify them so that appropriate action can be taken before catastrophic failure occurs. The

　　above effects cause a loss of battery capacity or charge. Any type of diagnostic should aim to identify these aging effects.

　　In the tests conducted, full spectrum measurements were made using electrochemical impedance spectroscopy (EISmeter analyzer from RWTH Aachen University); the battery is measured using a series of sinusoidal waveforms to measure the impedance across the spectrum. The measurement results are obtained by calculating the actual and imaginary voltage response parts for a given frequency using Fourier analysis. The complex impedance results are obtained by analyzing the magnitude and phase relationship of the voltage response to the excitation current.

　　This is impractical for the Sentinel solution, as the processing power required to do this would make any solution for continuous monitoring of the system unviable commercially. The challenge was therefore to develop a method that could measure using only one frequency but still obtain results comparable to those of the EISmeter.

　　Trend Analysis

　　As shown in the results, the two values ​​measured with the EISmeter and the Sentinel are in very good agreement. Although the value returned by the Sentinel is slightly higher, this can easily be compensated by calibration. However, for the purpose of battery diagnostics, this deviation is of relative rather than absolute importance. Since the measurements are made continuously, it is important to clearly see trend data from the results. These data, together with the temperature and voltage values, which are both measured using a single integrated circuit, form the information basis for the Sentinel solution.

　　Sentinel is the first monolithic integrated circuit (system on a chip) for monitoring VRLA and flooded batteries, capable of measuring internal temperature, voltage and standard impedance for both individual cells and entire batteries. Each Sentinel III module monitors individual cells or battery packs with nominal voltages between 0.9 and 16 volts, reporting data to a data logger called an S-BOX via a communications bus called the S-BUS.

　　The function of the Sentinel is to obtain key electrical parameters of the test to determine if the battery can function in the event of a main power failure.

　　A single serial bus can connect up to 250 Sentinel modules, set up in groups of up to six, and monitor up to six float/discharge currents, making installation extremely easy, just plug the plug into the socket using the pre-terminated data bus cable. 

　　In addition, Sentinel III is designed to be easy to install, taking about a quarter of the time required to install other systems. This is achieved through monolithic circuit design and simplified communication systems. Each independent unit uses LEM's proprietary communication bus called S-BUS bus, operates independently, but is directly controlled by the central intelligent unit called S-BOX; the monitor and data recorder have comprehensive alarm parameters and data storage devices (see Figure 4).

  
　　It is the detailed measurements coupled with intelligent data analysis that provide reliable reporting on the true battery condition and availability. Sentinel III provides accurate temperature, voltage and impedance data for cells or entire batteries. The software in the central data logging and analysis unit tracks data changes over time, extracts trend information, and provides users with real-time information on the performance of the backup battery after it is put into service. At the level of individual cells or entire batteries, the system identifies faulty battery components, generates alarms for complete failures, and requests manual inspection. Since the S-BOX box is also connected to a web server, all performance, trend and alarm data can be viewed via the Internet; non-emergency status updates are provided in the form of standard messages, allowing administrators to monitor the device from anywhere in the world.  　　Since the Sentinel itself is powered by the monitored battery, it is designed to remain in "sleep" mode most of the time and only "wake up" when taking measurements. The wake-up cycle takes less than 100 milliseconds and wakes up approximately every 5-10 minutes. Since the Sentinel III spreads the test load current across the internal resistors, the minimum impedance measurement cycle is 10 minutes to reduce internal temperature rise. This interval is short compared to the time scale of battery parameter changes, and in practice many operators will require longer intervals between impedance measurement cycles. Therefore, most of the time, the Sentinel consumes very little power from the main battery.