Detecting failed batteries has become particularly important in a number of key areas. One area, for instance, is to determine whether a battery has failed before a computer system attempts to rely upon it as a backup power supply following a failure of its primary power source.
There are several conventional methods used to measure the condition of rechargeable batteries, including: electrolyte specific gravity, gas-gauging, cell impedance measurements, and open circuit cell voltage. Some methods specifically apply to a given technology. For example, the electrolyte specific gravity methods apply to the specific gravity of the electrolyte applicable to wet cell batteries, where the electrolyte is accessible for testing. Gas-gauging methods apply only to applications where the battery is periodically discharged then recharged (e.g., a cyclic application). Other conventional methods, such as cell impedance methods, tend to require a significant amount of precision circuitry (and expense).
The open circuit method measures the condition of rechargeable batteries provided that the circuit voltage of an electrochemical system is at equilibrium. For a system to reach equilibrium, the battery, for twenty-four to one hundred and twenty hours, must neither receive a charge or current nor supply current to load. Accordingly, it is not practical to use the open circuit voltage method to measure the condition of batteries that power computer backup systems, since such batteries need to be continuously available. Therefore, it is unacceptable to use this method to test typical sealed lead-acid batteries used for computer backups and Uninterruptible Power Supply (UPS) applications.
Since batteries tend to lose charge while not in use, many battery backup power applications continuously provide just enough energy that slightly reverse biases the battery to overcome the battery's internal self-discharge losses and to maintain the battery's charge. Such reverse biasing is generally provided by placing a "float voltage" across the terminals of the battery.
Typical of conventional methods and apparatus that perform battery testing is the need to disconnect the battery from all connected electrical devices, especially lo the charging device before testing the battery. Further, because battery voltage tends to vary significantly when off-line, measuring off-line battery voltage generally does not conclusively establish whether the battery still has an acceptable charge. More particularly, Hawker Energy, a major manufacturer of typical sealed lead-acid batteries used in computer backup and UPS applications, indicates that the open is circuit voltage is an indicator of state of charge to an accuracy of .+-.20% for an off-line battery which has not been charged nor discharged for 24 hours. Hence, conventional testing methods that disconnect a battery for testing are unable to accurately determine battery voltage.
Still further, conventional testing system's need to disconnect a battery from all electrical devices is particularly troublesome if the battery serves as a backup power source, whether used for a telecommunications or computer system, a UPS, or an emergency lighting system that depends upon the battery functioning when needed to supply power. For instance, if a battery backup is not connected to a computer when the computer's primary power source fails, since there is neither a primary nor secondary backup power supply, such a computer system is likely to fail and cause fatal system errors. Such errors may corrupt data and software.
Since a computer may require a battery backup at a moment's notice, the battery should always be operational. Therefore, frequent battery tests are recommended to ensure that a battery is adequately charged when called upon to perform a power backup. However, because conventional battery testing systems generally need to have the battery disconnected from all electrical devices before testing the battery, frequent battery tests increase the likelihood that the battery backup will be unavailable when a primary power source fails. Without a charged battery backup, fatal system errors leading to lost and corrupted software and data could occur.
Conventional battery testing systems do not remedy the problems associated with testing a battery's charge while the battery is connected to a charging device. A reason for such failings is due to conventional systems' inability to distinguish between the voltage placed across the terminals of a battery by a battery charger and the battery's chemically induced voltage.
The purpose for placing a float voltage across the terminals of a battery is to prevent the natural degradation of the battery's charge, thereby prolonging the power and, thus, "life" of the battery. In order to charge a battery, the float voltage placed across the battery terminals must be greater than the voltage naturally generated by the battery's cells. Although helpful with prolonging the life of a battery, the float voltage tends to impede a conventional battery testing systems' ability to measure a battery's charge. Instead of measuring a battery's charge, conventional methods measure the greater of the two voltages, which is generally the float voltage, and are thus unable to determine when a battery fails. Consequently, conventional battery testing methods only determine whether the device providing the float voltage is working, but do not identify when a battery receiving the float voltage fails or is failing.
Also, typical of conventional battery testing systems is an inability to distinguish between the natural variability of voltage of each battery cell and a shorted-out battery cell. In a battery consisting of "n" cells in series, for example, a 48 volt battery consisting of 24 lead-acid cells, each with a potential of 2.0 volts, any one cell provides only 1/n of the total voltage. The battery terminal voltage under the range of charge and discharge conditions will typically vary from 40 to 56 volts in the above example (.+-.16.7% of the nominal 48 V), while the voltage of one cell represents only slightly more than 4% of the battery voltage. Hence, the effect of one bad cell is difficult to distinguish from the normal variance of battery terminal voltage in the conditions of use. Because conventional battery testing methods are unable to distinguish between a battery having cells with a voltage charge less than nominal and a battery having a shorted-out cell, the testing method assumes the worst case, i.e., shorted cells, and therefore classifies the battery as "failed." Consequently, such a battery is prematurely discarded and replaced regardless of whether the battery is operational and meets acceptable voltage and current requirements pursuant to specifications.
Further, cell imbalances during charging and discharging are normal occurrences. Individual battery cell manufacturing variances will cause a series string of battery cells to have slightly different voltages at any given time in the charge or discharge cycle. In order to overcome differences between cell voltages, the float-charge maintains a small overcharge current in order to ensure that the weakest cells in the string receive sufficient charge. Typical conventional battery testing methods do not consider the minimizing effects to cell imbalances caused by a float voltage placed across the terminals of a battery.
As a result, there has been a need for a battery testing method and apparatus that measures battery voltage while on-line to determine whether the battery has failed due to either shorted or open cells, avoids the need to take the battery off-line, avoids the errors caused by measuring float voltage, distinguishes between shorted cells, open cells and less than nominal average cell voltages, does not require periodic discharge/recharge cycles and is inexpensive and does not include complicated test circuitry.
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