Predicting Battery Life Accurately
Q: Is a voltage measurement the best predictor of battery life?
A: No. Batteries have an internal resistance or impedance to prevent them from operating at their full capacity. This means that the capacity of a battery to deliver voltage and amperage will vary depending on the load that you put on the battery.
When you measure the battery with a voltmeter, with the equipment disconnected and the load removed, a relatively small load is created by the voltmeter to allow the measurement. In many cases the battery recovers enough voltage so that the voltage reading will appear normal. This is especially true of nickel-based batteries.
Even a battery with high impedance may perform well if loaded with a low DC current such as a flashlight. With such a small load, virtually all of the stored energy can be retrieved and the deficiency of high impedance goes undetected. So when a higher load is placed on the battery it quickly loses it charge.
The best way to measure the true internal resistance of a battery is with a dedicated impedance meter. One method known as the DC load test measures the battery’s internal resistance by reading the voltage drops of two loads of different strengths. A large voltage drop indicates high resistance. The AC method measures the phase shift between voltage and current. The battery’s reactance and/or voltage deflections are used to calculate the impedance. Either test can identify batteries that would fail due to high internal resistance, even though the capacity may still be acceptable.
Temperature around the battery, the age of the battery, and the number of charge cycles all impact battery life. Large energy losses will occur through self-discharge if a battery is left in a hot vehicle.
Q: What specific requirements for power supplies are included in NFPA 731?
ANFPA 731 requires that power supplies are sized based upon the application and manufacturer’s requirements to provide adequate power for simultaneous use of all associated devices, such as readers, RTE motion detectors, locks, controllers and so forth. A safety factor of twenty-five percent is required to be added to the total power needs.
Site Codes and Cards
Q: Are site codes becoming obsolete?
A: Years ago sites codes were created to allow numbers to be reused by duplicating cards rather than increasing the available number of user codes. As long as duplicate use of a site code within a specific geographical area is avoided, a level of security is maintained. You also have to be able to easily get additional cards in that site code or need to maintain a sufficient inventory of cards. Because of these difficulties most systems now support multiple site codes or support a totally unique credential. This eliminates the need to maintain a card inventory for each site because cards from a general inventory can be used.
Brad Shipp is a former Executive Director and Training Director for the NBFAA where he authored several NTS courses, including the Access Control Certification course. His involvement in the access control industry dates back to 1974 and, in 1986, he became an instructor for the NBFAA National Training School. Shipp has served on several law enforcement, regulatory and industry association boards and has been honored for his service by the False Alarm Reduction Association and the International Association of Security and Investigative Regulators. Send in your access control questions to firstname.lastname@example.org.