A temporary disconnection of the battery from the system

July 18 [Thu], 2013, 10:51
The resistance of a battery provides useful information about its performance and detects hidden trouble spots. High resistance values are often the triggering point to replace an aging battery, and determining resistance is especially useful in checking stationary batteries. However, resistance comparison alone is not effective, because the value between batches of lead acid batteries can vary byBrand new RM791 eight percent. Because of this relatively wide tolerance, the resistance method only works effectively when comparing the values for a given battery from birth to retirement. Service crews are asked to take a snapshot of each cell at time of installation and then measure the subtle changes as the cells age. A 25 percent increase in resistance over the original reading hints to an overall performance drop of 20 percent.

Manufacturers of stationary batteries typically honor the warranty if the internal resistance increases by 50 percent. Their preference is to get true capacity readings by applying a full discharge. It is their belief that only a discharge can provide reliable readings and they ask users to perform the service once a year. While this advice has merit, a full discharge requires a temporary disconnection of the battery from the system, and on a large battery such a test takes an entire day to complete. In the real world, very few battery installations receive this type of service and most measurements are based on battery resistance readings.

Measuring the internal resistance is done by reading the voltage drop on a load current or by AC impedance. The results are in ohmic values. There is a notion that internal resistance is related to capacity, and this is false. The resistance of many batteries stays flat through most of the service life. Figure 1 shows theBrand new Vostro 1310 capacity fade and internal resistance of lithium-ion cells.


To estimate capacity and state-of-charge on the fly involves impedance trending by scanning a battery with frequencies ranging from less than one hertz to several thousand hertz. Read more about Testing Lead Acid Batteries.
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