How to test quartz watch battery capacity?

　　The capacity of a quartz watch battery is related to its volume; the larger the volume, the greater the capacity, which in turn determines the battery's service life. So, how can we test the capacity of a quartz watch battery?

　　Methods for testing quartz watch battery capacity

　　Once the battery voltage begins to drop, its service life is nearing its end. To avoid disrupting the normal operation of the watch, many quartz timepieces are equipped with a low-battery warning function. This is indicated by the second hand pausing for 4 seconds and then jumping forward 4 seconds in a rapid sequence. Once this occurs, the watch will typically continue to run for approximately 10 days, serving as a reminder that the battery needs to be replaced immediately. This feature is known as 'E.O.L' (End of Life), though it is not present in quartz watches without a centre second hand.

　　In reality, as with all batteries, the capacity of new batteries is not entirely uniform. Battery capacity is measured in milliampere-hours (mAh). To date, I have not heard of any instrument capable of quickly, non-destructively, and accurately testing the capacity of a silver-oxide battery. Over 20 years ago, I heard that there was a significant reward offered abroad for the invention of such a device, yet no one has managed to create one to this day.

　　When battery manufacturers conduct quality control inspections, they still rely on batch discharge testing. This involves connecting the battery to a load to create a closed circuit with a specific current. Once the battery voltage drops to a predetermined level, the actual capacity is calculated by multiplying the discharge current by the discharge time. Once the test is complete, the battery is considered spent.

　　Current methods for rapidly testing quartz watch batteries involve heavy-load discharge to observe the voltage drop under different currents. This is because, with use, the internal resistance of the battery increases; consequently, when the discharge current increases, the closed-circuit voltage drops sharply.

　　Some Swiss quartz timing machines include a heavy-load battery discharge testing function, with load resistance settings of 2000 ohms, 50 ohms, and 5 ohms (see photo). In practice, silver-oxide batteries used in watches have different applications, such as high-current output and low-current output types, which differ in their electrolyte composition. The former is typically used for LCD quartz watches with lights and alarms, while the latter is used for analogue quartz watches.

　　When the button is pressed, the load resistance is engaged, and the extent of the voltage drop is immediately observed. Different batteries should have different specifications, but generally, the voltage should not drop below 1.35V.

　　The cross-sectional area of the battery and the temperature during testing both affect the results; the larger the battery and the higher the ambient temperature, the less the voltage will drop.

　　Heavy-load battery testing can only provide a rough estimate of battery capacity rather than a precise numerical measurement. Furthermore, this test cannot be performed for long periods as it consumes a significant amount of the battery's energy.