When I measure a used alkaline battery with a tester, the needle swings in the opposite direction (indicates a negative voltage). Why?

2 November 2021

Although it does not occur in the unused state, when two or more alkaline batteries are connected in series and discharged, the battery with the smaller remaining capacity may show a negative voltage at the end of discharge. This is called turning.
As an easy-to-understand example, when two batteries A (battery with 50% remaining capacity) and battery B (battery with 100% remaining capacity) are discharged in series, when the capacity of battery A becomes zero, battery B There is still 50% of the capacity left. If the discharge is continued as it is, the battery A whose capacity has become zero will be forcibly discharged by the power of the battery B, and the battery A will be discharged to a negative potential and will be in a repolarization state.
Polarization is a phenomenon that occurs in batteries at the end of discharge, and is not an abnormal phenomenon for batteries. However, in a repolarized battery, gas is suddenly generated inside the battery and the internal pressure rises, so the gas discharge valve for burst prevention operates and the gas is released to the outside. At that time, the electrolyte may also be released (a phenomenon called “leakage”).
Repolarization can occur by leaving the battery in the device, so remove the used battery from the device immediately. Remove the battery even if you do not use the device for a long time. In addition, a combination of used batteries, a combination of old and new batteries (the capacity of the battery gradually decreases after it is manufactured, even if it is not discharged), a combination of different types of batteries (manganese dry batteries and alkaline batteries), different manufacturers. It may occur in combination with the battery of. When using multiple batteries, use the same brand of batteries with the same recommended expiration date.

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