When measuring a battery with a tester, it indicates plus and minus reversely. Why?

The reason could be that discharge balance was lost at the last period of discharge. OR when fresh and used batteries are mixed in the same appliance. As a consequence, the polarity reversal phenomenon occurred.

Polarity reversal may happen when more than two batteries are connected in series and used until the last moment of discharge.

When a large capacity of electric current beyond the limit leaves the battery, a minor difference in battery capacity could result in a serious decrease in electric balance - reversing the batteries’ polarity.

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