At what voltage should a 9 volt battery be replaced?

Related questions

1. Are lithium batteries better?
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3. Why do dead batteries bounce?
4. Why are lithium batteries not allowed on airplanes?
5. Does mass change when body is charged?
6. What voltage is a 1.5 volt battery considered dead?
7. What brand of battery lasts the longest?
8. What voltage are D batteries?
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