As zinc air hearing aid batteries operate differently than other batteries they have different characteristics. In winter, rooms may be drier, warmer or badly ventilated. These effects can disturb the performance of your batteries.
The results of a voltmeter vary depending on the type of voltmeter used. For a more accurate result, be sure to use a voltmeter with more than 1 megaohm input resistance for alkaline buttons batteries or a 10+ megaohm for lithium batteries.
Once a seal is peeled off, the battery starts ‘breathing’ and therefore gives of energy, even if it’s not used in a hearing aid. The battery therefore won’t last until the expiry date if the seal is removed.