Mercury oxide button cell batteries have been widely used in the past, but due to environmental concerns related to mercury content, their usage has been significantly reduced and even banned in some regions. These batteries typically had a voltage rating of around 1.35 volts when fresh. However, it's important to note that the use of mercury in batteries has been phased out due to its toxic nature and potential environmental impact, and alternative chemistries like silver oxide and alkaline have become more common for button cell batteries. Always make sure to check the specifications and labels on the specific battery you are using, as different chemistries can have different voltage ratings.