I have the BD Basal Thermometer. I turned it on the other day and the beeps seem WAY slower than I remember. (almost 4 seconds) That makes me think that the battery might need to be replaced - do you replace the batteries? If so, how often? Or, is that beep interval what you have with this?
http://www.bd.com/thermometers/images/products/digital_basal_pkg.jpg

If it does need a new battery, I'd compare the price of a whole new thermometer to just the battery. Those little ones tend to be expensive.

Good point, Erica. I think the thermometer was about $10 - $12 ?