Do not store the unit in non-breathable material i.e. plastic bag or damp gym bag.  If the battery needs changing a clear indication is that the display fades.  This may be apparent when the digits fade when the back light is used.  When changing the battery in your suunto heart rate monitor watch ensure this is done by a fully qualified service centre as the rubber seal should be check and the unit should be tested for water resistance every time it is opened.