One embodiment includes a battery cell electrode having a first material constructed and arranged to be charged and discharged and having a first potential versus state of charge relationship; a second material having a second potential versus state of charge relationship; wherein said second material is constructed and arranged to become active to transfer ions at a selected state of charge level to produce an observable change in measured potential from said first to said second potential versus relationship, and wherein the amount of the second material ranges from about 2 to about 30 weight percent of the battery cell electrode.