Should be a sticker on the side. The first character will be a number from zero through nine that corresponds to the last digit in the year in which the battery was manufactured. For example, a five would mean 2015. The second character is a letter that refers to the month the battery was made. The letters A through L directly correspond to January through December. For example, the letter 'C' would mean March. Putting it together, "5C" indicates a manufacturing date of March 2015. These characters can be interchangeable, so "5C" and "C5" mean the same thing.
