The present invention is directed to a method for analyzing the tail-end behavior of a lithium cell having a solid cathode. The tail of a longer-term accelerated discharge data (ADD) test is estimated from the tail of two shorter-term ADD tests. This is accomplished by first comparing the discharge tails of shorter-term ADD tests and determining angles or rotation that correspond to Rdc growth, and then trending rotation angles versus time to reach a give DoD. For example, the 18-month and 36-month ADD test tails are used to estimate the ADD test tail of a similarly constructed cell subjected to a longer-term ADD test, for example a 48-month ADD test.