1. An apparatus for estimating a battery life, the apparatus comprising: a battery monitor configured to monitor an output signal from a battery; a steady state detector configured to detect a first steady state in which the output signal stays in a steady state and a second steady state differing from the first steady state; and an estimator configured to estimate a battery life from a variation of the output signal in a transient state in which the output signal transitions from the first steady state to the second steady state.