Equilibrium of an electrochemical cell
You should consider the Nernst equation
$$
\Delta G = \Delta G^{\ominus} + RT\ln Q=- RT\ln Keq + RT\ln Q
 $$
At equilibrium deltaG is zero and Q the reaction coefficient is Keq also note
$$
\Delta G = -nFE
 $$
so E is zero at equilibrium. Outside equilibrium Q will move towards K decreasing E.