Angular amplitude of a simple pendulum is θ0. Maximum tension in the string for small θ0 will be:
mg1-cos θ0
mg1+ θ0
mg1-θ02
mg 1+ θ02
T-mg=mv2l But v2=2gl 1-cos θ0 So T=mg+2mg1- cos θ0 T=mg+2mg 2 sin2 θ02 T≈mg+4mgθ022 T≈mg1+θ02