According to the suggestion of Sader et al., who replaced the reduced surface potential Y = e psi(0)/kT by the effective reduced surface potential Y(h) = 4e(kappa h/2)tanh(-1) [e(-kappa h/2)tanh(Y14)] (where psi(0) is the surface potential of particles, k is Boltzmann’s constant, T is the absolute temperature, e is the proton charge, h is the distance of closet approach between the spheres, kappa is the Debye screening parameter), we return to the double layer interaction at moderate potential under constant surface potential and derive a formula for the interaction energy between two parallel plates, valid up to the moderate to high potential regime. Using an improved Derjaguin approximation we derive formulae for interaction energy and force for identical spherical colloidal particles, which are more accurate than the formulae of Sader et al. at moderate potential.