  We apply the quantum-defect theory for $-1/R^4$ potential to study the
resonant charge exchange process. We show that by taking advantage of the
partial-wave-insensitive nature of the formulation, resonant charge exchange of
the type of $^1$S+$^2$S can be accurately described over a wide range of
energies using only three parameters, such as the \textit{gerade} and the
\textit{ungerade} $s$ wave scattering lengths, and the atomic polarizability,
even at energies where many partial waves contribute to the cross sections. The
parameters can be determined experimentally, without having to rely on accurate
potential energy surfaces, of which few exist for ion-atom systems. The theory
further relates ultracold interactions to interactions at much higher
temperatures.
