  We develop an analytical ringdown waveform model, including both the
fundamental and the overtone quasinormal modes, for charged black holes and
show that it is precise enough to analyze current gravitational wave data.
Applying this waveform model to GW150914, the charge to mass ratio of the
remnant black hole ($\lambda_f$) has been constrained with the sole ringdown
gravitational wave data for the first time and the $90\%$ upper limit is
$\lambda_f\leq 0.38$. Correspondingly, the deviation parameter of the
scalar-tensor vector gravity ($\alpha_{\rm s}$) is limited to be $\alpha_{\rm
s}\leq 0.17$. Our approach can be directly applied to other binary black hole
mergers with loud ringdown radiation.
