  We construct a template to model the post-merger phase of a binary black hole
coalescence in the presence of a remnant $U(1)$ charge. We include the
quasi-normal modes typically dominant during a binary black hole coalescence,
$(\ell,m,n) = \{(2,2,0), (2,2,1)\}$ and also present analytical fits for the
quasinormal mode frequencies of a Kerr-Newman black hole in terms of its spin
and charge, here also including the $(3,3,0)$ mode. Aside from astrophysical
electric charge, our template can accommodate extensions of the Standard Model,
such as a dark photon. Applying the model to LIGO-Virgo detections, we find
that we are unable to distinguish between the charged and uncharged hypotheses
from a purely post-merger analysis of the current events. However, restricting
the mass and spin to values compatible with the analysis of the full signal, we
obtain a 90th percentile bound $\bar{q} < 0.33$ on the black hole
charge-to-mass ratio, for the most favorable case of GW150914. Under similar
assumptions, by simulating a typical loud signal observed by the LIGO-Virgo
network at its design sensitivity, we assess that this model can provide a
robust measurement of the charge-to-mass ratio only for values $\bar{q} \gtrsim
0.5$; here we also assume that the mode amplitudes are similar to the uncharged
case in creating our simulated signal. Lower values, down to $\bar{q} \sim
0.3$, could instead be detected when evaluating the consistency of the
pre-merger and post-merger emission.
