  We perform general-relativistic simulations of charged black holes targeting
GW150914. We show that the inspiral is most efficient for detecting black hole
charge through gravitational waves and that GW150914 is compatible with having
charge-to-mass ratio as high as 0.3. Our work applies to electric and magnetic
charge, and to theories with black holes endowed with U(1) (hidden or dark)
charges. Using our results we place an upper bound on the deviation from
general relativity in the dynamical, strong-field regime of the so-called
theory of MOdified Gravity (MOG).
