  Direct detection of black hole-neutron star pairs is anticipated with the
advent of aLIGO. Electromagnetic counterparts may be crucial for a confident
gravitational-wave detection as well as for extraction of astronomical
information. Yet black hole-neutron star pairs are notoriously dark and so
inaccessible to telescopes. Contrary to this expectation, a bright
electromagnetic transient can occur in the final moments before merger as long
as the neutron star is highly magnetized. The orbital motion of the neutron
star magnet creates a Faraday flux and corresponding power available for
luminosity. A spectrum of curvature radiation ramps up until the rapid
injection of energy ignites a fireball, which would appear as an energetic
blackbody peaking in the x ray to $\gamma$ rays for neutron star field
strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\odot}
$ black hole. The fireball event may last from a few milliseconds to a few
seconds depending on the neutron star magnetic-field strength, and may be
observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per
year for neutron star field strengths $\gtrsim 10^{14}$G. We also discuss a
possible decaying post-merger event which could accompany this signal. As an
electromagnetic counterpart to these otherwise dark pairs, the black-hole
battery should be of great value to the development of multi-messenger
astronomy in the era of aLIGO.
