  Most black holes (BHs) will absorb a neutron star (NS) companion fully
intact, without tidal disruption, suggesting the pair will remain dark to
telescopes. Even without tidal disruption, electromagnetic luminosity is
generated from the battery phase of the binary when the BH interacts with the
NS magnetic field. Originally the luminosity was expected in high-energy X-rays
or gamma-rays, however we conjecture that some of the battery power is emitted
in the radio bandwidth. While the luminosity and timescale are suggestive of
fast radio bursts (FRBs; millisecond-scale radio transients) NS--BH coalescence
rates are too low to make these a primary FRB source. Instead, we propose the
transients form a FRB sub-population, distinguishable by a double peak with a
precursor. The rapid ramp-up in luminosity manifests as a precursor to the
burst which is $20\%-80\%$ as luminous, given 0.5~ms timing resolution. The
main burst is from the peak luminosity before merger. The post-merger burst
follows from the NS magnetic field migration to the BH, causing a shock. NS--BH
pairs are especially desirable for ground-based gravitational wave (GW)
observatories since the pair might not otherwise be detected, with
electromagnetic counterparts greatly augmenting the scientific leverage beyond
the GW signal. Valuably, the electromagnetic signal can break degeneracies in
the parameters encoded in the GW as well as probe the NS magnetic field
strength, yielding insights into open problems in NS magnetic field decay.
