  Recent numerical simulations have found that the Cauchy horizon inside
spherical charged black holes, when perturbed nonlinearly by a
self-gravitating, minimally-coupled, massless, spherically-symmetric scalar
field, turns into a null weak singularity which focuses monotonically to $r=0$
at late times, where the singularity becomes spacelike.
  Our main objective is to study this spacelike singularity. We study
analytically the spherically-symmetric Einstein-Maxwell-scalar equations
asymptotically near the singularity. We obtain a series-expansion solution for
the metric functions and for the scalar field near $r=0$ under the simplifying
assumption of homogeneity. Namely, we neglect spatial derivatives and keep only
temporal derivatives. We find that there indeed exists a generic spacelike
singularity solution for these equations (in the sense that the solution
depends on enough free parameters), with similar properties to those found in
the numerical simulations. This singularity is strong in the Tipler sense,
namely, every extended object would inevitably be crushed to zero volume. In
this sense this is a similar singularity to the spacelike singularity inside
uncharged spherical black holes. On the other hand, there are some important
differences between the two cases. Our model can also be extended to the more
general inhomogeneous case.
  The question of whether the same kind of singularity evolves in more
realistic models (of a spinning black hole coupled to gravitational
perturbations) is still an open question.
