  Applying an operator product expansion approach we update the Standard Model
prediction of the $B_c$ lifetime from over 20 years ago. The non-perturbative
velocity expansion is carried out up to third order in the relative velocity of
the heavy quarks. The scheme dependence is studied using three different mass
schemes for the $\bar b$ and $c$ quarks, resulting in three different values
consistent with each other and with experiment. Special focus has been laid on
renormalon cancellation in the computation. Uncertainties resulting from scale
dependence, neglecting the strange quark mass, non-perturbative matrix elements
and parametric uncertainties are discussed in detail. The resulting
uncertainties are still rather large compared to the experimental ones, and
therefore do not allow for clear-cut conclusions concerning New Physics effects
in the $B_c$ decay.
