The axial charge in the weak nucleon axial vector current ha, been precisely determined by measuring the alignment correlation terms in the beta-ray angular distributions of the purely spin aligned mirror pair nuclei B-12(l(pi) = 1(+), T-1/2 = 20.2 ms) and N-12(l(pi) = 1(+), T-1/2 = 11.0 ms). The axial charge was determined to be = 4.96+/-0.09 (stat.) +/- 0.05 (syst.) at a 90% confidence level. The previously obtained data in the year 1996 was reanalyzed to be added to the present result, The combined result is y = 4.90+/-0.10 at a 90% confidence level. The axial charge is enhanced as much as (72+/-4)% from the impulse approximation model calculated value y(1A)= 2.85. Calculations using the impulse approximation with the soft-pi contribution y(th) = 4.15 explain the data quite well but not completely. If we introduce the in-medium mass renormalization the unexplained part of the experimental result corresponds to an in nuclear-medium nucleon mass reduction of (16+/-4)% at a 90% confidence level relative to tire free nucleon mass, at the place where the decaying nucleon resides.