The electric quadrupole coupling constant of the Al-31 ground state is measured to be nu(Q) = vertical bar eQV(zz)/h vertical bar = 2196(21) kHz using two different beta-NMR (Nuclear Magnetic Resonance) techniques. For the first time, a direct comparison is made between the continuous If technique and the adiabatic fast passage method. The obtained coupling constants of both methods are in excellent agreement with each other and a precise value for the quadrupole moment of Al-31 has been deduced: vertical bar Q(Al-31)vertical bar = 134.0(16) mb. Comparison of this value with large-scale shell-model calculations in the sd and sdpf valence spaces Suggests that the Al-31 ground state is dominated by normal sd-shell configurations with a possible small contribution of intruder states. The obtained value for vertical bar Q(Al-31)vertical bar and a compilation of measured quadrupole moments of odd-Z even-N isotopes in comparison with shell-model calculations shows that the proton effective charge e(p) = 1.1e provides a much better description of the nuclear properties in the sd-shell than the adopted value e(p) = 1.3e. (C) 2009 Elsevier B.V. All rights reserved.