In an inhomogeneously-doped magnetic semiconductor, spin relaxation time T1 can be determined by all-electrical measurements. Nonequilibrium spin injected in a magnetic p–n junction gives rise to the spin-voltaic effect, in which the nonequilibrium spin-induced charge current is very sensitive to T1 and can flow even at no applied bias. It is proposed that T1 can be determined by measuring the I–V characteristics in such a geometry. In a magnetic p–n junction, for which the results can be calculated analytically, it is also possible to extract the g-factor and the degree of injected-carrier spin polarization.