  Charge and spin transport in spintronics devices can be described by a spin
diffusion equation suitable for modelling scales much larger than the
scattering and atomic scales. This work concerns the coarse graining procedure
used to compute the coefficients of the diffusion equation, which are sensitive
the details of individual atoms and impurities. We show with two simple
examples that in spintronics devices which have both a spin-orbit interaction
and magnetization, standard coarse graining can easily obtain diffusion
equations which fail to conserve electronic charge. The same failure can occur
in systems with both a spin-orbit interaction and orbital polarization. We show
that linear response theory, coupled with the self-consistent Born
approximation and ladder diagrams, offers an improved way of calculating
diffusion equations. We show that the resulting equations satisfy a
Ward-Takahashi identity that guarantees charge conservation.
