  As the spin-triplet superconductivity arises from the condensation of spinful
Cooper pairs, its full characterization requires not only charge ordering, but
also spin ordering. For a two-dimensional (2D) easy-plane spin-triplet
superconductor, this na\"{i}vely seems to suggest the possibility of two
distinct Berezinskii-Kosterlitz-Thouless (BKT) phase transitions, one in the
charge sector and the other in the spin sector. However, it has been recognized
that there are actually three possible BKT transitions, involving the
deconfinement of, respectively, the conventional vortices, the merons and the
half-quantum vortices with vorticity in both the charge and the spin current.
We show how all the transitions can be characterized by the relation between
the voltage drop and the spin-polarized current bias. This study reveals that,
due to the hitherto unexamined transport of half-quantum vortices, there is an
upper bound on the spin supercurrent in a quasi-long range ordered spin-triplet
superconductor, which provides a means for half-quantum vortex detection via
transport measurements and deeper understanding of fluctuation effects in
superconductor-based spintronic devices.
