  The baryonic Tully-Fisher relation (BTF) is a fundamental relation between
baryonic mass and maximum rotation velocity. It can be used to estimate
distances, as well as to constrain the properties of dark matter and its
relation with the visible matter. In this paper, we explore if extremely
low-mass dwarf galaxies follow the same BTF relation as high-mass galaxies. We
quantify the scatter in the BTF relation and use this to constrain the allowed
elongations of dark matter halo potentials. We obtained HI synthesis data of 11
dwarf galaxies and derive several independent estimates for the maximum
rotation velocity. Constructing a BTF relation using data from the literature
for the high-mass end, and galaxies with detected rotation from our sample for
the low-mass end results in a BTF with a scatter of 0.33 mag. This scatter
constrains the ellipticities of the potentials in the plane of the disks of the
galaxies to an upper limit of 0-0.06 indicating that dwarf galaxies are at most
only mildly tri-axial. Our results indicate that the BTF relation is a
fundamental relation which all rotationally dominated galaxies seem to follow.
