  An investigation of magnetic fields generated in an expanding bubble of
plasma with misaligned temperature and density gradients (driving the Biermann
battery mechanism) is performed. With gradient scales $L$, large-scale magnetic
fields are generated by the Biermann battery mechanism with plasma $\beta \sim
1$, as long as $L$ is comparable to the ion inertial length $d_i$. For larger
system sizes, $L/d_e > 100$ (where $d_e$ is the electron inertial length), the
Weibel instability generates magnetic fields of similar magnitude but with
wavenumber $k d_e \sim 0.2$. In both cases, the growth and saturation of these
fields have a weak dependence on mass ratio $m_i/m_e$, indicating electron
mediated physics. A scan in system size is performed at $m_i/m_e = 2000$,
showing agreement with previous results with $m_i/m_e = 25$. In addition, the
instability found at large system sizes is quantitatively demonstrated to be
the Weibel instability. Furthermore, magnetic and electric energy spectra at
scales below the electron Larmor radius are found to exhibit power law behavior
with spectral indices $-16/3$ and $-7/3$, respectively.
