  We have performed global fits of $f_{\pi}$ and $m_{\pi}$, from a variety of
RBC-UKQCD domain wall fermion ensembles, to $SU(2)$ partially quenched chiral
perturbation theory at NNLO. We report values for 9 NLO and 8 linearly
independent combinations of NNLO partially quenched low energy constants, which
we compare to other lattice and phenomenological determinations. We discuss the
convergence of the expansion and use our large set of low energy constants to
make predictions for the pion mass splitting due to QCD isospin breaking
effects and the s-wave $\pi \pi$ scattering lengths. We conclude that, for the
range of pseudoscalar masses explored in this work, $115~\mathrm{MeV} \lesssim
m_{\rm PS} \lesssim 430~\mathrm{MeV}$, the NNLO $SU(2)$ expansion is quite
robust and can fit lattice data with percent-scale accuracy.
