  Nanodiamonds containing nitrogen-vacancy (NV) centers offer a versatile
platform for sensing applications spanning from nanomagnetism to in-vivo
monitoring of cellular processes. In many cases, however, weak optical signals
and poor contrast demand long acquisition times that prevent the measurement of
environmental dynamics. Here, we demonstrate the ability to perform fast,
high-contrast optical measurements of charge distributions in ensembles of NV
centers in nanodiamonds and use the technique to improve the spin readout
signal-to-noise ratio through spin-to-charge conversion. A study of 38
nanodiamonds, each hosting 10-15 NV centers with an average diameter of 40 nm,
uncovers complex, multiple-timescale dynamics due to radiative and
non-radiative ionization and recombination processes. Nonetheless, the
nanodiamonds universally exhibit charge-dependent photoluminescence contrasts
and the potential for enhanced spin readout using spin-to-charge conversion. We
use the technique to speed up a $T_1$ relaxometry measurement by a factor of
five.
