  We investigate the upper bound of charge diffusion constant in holography.
For this purpose, we apply the conjectured upper bound proposal related to the
equilibration scales ($\omega_{\text{eq}}, k_{\text{eq}}$) to the
Einstein-Maxwell-Axion model. ($\omega_{\text{eq}}, k_{\text{eq}}$) is defined
as the collision point between the diffusive hydrodynamic mode and the first
non-hydrodynamic mode, giving rise to the upper bound of the diffusion constant
$D$ at low temperature $T$ as $D = \omega_{\text{eq}}/k_{\text{eq}}^2$. We show
that the upper bound proposal also works for the charge diffusion and
($\omega_{\text{eq}}, k_{\text{eq}}$), at low $T$, is determined by $D$ and the
scaling dimension $\Delta(0)$ of an infra-red operator as $(\omega_{\text{eq}},
\, k_{\text{eq}}^2) \,=\, (2 \pi T \Delta(0) \,, \omega_{\text{eq}}/D)$, as for
other diffusion constants. However, for the charge diffusion, we find that the
collision occurs at real $k_{\text{eq}}$, while it is complex for other
diffusions. In order to examine the universality of the conjectured upper
bound, we also introduce a higher derivative coupling to the
Einstein-Maxwell-Axion model. This coupling is particularly interesting since
it leads to the violation of the \textit{lower} bound of the charge diffusion
constant so the correction may also have effects on the \textit{upper} bound of
the charge diffusion. We find that the higher derivative coupling does not
affect the upper bound so that the conjectured upper bound would not be easily
violated.
