Context : Recent work identified a growth barrier for dust coagulation that originates in the electric repulsion between colliding particles .
Depending on its charge state , dust material may have the potential to control key processes towards planet formation such as MHD ( magnetohydrodynamic ) turbulence and grain growth which are coupled in a two-way process .

Aims : We quantify the grain charging at different stages of disc evolution and differentiate between two very extreme cases : compact spherical grains and aggregates with fractal dimension D _ { f } = 2 .

Methods : Applying a simple chemical network that accounts for collisional charging of grains , we provide a semi-analytical solution .
This allowed us to calculate the equilibrium population of grain charges and the ionisation fraction efficiently .
The grain charging was evaluated for different dynamical environments ranging from static to non-stationary disc configurations .

Results : The results show that the adsorption/desorption of neutral gas-phase heavy metals , such as magnesium , effects the charging state of grains .
The greater the difference between the thermal velocities of the metal and the dominant molecular ion , the greater the change in the mean grain charge .
Agglomerates have more negative excess charge on average than compact spherical particles of the same mass .
The rise in the mean grain charge is proportional to N ^ { 1 / 6 } in the ion-dust limit .
We find that grain charging in a non-stationary disc environment is expected to lead to similar results .

Conclusions : The results indicate that the dust growth and settling in regions where the dust growth is limited by the so-called ” electro-static barrier ” do not prevent the dust material from remaining the dominant charge carrier .