The discharge mechanism of charging microparticles in the charge case of the injector of an electrostatic accelerator is analyzed. The processes of origination and spread of the plasma discharge that occurs as the microparticle approaches the charging electrode are discussed. To determine the charge of the particles obtained as a result of microdischarge the problem of expansion of multi-component plasma under the influence of an electrostatic field is solved. A system of equations that describe the dynamics of the expanding plasma bunch, ionization and recombination processes, taking into account the influence of the electrostatic field on the plasmoid surface current, is solved. The described system of differential equations makes it possible to find the degree of ionization of chemical elements contained in the plasma bunch and the amount of ions as a function of time and chemical composition. The dependences of the degree of ionization of ions on the composition of the plasma bunch and the voltage on the charging electrode are obtained. The dependence of the particle charge on the voltage of the needle electrode is shown.