The Reservoir node defines a current balance and a material balance for the salt, here /, in a free electrolyte domain.

The settings of the Electrolyte conductivity σl (SI unit: S/m) and the Electrolyte salt diffusivity Dl (SI unit: m2/s) values defines the electrolyte salt transport due to migration and diffusion.

Also convective transport can be of importance in lead acid cells. You may set the Mean surface-averaged velocity expression to use analytical expressions for the velocity, based on the volume changes and the electrolyte current for either the Negative electrode main reaction or Positive electrode main reaction, or use User defined for an arbitrary expression for the Velocity u (SI unit: m/s)