We have run kinetic ring current simulations with different plasma boundary conditions, but preserving a constant plasma pressure, during a "test" time interval conducive to a storm. Although the energy density of the injected plasma is kept constant, the resulting proton ring current energy is significantly increased as the injected plasma is colder and denser. Investigation of the separate effects of density and temperature shows that (1) ring current strength is primarily controlled by density, and that (2) colder plasma leads to a ring current peak closer to Earth and to midnight, but density has no effect. While cold plasma may convect deeper inside the ring current region, hotter plasma is more subject to magnetic drifts and quickly drifts toward dusk with lower energization. These results support the idea that the presence of cold and dense plasma in the magnetotail may lead to increased ring current during an ensuing storm.