A simple theoretical model is presented to simulate the galvanostatic discharge behavior of the Ni-composite graphite electrode. The discharge profiles predicted by using a constant diffusion coefficient (CDC) and by a varied diffusion coefficient (VDC) are compared in this paper. The results show that, the VDC model can be simplified to the CDC for discharge rates less than 2C for a 5μm particle. Also, an approximate analytical solution is presented for VDC model, which is found to be valid for discharge rates up to 6C. Exchange current and diffusion coefficient for the lithium-diffusion are predicted.