Bi-functional electrode materials, composed with capacitive activated carbon (AC) and battery electrode material, possess higher power performance than traditional battery electrode materials. Negative electrodes of lead acid battery with AC additives (lead-carbon electrode), compared with traditional lead negative electrode, is of much better charge acceptance, and is suitable for the energy storage in hybrid electrical vehicles. In this paper, we discussed the electrochemical processes on AC in lead-carbon electrode. In the charge process of lead-carbon electrode, lead electrodeposits on AC at higher potentials than the open circuit potential of lead, and the large surface area of AC provides extra active sites for lead electrodeposition and the growth of electrochemical active surface area of lead. Namely, AC acts as electron bumper and electron distributor in the charge process of lead-carbon electrode. Most importantly, the hydrogen evolution reaction (HER) on AC is prohibited by the reversible hydrogen adsorption at high charge rates, meanwhile the adsorbed hydrogen contributed to the charge process.