An aqueous solution of carbon colloid was prepared by electrochemical oxidation of a graphite anode. It was found that the addition of this colloid into the electrolyte of lead acid batteries enhanced the charge-discharge capacity and extended life. The mechanism for this could be as follows: As a result of electrochemical oxidation the surface of the carbon particles is subjected to a chemical modification, and is endowed with hydrophilic groups such as carboxyl. This was confirmed with ESCA spectroscopy. When the carbon colloid is added to an electrolyte in a lead acid battery, the cathode (PbO2) is electrochemically doped with the carbon particles. This is supported by the results of voltammetric and optical absorption spectroscopic studies. Since PbO2 is a semiconductor and is used in a granular form, some of the particles probably do not have good electrical connection, and are not fully utilized. It is expected that doped carbon particles could provide electrical connection to non-utilized PbO2 particles, which enhances the charge-discharge capacity and extends the life of lead acid batteries. (C) 2000 Elsevier Science S.A. All rights reserved.