Photovoltaic systems connected to lead-acid batteries represent particularly convenient solutions for the so-called solar home system (SHS). Batteries for photovoltaic installations generally suffer from two typical problems, electrolyte stratification, which causes irreversible sulfating of the plates when the battery is not fully charged, and softening of the positive active mass, when the battery is completely charged. Since the low cost of the entire system is one of the objectives of SHS applications, starting-lighting-ignition batteries, based on flooded plane-plate technology with lead calcium grids, can been considered as good candidates. The present work was mainly aimed at clarifying the phenomena concerning the influence of phosphoric acid and colloidal silica on the behavior of the positive active mass of the lead-acid battery. Due to the complexity of the real system, the investigation was carried out with pure lead dioxide electrodeposited onto flat gold electrodes. The research activity was not limited to laboratory-scale electrodes, but included measurements on batteries and plates with the primary scope of developing low-cost batteries for photovoltaic applications. The obtained results are discussed on the basis of the laboratory tests with electrodeposited lead dioxide. It is also shown that a new acid formulation using 4% of silica and 2.2% of phosphoric acid, tested in standard automotive batteries under seasonal cycling operation, leads to improvements in low-cost batteries applications in solar home systems. Colloidal silica has a beneficial effect in preventing the stratification of the electrolyte while the presence of phosphoric acid delays the softening of the active material. (C) 2003 The Electrochemical Society.