In order to investigate the behaviour of valve-regulated lead/acid batteries in solar power applications, gel and AGM batteries were installed in different solar power systems. Each system is divided into several groups and each group has the same battery type, the same loading and the same solar generator. The only difference is the charge/discharge strategy. A key result after 2 years of testing is that the charge strategies which are typically used today in the field cannot charge the batteries completely. However, if the batteries were charged intensively afterwards they returned to full capacity. This means that there is a problem of undercharging in the field. Improving the charge/ discharge strategy can, therefore, extend the service life and the energy turnover of VRLA batteries in solar power applications. Moreover, some existing VRLA battery types were modified with regard to the amount of electrolyte and phosphoric acid. These versions were investigated in the laboratory and are included in the field tests. (C) 2001 Elsevier Science B.V. All rights reserved.