Results were presented on examinations of two valve-regulated lead-acid (VRLA) batteries AGM SVT 300 after six-year exploitation in two industrial systems of uninterruptible power supply. Batteries of the same type, with the nominal service life of twelve years, operated with similar load and the number of cells, but in various temperature conditions. Battery B1, operating without a proper airconditioning for a long time, expressed failure of three battery cells (out of total 106). These failures were followed by significant characteristic degradation of another four cells after completion of the charging process of the previously deeply discharged battery. In the case of the battery B2, exploited with good airconditioning , characteristics of the 100 battery cells (out of total 102) were spotless during the discharge of the 80% of nominal capacity, as well as during the following charging. From two cells from the battery circuit, not being from the same batch as the remaining hundred cells, was discharged only 30% of the nominal capacity. Voltage variations of the weak battery cells were analyzed, as well as their failure mechanisms. Implemented procedure for equalization of the VRLA battery's cells after accomplishing charging process was described. A practice of batteries operation with rectifiers that would enable their operation during the entire declared service life was proposed.