This paper presents the results from a comparative study between two types of valve-regulated lead-acid battery cells, with uncoated and polymer composite coated absorptive glass mat (AGM) separators. The volt-ampere characteristics of the studied cells, recorded at different ambient temperatures, show that the cells with polymer coated separators have significantly lower overcharge (recombinant) current than the cells with conventional untreated AGM separator. During overcharge, the higher recombinant current in the cells with plain separator leads to higher cell temperature than that of the cells with polymer coated AGM separator. The possibility to avoid thermal runaway (TR) is also illustrated during polarization of the cells at 2.65 V. After 320 h, a conventional cell has C/4 current (trend to TR), while the cells with composite coating sustain low (C/26) constant current for long period of time (at least 650 h). The cycle life test indicates stable operation of the cells with coated separator, while the conventional cell reaches high recombinant current and thus, it is susceptible to thermal runaway phenomena.