At the nuclear power plant in Forsmark, several different securitysystems are used. One of these systems comprises uninterruptiblepower supply (UPS) and consists of lead-acid batteries. The batteries are dimensioned to supply the load for two hours and are constantly charged. This to ensure they’re fully charged in case the regular power source fails. Discoveries have been made that the lifetime of the batteries are shorter than calculated and this thesis examines if the environmental temperature is the reason. Higher temperatures will cause the batteries to corrode more easily which leads to a decrease of the capacity of the batteries. The temperature have been logged in selected areas during two weeks in April 2018. The results have been compared to the meteorological temperature and analyzed together with capacity tests of the batteries. The batteries that have inferior results in the capacity tests are placed in spaces with high temperatures and needs to be cooled as soon as possible. In longterm, installation of a proper cooling system is recommended. To measure the temperature during several periods throughout the year ensures if the meteorological temperature affects the environment of the batteries. If so, more spaces where batteries are placed may need cooling during the hotter period of the year.