The production of electricity with renewable resources has also necessitated the storage of energy. This need, which is satisfied by lead acid batteries, is now being replaced by lithium battery cells. But the necessity of monitoring and managing the lithium battery cells also lead to the design of battery management systems. In this study, firstly, battery management systems, general features and basic concepts were emphasized, and then a battery management system with passive balancing and IoT feature was designed on the purpose of controlling a 4 cell lithium iron phosphate battery pack. Laboratory and field tests of the designed equipment have been performed and it has been seen that the switching between the working modes, the charge-discharge control and the balancing feature have been successfully performed according to the obtained results.