The purpose of this thesis is to study the topology and control strategy of a battery energy storage system (BESS) with parallel paths. Firstly, a single battery pack and its dedicated dc-dc converter form a sub-system. A group of these sub-systems is combined at a common dc-link, while the latter connects to a voltage source converter (VSC) converting dc power to ac. The employment of the dc-dc converters allows the batteries with distinct characteristics, i.e., various types and different ages. Therefore, this topology brings excellent convenience to re-use the batteries retired from the electric vehicles (EV) in the power systems. However, it becomes a significant problem for how to control these different batteries in the parallel paths to prolong the life of the batteries. In order to solve the problems mentioned above, this study develops an average-value model of the parallel BESS and validates it by comparing it with a fully detailed model in PSCAD/EMTDC. The stability boundary of each sub-system is investigated through the established model in steady-state. Besides, this non-linear model can be further linearized into a linear model, and small-signal stability analysis can be conducted to investigate the influence caused by the change of the system parameters. From the stability boundary obtained before, a control strategy is proposed to regulate these different batteries and extend the useful life of the older batteries by considering the factors such as maximum power limits and the status of the individual battery. Simulations in the PSCAD/EMTDC validate the effectiveness of the control strategy.