This paper is aiming to address a decentralized dynamic safety control issue for battery energy storage system in DC microgrids. A novel dynamic control barrier function (DCBF) based on nonlinear disturbance observers is devised in the decentralized converter control to restrain the current value while ensuring DC bus voltage regulation. First, a DCBF dependent on load variation is built in order to restrain the current value of each energy storage unit triggered by sudden load changes. Second, a decentralized composite control law, incorporating the DCBF via quadratic program (QP), achieves the objective of current constraint while ensuring the large-signal stability of the DC microgrids. The efficacy of the proposed method has been validated through both simulations and experimental comparison tests.