Abstract: With the promotion of lithium-ion batteries and the development of hybrid energy storage technologies, the difference of energy storage units in the energy storage system becomes more and more significant, which puts forward challenges for power allocation. In order to reduce the aging cost of energy storage systems during operation, this paper proposes a power allocation strategy based on the cycle-based aging feature of each energy storage unit to reduce aging costs. An online rainflow counting method is proposed for real-time updating of power-cycle mapping relationships, then the real-time subgradients of the aging cost with respect to charging and discharging power are obtained by using the chain rule of differentiation. The reciprocal of real-time subgradients is used as power allocation weight to achieve equalizing aging costs of energy storage units. A transferrable power allocation algorithm is further developed, which allocates power according to weights while considering power limits and the state of charge (SOC) boundary of energy storage units. In an energy storage system participating in secondary frequency regulation, the proposed strategy can reduce the aging cost by 8.08% and 10.34%, respectively, compared with two commonly used power allocation methods (based on maximum power and residual energy).