To ensure optimal and economical battery operation, it is necessary to consider its lifetime-limiting aspects, e.g., performance degradation and degradation costs. Battery performance degradation is commonly assessed by offline lifetime models. They are suitable for battery planning and performance monitoring, but cannot be used in real-time operation. Therefore, in this letter, an incremental degradation cost estimation method for optimal battery real-time operation is proposed. It enables battery degradation evaluation for any time resolution and set of operating conditions during the real-time operation.