—Since sensor nodes use batteries as their source of energy, energy-based routing becomes an important requirement to extend network lifetime. This routing is done using the nodes remaining energy information. Nonetheless, this information cannot be estimated using built-in primitives in nodes powered by a lithium-ion battery. In this paper, we introduce an efficient battery model that estimates the remaining energy of a node independently of its battery chemistry. Our model is based on an existing battery model that uses the current consumption during a state and its duration to estimate the battery remaining energy. The original model is very accurate, however, it cannot be implemented in a sensor node as it requires complex computations and large lookup tables. Therefore, we present a recursive approximation of this model that requires low memory and simple computation, while maintaining the original model accuracy. Through extensive real battery discharge measurements, we demons...