The use of batteries as a backup in case of power outages is a common practice adopted by telecommunications companies that need to keep their services always active. Besides, those batteries can also be used for other purposes such as participating in the energy market in order to reduce the electricity bill, as long as the safety usage rules are respected. In this context, batteries can be used when the energy costs more and recharged when the energy costs less, which is known as the demand-response mechanism. Our focus in this work is to optimize the use of batteries installed for backup to participate in the demand-response mechanism, in order to reduce the total energy cost for the company. We formally state the related optimization problem and propose two solving approaches to address it: a mixed-integer program and a heuristic to solve large instances. Simulations based on real data of a French telecommunications operator prove the relevance of using batteries to reduce the energy cost for the company by participating in the demandresponse mechanism. The proposed heuristic proves to be economically relevant and computationally efficient, being a good alternative to a mixed-integer program for large-scale problems.