## 4 Discussion  
This study carefully analyzed the exploitation of battery energy storage systems to provide frequency regulation. The evolution of the power system requires reliable and rapid control of frequency deviation. BESS can provide very quick ancillary services; however, their limited energy reservoir must be taken into account when providing these services. This can be done by defining innovative requirements that implement degrees of freedom for SoC management. These DoF are used by the asset manager as a room to develop a control strategy able to restore battery SoC towards the target value. An effective SoC restoration strategy guarantees the reliability of provision while limiting its cost. Nonperformance is measured: the lower NP is the higher is the reliability of provision. The cost of restoration is assessed in terms of the costs of the energy flows for managing SoC and the decrease in BESS life associated with larger energy flow. Some DoF has shown effectiveness in decreasing NP while keeping acceptable IRR. Energy flows related to SoC restoration are not negligible, ranging from 25% to 45% of the energy dedicated to services provision. They must be included in the assessment while evaluating the design of an innovative product traded on balancing markets. This paper can support NRAs, market and system operators in selecting the appropriate DoF for each product. This could lead to a suitable framework both for guaranteeing accurate respect of dispatching orders and for having a positive return on investment. The comparison among SoC restoration strategies showed that both strategies based on dead-band (DB) and on minimum guaranteed available energy (AE and DT) are able to decrease NP below acceptable values. Instead, the absence of DoF obliges the BESS operator to interrupt the provision of the service, thus leading to higher NP. The possibility of providing over or under-regulation to manage SoC, as implemented nowadays in some EU experiences, does not prevent SoC to saturate often leading to very high NP. Among acceptable strategies, DB is hard to show a positive rate of return, due to high SoC management costs and shortened battery life. Mechanisms that offer larger DoF, such as AE and DT, present IRR greater than 4%. Figure 10: SoC distribution during the yearly simulation for the active SoC restoration strategies studied: (**a**) Base case; (**b**) No DoF case; (**c**) O/U case; (**d**) DB case; (**e**) AE case; (**f**) DT case. Y-axis represents the fraction of time in the year in which SoC is at a particular level. Still, investment in batteries presents a return below the hurdle rates. Further works could investigate the possibility of provision of multiple services simultaneously (also including innovative ancillary service, such as very quick primary frequency regulation, which may be useful in the future power system with low inertia), to provide both SoC management and revenue stacking [46]. This is the case of \"passive\" SoC restoration strategies, that take advantage of the provision of further service to manage SoC. This can provide reliability of the system and largely decrease restoration costs. A study in this direction could investigate if any of the market or grid rules represent a barrier for BESS [47; 48; 49]. For instance, some issues arise when it comes to measuring, monitoring and valorizing the simultaneous provision of multiple services. Some other obstacles can be represented by a feature of market products, such as minimum bid size, distance from market closure to delivery time, symmetry and time definition of products. Furthermore, it could show if there is any way of providing a return on an investment in BESS by participation in the market only, so that no other forms of remuneration must be put in place by the system to benefit from the fast and precise regulation of BESS. Conceptualization, C.D.P., M.A. and M.M. ; methodology, M.A., C.D.P. and M.M. ; software, C.D.P. and M.A. ; validation, M.M. and G.R. ; data curation, A.R. and G.R.