*3.1. Energy Storage System Technologies for Micro-Grids*

The energy storage technology, that is presently widely acknowledged as the best to use in micro-grids, is based on electrochemical components, in particular rechargeable batteries [12]. Li-ion chemistry is emerging and consolidated as the most used battery technology. The intrinsic advantages of Li-ion batteries with respect to lead-acid stand as the larger values of the energy and power densities with respect to both unit weight (Wh/kg and W/kg) and unit volume (Wh/L and W/L) [46]. The net results are ESSs that are more compact in size and weight, more efficient for the larger number of charge/discharge cycles possible, and even cost-effective in long-term applications. These advantages are only marginally counterbalanced by the larger cost of the battery cells and the poor intrinsic safety of some sub-chemistries of the Li-ion family, which requires an accurate monitoring and management of the battery, in order to keep every cell constituting it in its Safe Operating Area (SOA) [47].

If the Li-ion cell costs are progressively decreasing due to the everyday larger number of applications in which they are used, the development of BMSs, capable of optimally monitoring and managing a Li-ion battery, is still open [48]. Several electronic circuits mainly based on application specific integrated circuits (e.g., battery monitor ICs) are being developed and utilized as BMSs [49–51]. The main and fundamental function of a BMS is to measure most of the battery quantities, and in particular, the voltage and temperature of each cell, the battery current and any other useful parameter, and to operate to maintain every cell of the battery inside the SOA. It is well known that exceeding the SOA limits may lead to dangerous and even catastrophic consequences [52]. Therefore, the BMS must be able to limit or even interrupt the battery current when a potentially dangerous situation is detected. Moreover, the BMS is required to carry out other more advanced functions, such as the charge balancing between the cells to compensate possible mismatches, the estimate of the residual charge available in the battery, the estimate of the state of health of the battery cells, and to perform all the diagnostic functions useful to reveal the battery status and prolong battery life [53]. The battery temperature management is another task in charge of a BMS, usually carried out by activating fans for the battery cooling or even heaters, when the battery operates at very low temperatures and needs to be pre-heated before being fully operational. BMSs are sometimes also asked to dynamically manage the battery architecture [54,55], by controlling the parallelization of battery strings when the battery architecture is modular, and the battery capacity in ampere-hour can be selected and adapted to the application requirements by choosing the number of strings to be connected in parallel. Even part of the battery module series, that increase the battery voltage, can be excluded by the BMS with a bypass switch, in order to allow the module substitution or maintenance without interrupting the energy storage service [56]. Finally, the BMS provides the communication functions required to insert the ESS in the micro-grid network, by which, the grid management functions and the policies regarding the energy flux management, are carried out. In conclusion, the BMS is an electronic system that provides

very powerful and fundamental functions. The BMS realization is based on electronic platforms that must be sufficiently powerful but, at the same time, compatible in size and cost affordable for the considered application. Microcontroller-based and even-FPGA (Field Programmable Gate Array) based platforms, when complex algorithms have to be executed, are the most useful solutions as BMS realizations [57]. Figure 4 shows the main functions performed by a BMS, grouped in the three fields *Appl. Sci.*  of Safety and Lifetime Extension, Battery State Estimation and System Integration. **2019**, *11*, x FOR PEER REVIEW 8 of 18

**Figure 4.** Main functions performed by a Battery Management System (BMS). **Figure 4.** Main functions performed by a Battery Management System (BMS).
