# Combination of LiCs and EDLCs with Batteries:
## 1 Introduction  
One of the biggest challenges towards achieving efficient and effective energy use in electric vehicles (EVs) and other renewable energy technologies is the energy storage system [1; 2]. Batteries, though being improved with newer technologies, are still not capable of meeting the load requirement while retaining their lifespan. Hybrid energy storage systems have been proposed in the literature to solve this problem [3; 4; 5]. It has been proposed previously to combine high power dense energy storage alternatives with batteries that are high energy dense. Supercapacitors (SCs) and hybrid capacitors (HCs) are quite similar energy storage devices, as they are both double-layer capacitors [6; 7]. However, SCs have a higher power density and a lower energy density than HCs [8]. In battery and SC hybrid systems, there has been a reported downside due to the poor energy density of SCs, while in battery and HC systems, the low power density of HCs has been reported to pose a challenge [9; 10]. Generally, when energy storage hybridisation is considered, the more conventional approach is to combine two energy storages, one high energy dense and one high power dense storage; this has been the trend of most of the research in the area of energy storage hybridisation [11]. More often than not, the hybrid energy storage would either be a combination of battery and SC or battery and HC [12; 13; 14]. Starting with the battery and SC hybridisation, much work has been done in this aspect for different applications, amounting to a rather rapid adoption of the topology, although it possesses certain limitations. In reference [15], a Pb-acid battery of 12 V was hybridised with an SC bank of 12 cells in series, with each cell having a capacitance of 310 F through a multiple input non-isolated bidirectional half-bridge DC-DC converter. The target application was for use in an EV; however, the test was carried out using an electronic load. The authors were able to achieve up to 75% discharge of the SC bank through the DC-DC converter and consequently achieved an efficient use of the battery. The authors proposed that future work will focus on the analysis of their system's efficiency at high voltage condition; also, the DC-DC converter they used needed more work to improve its efficiency and cost-benefit ratio. A similar application was considered by [16]; the authors also proposed the hybridisation of the battery and SC, but their approach involved using only one DC-DC converter. The battery was connected at the input of the DC-DC converter, and SC was placed parallel to the output of the DC-DC converter, which was then placed parallel to the load such that SC was always performing peak power shaving for the battery. The downside of this topology is that the battery will always be in use, and so, there will be constant energy flowing from the battery towards SC and the load, which will amount to an inefficient use of energy because of the losses in charging SC from the battery. In reference [17], a Li-ion battery and SCs were used in a hybrid energy storage system with the application of a fuzzy logic controller (FLC). The authors used an isolated dual active bridge converter for each of the energy storages. They reported to have achieved fast charging; however, the research trend is moving away from the use of transformer isolation in the hybridisation of energy storages because the weight of the transformer further adds to the weight of the entire system, especially in high voltage applications. This consequently increases the energy and power requirements of the system, especially for systems involving mobility such as electric wheelchairs and electric vehicles. Several other combinations of battery and SCs for locomotive and stationary applications were presented in [18; 19; 20; 21; 22; 23; 24; 25]. Although LiCs have proven to present significant advantages over SCs [26], their adoption has been quite slow [27]; this is very likely due to the very huge cost of HCs as compared to SCs [28].