## 2 State-of-the-Art Literature Analysis  
This section discusses the recent literature available within the HESS comprising LIB and LAB. Additionally, different battery management strategies and their control are further discussed. Taking into account the existing ICEVs around the world now, authors in [30; 31] suggest that further improvements in ICE can help in decreasing the greenhouse-gas emissions that are currently caused by TVs. This reduction can be achieved through technological advancements, which include enhancing engine efficiency, incorporating hybrid energy storage sources, and using renewable fuels that contain a lower carbon footprint (i.e., Green hydrogen). The effect of lead-acid battery's short lifespan poses challenges in recent ICEVs. Although LIB has shown great progress and as a solution for SLI application to potentially replace LABs, these batteries have several challenges, including; higher cost, which is not competitive to LABs, their recycling efficiency is low, and in terms of safety, the high power demands in ICEVs may vary from the high current over short periods to deep discharge cycles, and it is unknown at this stage as to how these batteries will perform under these circumstances [5; 27]. Many studies have proposed different solutions for improving LAB's lifespan for use in TVs, such as in [32], where the authors combine LAB with a supercapacitor using a battery semi-active cascaded topology approach. This topology connects the battery to the bidirectional DC-DC converter input, and the supercapacitor is connected to the converter's output. The converter is controlled by a fractional-order proportional-integral-derivative (FOPID) controller to properly charge and discharge the HESS. The HESS operation is optimised by using an atom search algorithm to enhance the LAB lifespan. The study also compared the FOPID with fuzzy logic controller for enhancing battery lifespan. FOPID shows superior performance as compared to fuzzy logic control during HEV operation. On the other hand, [29] proposes a double switching-based control for a combination of LAB and LIB energy storage systems for EVs to explore the batteries' pros and cons. This control selects the battery to be used during different vehicle operation modes under specified conditions and limits. The outcomes of the experimental study show that the HESS saves up to 68.62% of LABs and 29.48% of LIBs energy as compared to a single use of LAB, thus enhancing vehicle travelling range. Moreover, the LIB is used as an auxiliary source to support the LAB. In addition, the proposed system has a lower mass and lower price when compared to a single battery system. However, the study did not provide a cost analysis of the system compared to a single battery system. In [33], a combination of LIB, LAB, and a supercapacitor is proposed to overcome the short vehicle range and longer charging time challenges in EVs. The authors use a bidirectional cuk converter and an energy management system (EMS) to improve the HESS efficiency by providing efficient power-sharing amongst the ESSs. In this study, LIB is considered the main energy source, LAB is the auxiliary source, and SC provides LAB support to handle rapid power dynamics during vehicle acceleration and braking. The EMS uses two-stage pulse width modulation (PWM) signals and a simple PI control providing charging to the batteries using a constant-current-constant voltage (CC-CV) method. Furthermore, the authors in [34; 35] propose a combination of LAB and a supercapacitor using a fully active topology approach for TVs. In this study, the authors emphasise that allowing the supercapacitor to absorb high transient power requirements from the TV can improve LAB performance significantly in terms of lifespan. Therefore, despite the rich, relevant literature on the hybridisation of LIB and LAB, there is little attention to the use of HESS for providing starting functions or as an SLI for ICEVs. The current literature focuses more on solving a limited range of issues in EVs.