Energies 2023, 16, 623. https://doi.org/10.3390/en16020623 www.mdpi.com/journal/energies Article Age Estimation of a Hybrid Energy Storage System for Vehicular Start–Stop † Emad Manla 1, * and Adel Nasiri 2 1 College of Engineering, West Texas A&M University, Canyon, TX 79016, USA 2 College of Engineering and Computing, University of South Carolina, Columbia, SC 29208, USA * Correspondence: emanla@wtamu.edu † This paper is an extended version of our paper published in the 2015 IEEE Energy Conversion Congress and Exposition conference (ECCE), Montreal, QC, Canada; pp. 6199–6205. Abstract: Ultracapacitors are energy storage devices that have shown outstanding capability in a vast spectrum of applications, mainly in energy storage systems required to deliver short bursts of electrical energy. Ultracapacitors possess high power density while batteries possess high energy density. In this paper, a hybrid energy storage device comprising a lithium-ion ultracapacitor mod- ule and a lead acid battery was modeled, built, and tested for vehicular start–stop application, which requires a much larger number of engine cranking events than conventional vehicles. The combina- tion of a lead acid battery with Li-ion ultracapacitors was chosen due to the fact that the vast major- ity of vehicles utilize lead acid batteries to crank the internal combustion engine. This allows retro- fitting this hybrid setup in conventional vehicles along with the start–stop feature without inflicting damage to the already installed lead acid battery. The start–stop feature puts high stress on the lead acid battery, contributing to its faster aging. This feature is commonly found in hybrid vehicles to save the unnecessarily burned fuel during idling. This paper discusses aging of the lead acid battery as a result of being used in hybrid vehicles equipped with start–stop when used alone versus when used in the hybrid setup. The paper shows cranking tests performed on a number of cars to obtain voltage, current, power, and energy requirements for combustion engine cranking. Mathematical derivation, analysis, and an energy storage age estimation method are also presented. A set of crank- ing events followed by capacity checks performed on two automobile energy storage systems, one being a lead acid battery alone and the other being the proposed hybrid module, show the ad- vantage of integrating the ultracapacitor module with the lead acid battery to extend its life span almost fivefold in a hybrid automobile. Keywords: ultracapacitor; lead acid battery; modeling; aging study; internal combustion engine cranking; energy storage system 1. Introduction A hybrid energy storage system (ESS) made up of a lead acid battery and an ultraca- pacitor module [1,2] is modeled, simulated, built, tested, and presented in this paper. This ESS can replace lead acid batteries found in conventional vehicles in order to integrate the start–stop feature aiming to improve the mile-per-gallon (mpg) rating without shortening the life span of lead acid batteries through extreme repetitive engine cranking that the start–stop application introduces. Ultracapacitors are power-dense devices that have been considered for electric and hybrid vehicles for numerous advantages. The authors of [3–5] discussed the advantages of ultracapacitor use in propulsion systems to meet peak power demands. Integration of lead acid batteries with ultracapacitors for vehicular applications has been discussed in simulations and experiments [6–8]. Control schemes have also been discussed for ultra- capacitors [9–11]. The long-term increase in demand for oil and gas stimulates the need Citation: Manla, E.; Nasiri, A. Age Estimation of a Hybrid Energy Storage System for Vehicular Start–Stop. 2023, 16, 623. https:// doi.org/10.3390/en16020623 Academic Editors: Eklas Hossain, Rajvikram Madurai Elavarasan and Gauri Shankar Received: 30 November 2022 Revised: 19 December 2022 Accepted: 27 December 2022 Published: 4 January 2023 Copyright: © 2023 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).