Energy Storage Systems in Electrified Transportation
This article explains how battery packs utilize an energy management system for protection, control, and estimation.
Electrification is the most promising solution to enable a more sustainable and environmentally friendly transportation system. Traditionally, electrical energy storage for vehicle applications has been limited to starting lighting ignition (SLI) sub-systems. However, the increase in vehicle electrification has led to the rise in the energy, power, and cycling requirements of vehicle energy storage systems. The battery pack plays a critical role in electrified powertrains. In the battery pack, a significant amount of energy is stored and is potentially harmful if released quickly. Read on to learn more about the energy storage systems used in electrified transportation.
Overview
Battery packs utilize an energy management system that enables protection, control, and estimation [1]. In a battery pack, cells must be protected from operation in too low or too high temperatures, which may cause fast aging, deterioration, and damage. Similarly, excessive current can lead to damage, depletion of charge, and overcharging (stress due to high voltage). The risks incurred due to undervoltage and overvoltage can be minimized by keeping the state of charge (SOC) of each cell well balanced. Preferably, identical batteries are chosen to form a battery pack, and they may be configured in series, parallel, or a mixture of both configurations to deliver desired voltage, capacity, or power density.
Balancing helps in maximizing the effective capacity of the battery stack. Cell balancing is the process of equalizing the voltages and state of charge among the cells when they are at full charge. One of the means of cell balancing is to employ dissipative hardware that transforms excess SOC into heat. Nondissipative topologies are based on DC-DC converters, and they facilitate charge movement from cells with high SOC to cells with low SOC, thus reducing the energy losses significantly [1]. The SOC of a cell is, in general, not directly measurable, so the battery management system actuates balancing currents based on an SOC estimate or is estimated empirically.
Energy storage systems or batteries form a crucial part of transportation electrification. The study of these storage systems includes the understanding of battery electrochemistry, characteristics of the battery cells, critical parameters including cycle life, cost, power, and energy dynamics, charge or discharge characteristics, electrical circuit modeling, cell balancing, battery management system , and modeling and simulation of battery systems [2]. Some of the commonly employed energy storage technologies are flooded lead-acid (FLA) cells, valve-regulated lead-acid (VRLA) batteries, and nickel-metal hydride (NiMH) batteries. A graphical comparison of different energy storage technologies in the form of a cost augmented three-dimensional diagram is shown in Figure 1 [1].
Figure 1. Cost augmented three-dimensional Ragone diagram comparing several energy storage technologies [1]
Energy Storage Systems in Electrified Transportation
The increase in vehicle electrification has led to enabling efficient electric mobility along with maintaining faster response. The other secondary conveniences that come with this change include at-home charging, vehicle-to-home (V2H) backup power, upcoming vehicle-to-grid (V2G) infrastructure support, and wireless charging [1]. The choice of energy storage technology depends on various factors like vehicle platform and its degree of electrification. It also affects the design of the energy management system (EMS) and how it is integrated into the vehicle. These EMS or BMS are tasked with interconnecting multiple cells, estimating system state, diagnosing fault conditions, reporting the availability of power and energy, and communicating with other vehicular systems like on-board or off-board charger, infotainment, and traction control systems [1].
There have been several energy storage technologies used for specific applications and have pros and cons in terms of usage. FLA technology is mature and highly recyclable but suffers from factors like limited cycle-life and depth-of-discharge. There are enhanced FLA (EFLA) batteries that possess a double life-cycle to that of FLA, thus making them ideal for most basic start-stop hybrid platforms [1]. VRLA (also known as sealed lead-acid or SLA) batteries support applications that demand increased power and cycle life. This enables them to handle small amounts of traction and regenerative braking energy. However, the VRLA technology is less mature and more expensive as compared to the EFLA technology.
NiMH battery technology is relatively mature and has proven longevity. It has been employed in HEVs for several years now. The power or energy capabilities are typically double or triple as compared to lead-acid. However, it has a significant drawback of high self-discharge which limits them to power-oriented applications such as mild and full hybrids. ZEBRA batteries are commercially available and are based on sodium nickel chloride (Na-Ni-Cl) electrochemistry. This technology is mature and has greater energy density, better cycle life, lower cost, and is insensitive to ambient temperature, making it suitable for extreme climates. Lithium-ion-based cells continue to dominate the consumer portable electronics market and are preferred for PHEVs and EVs.
Key references:
- Berker et. al., Making the Case for Electrified Transportation, 2015.
- Dragan Maksimovic et. al., Power Electronics for Electric Drive Vehicles, 2013.
