7.1 Battery cell and pack costs and characteristics
Lithium ion (Li-Ion) battery chemistry represents the technology of choice for electric vehicles today and for the forseeable future. Li-Ion batteries have overtaken nickel-metal hydride (NiMH) batteries because of their better specific energy and power density qualities. The relationship between power delivery and specific energy density is very important for the performance of the vehicle. Figure 7.2 maps the specific power density relative to specific energy density for different electricity storage options. The fact that Li-Ion batteries are to the right of the frontier of options shows why they have become the focus of PHEV and EV battery development.39
A delicate equilibrium needs to be found to determine the optimal battery chemistry and size. The need for high power, acceleration and optimal regenerative breaking has to be balanced against large energy storage requirements (i.e. a high specific energy density) that increases the vehicle range.
Li-Ion batteries come in a range of different chemistries, so the term Li-Ion battery refers to the Li-Ion family of battery chemistries that induce lithium ions to move between the anode and cathode in a battery.
Within the family of lithium-ion battery chemistries, each specific technology usually has a number of trade-offs between power density, specific energy density, safety, cycle life and performance, and costs (Table 7.1). The original Li-Ion chemistries developed for consumer applications have proven too expensive for automotive uses, given their large share of the total cost of the vehicle. This has spurred the development and deployment of alternative, cheaper Li-Ion chemistries with more suitable thermal characteristics better adapted to automotive applications. These include lithium-iron-phosphate (LFP), lithium-manganese-oxide spinel (LMO), and nickel-cobalt-aluminium (NCA) (NPC, 2012). To date, no dominant chemistry has emerged, but deployment for automotive applications is still in its infancy and further experience will prove invaluable in improving performance and reducing costs.
Figure 7.2: Specific power versus specific energy density for different electricity storage solutions
Source: based on Johnson Controls, published in NPC, 2012.
Table 7.1: Li-Ion battery characteristics by chemistry
Source: NPC, 2012.
The key parameters a vehicle designer must take into account when considering a battery are costs, the spe-cific energy density of the battery and the relationship with power charge and discharge. The specific energy density can be measured in two ways; either in terms of energy per unit mass (e.g. Wh/kg) or volume (e.g. Wh/ litre). Depending on the design constraints, one metric will be more useful than another. For light-duty vehicles, weight is always an issue, but for PHEVs where space is at a premium, density per unit of volume may be the main consideration.
An additional complication for vehicle manufacturers is that battery cell performance in terms of specific energy density is higher than the overall battery pack density. This is due to the additional weight of the protective casing, thermal energy management systems and controls. In the recent past, the additional load from these components would double the weight of the battery pack from the battery cell weight, thereby halving the specific energy density (NPC, 2012). Further R&D and deployment should help to reduce this weight burden.
In addition to battery costs, the estimated economic battery life is critical to determining the overall cost of driving an electric vehicle. Battery performance degrades over time with the number of cycles (charge/ discharge cycles) performed. Maximising the number of cycles a battery can perform before it deteriorates to a point it needs replacement will significantly enhance the economics of PHEV and EVs. To maximise the life of a battery, the swing in the state-of-charge (SOC) is typically limited to 40-80%. Thus the effective cost of electricity available for driving is higher than the nameplate cost, as only 40-80% of the battery charge is made available. For instance, a battery pack that costs USD 500/kWh, but that charges and discharges over only 60% of its capacity would have an effective cost of USD 833/kWh.
The other key operational area for maximising battery life is the thermal energy management system for the battery. The main problem is that battery life is significantly reduced for Li-Ion batteries when the battery operates at high temperatures. Ensuring the battery temperature remains as close to design goals as possible is therefore essential if battery life is to meet expectations. Most PHEVs today use a liquid thermal energy management strategy, but this is expensive. Some use cheaper forced air systems, notably the Nissan Leaf and Mitsubishi "I". In contrast, very cold temperatures do not have a significant impact on battery life, but do have a significant impact on the power availability of the battery. The significantly reduced power availability will limit acceleration, top speed and vehicle range. This is an important issue for EVs that rely exclusively on their battery for power. It is less of a problem for PHEVs because the ICE can be used to offset the lack of battery power. The thermal energy generated from battery operation can be used to raise the operating temperature when using a smart battery thermal management system, rather than rejecting the heat. However, for EVs this may need to be supplemented by an alternative source of heat prior to operation or during operation.
All these variables will interact with driving patterns and charging patterns to produce a wide range of operating profiles for the charge/discharge cycles of a PHEV or EV battery pack. Although Li-Ion batteries have been shown to be able to achieve 4 000 to 5 000 deep cycles in the laboratory and 1 000 in commercial operation, it is difficult to translate this into mileage. However, vehicle manufacturers are moving to remove this uncertainty from consumers and offering battery warranties or battery leasing options to transfer the risk from the owner of the vehicle.
The estimate of battery pack costs for EVs in 2012 varies quite widely depending on the literature, but is typically USD 500-800/kWh (Reuters, 2012; BNEF, 2012; McK-insey, 2012; Element Energy, 2012a; and IEA, 2013). The average battery cell costs are USD 400/kWh, but they vary widely depending on the scale of production. The build-up into battery packs adds 50-100% to the cell costs (Element Energy, 2012a).
The total cost for a battery pack with a capacity of 22 kWh for a mid-size EV is dominated by the battery cells themselves, which account for just over half of the total cost (Figure 7.3). The balance of the costs are split between a range of components, with only the margin and warranty share exceeding 10% of the total.
Figure 7.3: Total cost breakdown for a 22 kWh Li-Ion battery pack for a 100% electric EV, 2012
Source: Element Energy, 2012.
The battery packs for PHEVs can be twice, or even three times as expensive per kWh of storage for small batteries, than the battery packs of EVs (Figure 7.4). Due to their higher power performance, the battery cells themselves carry a price premium of around 30% per kWh compared to larger EV batteries (Element Energy, 2012a). The smaller size of PHEV batteries also tends to raise the percentage difference for the balance of system costs for the battery pack. Finally, the battery management system is significantly more expensive. This is because it requires greater flexibility (more sophistication and components) to shift energy to and from the battery more regularly and in a wide range of operating modes. There is also a need to more actively manage the battery cells and state of charge. All these factors increase the specific cost of the battery pack relative to that of an EV. The main advantage of PHEVs, that of smaller battery packs, is therefore offset to quite a large degree by these higher unit costs.
39 Ongoing research is focused on new battery technologies with significantly improved performance compared to today's Li-Ion batteries' specific energy densities and power outputs. These performance improvements are needed to allow EVs to go mainstream; an EV with today's battery technology with a 400 km range would have twice the kerb weight of a conventional ICE vehicle (NPC,2012).