With concerns growing about the contribution of fossil-fuelled trucks and cars to global warming, the automotive industry moved seemingly unanimously to a single battery chemistry for their electrified vehicle platforms: lithium-ion (Li-Ion).
When the home rechargeable battery market, previously dominated by nickel-cadmium (NiCad) and then nickel-metal-hydride (NiMH), adopted Li-Ion batteries, it transformed the world of portable electronics. Compared to its forebears, at 100 to 250 Wh/kg, Li-Ion provides exceptional levels of energy density, offering double that of typical NiMH technology, and four times more than NiCad. In contrast, fossil fuels provide around 12,000 Wh/kg. However, electrical power can be converted into motion more efficiently than burning fossil fuel, and the use of regenerative braking technology can recharge the energy source, something not possible in a petrol or diesel vehicle.
Li-Ion batteries
Li-Ion is a catch-all name for a wide variety of chemistries that include lithium manganese titanate, lithium manganese oxide, and doped lithium nano phosphate, to name just a few. Each variant aims to optimise the energy density, balancing it against the safety of the cell under abuse, cost, and performance in terms of power delivered. All these and more chemistries are used in the electric vehicle (EV) models available today.
The EV battery itself it a complex system, made of many different components typically termed cell, module and pack. The cell is the basic battery unit that functions and looks the same as any cylindrical or flat battery found in consumer goods such as toys and portable devices. Li-Ion cells typically have an open-circuit voltage of around 3.2 V to 3.7 V, and a charging voltage of 3.6 V to 4.2 V, depending on chemistry.
Modules consist of multiple cells within a mechanical frame, protecting them against knocks, vibrations and heat, and the pack consists of multiple modules - this is where the level of complexity rises significantly. In the context of an EV: the cells must be protected in the event of a crash. The high levels of energy stored could cause fire or explosion if a crash were to result in a short circuit. From an electrical standpoint, the pack requires a battery management system (BMS). The BMS monitors the charge state of the battery, potentially down to each individual cell, the temperature while in operation, as well as the discharge rate. This information is shared with the EV's other systems to provide battery status information to the driver via the dashboard.
Battery capacity
Battery packs vary in capacity, matching the needs of the vehicle (size, weight, intended use). Unsurprisingly, there is a direct correlation between capacity and EV range with a fully charged battery. For example, the two-seater Renault Twizzy features a 6.1 kWh battery and offers a range of 90 km (56 miles). The Nissan Leaf ZE1, for comparison, features a 40 kWh battery and achieves a range of around 250 km (155 miles).
What does this mean for the driver? If smartphones are used as a benchmark, operating an EV down to single-digit battery capacity would be a risky business. Instead, EVs display battery capacity in terms of range (km/miles) rather than actual charge level. In the context of driving, this is a more useful figure than a percentage charge level. Furthermore, the capacity of a battery reduces over time based on the number of charge cycles, how aggressively the charging and discharging is, and the temperature at which it is stored and charged.
According to Tesla1, to extend battery life, they avoid allowing the cells in their battery packs to reach peak voltage during charging, and minimum voltage during discharge. They also try to keep charging rates below C/2 (half the specified charge/discharge rate). They also heat the battery pack to ensure charging below 0°C does not occur.
As a result of such approaches, car makers expect to be able to support at least 6,000 charging cycles or ten years of operation. But what happens after this? Luckily, Li-Ion batteries are less hazardous to the environment than their forebears when discarded. Lithium is also a rare metal, so expect an emphasis on recycling and recovery. However, due to volatility in the market price, governments may need to subsidise recycling programmes. Furthermore, although the cells may no longer provide the expected range of performance in an EV, the cells, modules and even packs may still be useful in other contexts. Energy storage, either in a grid-tied or home context, may see these clever power systems used for decades, rather than just years.
The next blog in the series will look at: The Electric Powertrain
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