7.1 Battery and motor developments

Batteries

Lead-acid batteries have been used in battery electric vehicles since the 1890s. They have been the standard technology for vehicle starting and lighting ever since.

Battery electric vehicles using lead-acid batteries have been produced right through the 20^th century. They have had the advantages of silence, relative reliability and being easy to drive. However, the critical disadvantages compared to fossil-fuelled vehicles have been the limited speed and range.

The importance of energy density

Petrol and diesel fuel have a very high energy density, both in terms of energy per unit weight and energy per unit volume, as shown in Table 5.

The stored energy density of electricity in batteries is far lower. Even for the best lithium ion batteries, the energy density per kilogram is a factor of 60 lower than that for petrol.

Many types of rechargeable battery have been developed. Nickel cadmium (Ni-Cd) batteries were popular in the 1980s and nickel metal-hydride (NiMh) batteries were widely used in the 1990s. Figure 10 shows their energy densities, both in terms of stored energy per unit weight and energy per unit volume. Note the large increase in energy density as the technologies have progressed from lead-acid through to nickel metal hydride and lithium-ion batteries.

Figure 10: The energy density of different types of rechargeable batteries

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This chart shows the energy density of four different types of rechargeable battery. It has a y- axis labelled ‘volumetric energy density’ in watt-hours per litre, with a scale running from zero to 400. A red upward-pointing arrow next to the axis is labelled smaller size. The x- axis is labelled ‘specific energy density’ in watt-hours per kilogram, with a scale running from zero to 250. A red arrow pointing to the right is labelled ‘lighter weight’. There are four coloured shapes indicating the properties of the batteries. A blue circle is labelled ‘Lead acid’. The range of volumetric energy densities is between 30 and 90 watt hours per litre and that for the specific energy density between 20 and 60 watt hours per kilogram. A green ellipse is labelled ‘nickel cadmium’. The range of volumetric energy densities is between 70 and 180 watt hours per litre and that for the specific energy density between 40 and 90 watt hours per kilogram. A blue ellipse is labelled ‘nickel metal hydride’. The range of volumetric energy densities is between 110 and 290 watt hours per litre and that for the specific energy density between 85 and 130 watt hours per kilogram. An orange ellipse is labelled ‘lithium ion’. The range of volumetric energy densities is between 240 and 380 watt hours per litre and that for the specific energy density between 125 and 215 watt hours per kilogram.

Figure 10: The energy density of different types of rechargeable batteries

The expansion in the use of plug-in hybrid electric vehicles (PHEVs) and full battery-electric vehicles (BEVs) since 2010 has been made possible by the high energy density of lithium-ion technology, coupled with extraordinary feats of mass production. Each cell of a lithium battery has a voltage of about 3.7 volts, so a typical 400 volt vehicle battery module will require over 100 cells. A whole battery may have 1000 or more. The large-scale production plants for these cells need to produce them in quantities of billions and the total energy storage produced per year can be measured in gigawatt-hours. The US Tesla company has thus called them gigafactories.

The cost of lithium-ion batteries has fallen from over US$1200 per kWh of electricity stored in 2010 to about US$130 in 2021 (Henze, 2021). Since the battery can make up a half of the total cost of a battery electric vehicle, its life expectancy is an important consideration. A typical warranty on BEV batteries is for a life of 8 years.

Electric motors

The first-generation electric vehicles used direct current (DC) motors, but more recent cars convert the direct current to alternating current (AC) using an electronic inverter, which then drives a variable speed induction motor. These motors are lighter and have a higher efficiency.

Environmental problems of materials

The rapid development of a new technology has raised questions about the toxicity and sustainability of the materials used.

Lithium-ion batteries do not contain highly toxic elements such as lead or cadmium. In 2023, world lithium reserves were estimated to be about 26 million tonnes (about a third in Chile). The reserve/production (R/P) ratio (i.e. the number of years the reserves will last if extracted at the current rate) was over 100 years.

However, the most common type of lithium-ion battery also contains the element cobalt, which is a by-product of copper mining. World cobalt reserves in 2023 were only about 10 million tonnes, with an R/P ratio of only 54 years. A half of the reserves were in the Democratic Republic of the Congo, which has had a long history of politic unrest. Newer low cobalt batteries contain 75–90% less cobalt than earlier generations of batteries, although they use twice as much nickel (which is also in short supply). This perhaps stresses the need for cobalt and nickel recycling from used battery packs.

The high efficiency electric motors used in electric vehicles have also created an increased demand for a range of ‘key elements’ such as copper and nickel required for basic wiring. Other ‘critical rare elements’ include cobalt, neodymium and samarium used in the manufacture of powerful magnets.

The need for such elements in batteries and motors could give rise to future ‘resource wars’ similar to those over oil and gas supplies in the past (and present). More optimistically, new battery technologies could be developed, such as lithium-sulfur or lithium-air, which are not so dependent on critical rare elements.