technologies of electric and hybrid vehicles

Peter Coombes of Tech-Club looks at how the benefits and challenges of battery technology define electric vehicles, and shape your future

Published:  13 November, 2017

Having recently presented short seminars about electric vehicle technology at Top Tech Live, and at some other trade events, it has become clear that technicians are only slowly beginning to delve into the
world of electric and hybrid vehicle technologies.   

Whether we like it or not, electrically propelled vehicles, including hybrid vehicles, have been significantly growing in numbers in recent years; and although the impact in the independent repair sector is so far relatively small, it won’t be too long before we see electric and hybrid vehicles much more frequently in workshops and for MOT tests.

If we take a snapshot view of UK registrations over recent months, alternative fuel vehicle sales have increased by around 45% to 50% compared to the same months of 2016, with the biggest increases being represented by pure electric vehicles and by petrol/electric hybrids. But if you think that focusing on conventionally propelled cars such as Jaguars will keep you away from electrically propelled vehicles, then it’s worth noting that registrations so far this year for alternative fuelled vehicles (primarily electric and petrol/hybrid-electric vehicles) are greater than Jaguar car sales. Of course, Jaguar is about to launch a fully electric vehicle for 2018 too, with most other manufacturers already producing electric and hybrid vehicles. So over the coming issues we are going to look at some of the main technologies that feature on the pure electric and hybrid electric vehicles.

Rechargeable lead-acid batteries have provided an inexpensive and effective way of storing energy for vehicle electrical systems for 100 years or more. However, to power electric vehicles for acceptable distances with good performance would require unacceptably large and heavy lead-acid batteries; and this has been a significant factor in restricting developments of electric vehicles. But then, along with the increasing environmental and political pressures to reduce energy consumption and improve emissions, there was a massive explosion in consumer electronics (such as laptops, phones etc); and because consumer electronics also required lightweight batteries, this helped to accelerate battery development.

Initially, nickel-metal hydride batteries (NiMH) provided a lighter solution compared to Lead-Acid batteries; and from the late 1990s the NiMH batteries became dominant on hybrid vehicles such as the Toyota Prius.  A further development then revolved around lithium based batteries that offered even lighter weight for the same energy storage capacity; and because a pure electric vehicle relies totally on batteries for stored energy, the lighter lithium based batteries enabled pure electric vehicles to achieve improved driving range and performance. Although there are some variations of lithium batteries, the most widely used type is lithium-ion (Li-ion) that currently provide high capacity energy storage in the lightest and smallest possible battery pack, and at a viable (and reducing) cost.

The Li-ion King
The basic principle of operation for NiMH and Li-ion batteries is much the same as for a lead-acid battery because they all store energy in chemical form; and (as a very simple explanation) this energy is released as positive and negative particles that then gather on two different types of plates in the battery cells. Importantly, the negative and positive particles are attracted to each other, and if a piece of wire and a light bulb (or any other electrical device) is used to connect the different plates together, electrically charged particles will then flow between the plates and through the wire and the bulb.

The critical factor for electric vehicles (and portable consumer electronics) is how much energy can be stored or delivered for a specific weight of battery, which is usually quoted for one kilogram of battery weight. The amount of energy for each kilogram of battery is then usually identified as ‘watts per hour,’ which again as a simplistic explanation is a guide to how much power or energy can be delivered at a constant rate for one hour. So we now have kilowatts being delivered for a period of one hour for each kilogram of battery weight, which is usually referred to as energy density or specific energy.

For comparison, a lead-acid battery has a specific energy in the region of 30-45 watt-hours/kilogram, whereas a nickel based NiMH battery is more likely to be 80-110 watt-hours/ kilogram; but modern lithium (Li-ion) batteries can be in excess of 200 watt-hours/kilogram. A lithium battery can store or deliver more than four times the energy for the same weight as a lead-acid battery; or alternatively, a Li-ion battery could be one quarter of the weight but provide the same amount of energy. What is then interesting to note is that if we compare a Li-ion battery of one kilogram to one kilogram of petrol or diesel fuel, the petrol/diesel can actually store approximately 100 times more energy than the Li-ion battery (or 400 times more than a lead acid battery).
On this reckoning, how can an electrically propelled vehicle be competitive with a vehicle powered by an internal combustion engine when there is so much less stored energy compared to the same weight of fuel? There are a few reasons, but one benefit of electric power is that electric motors are so much more efficient at converting energy into propulsion compared to petrol/diesel engines and transmission systems.

Although the battery pack might be relatively heavy, the electric motor and transmission systems of pure electric vehicles are much lighter than the internal combustion engine and transmission systems that they replace. More of this in the next issue.

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