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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  • Electric future shock  

    The need to adapt to changing vehicle technology is one of the main challenges of our time in the sector. Increasing connectivity and a vastly more complicated conventional vehicle provide a whole raft of obstacles on their own, before you even get to the rise of electric vehicles and hybrids.

    Add to that a more uncertain legislative environment resulting from rules not quite keeping up with the technology coming in, and you’ve got yourself a whole host of issues that the entire industry needs to stay on top of if it is going to continue to offer a sterling service to customers.

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    The key, as it has been in the past, is access. In this case, the right to be able to repair vehicles. Think that’s all sorted? Perhaps not:  “The rise of the electric cars and vehicles is something that could hit the automotive aftermarket hard – in particular, independent garages.

    “Many, if not all, electric vehicles invalidate their manufacturer warranty if essential work is carried out on the electrical systems by someone other than the main dealer. What’s more, many cars with batteries, such as the Mitsubishi Outlander PHEV, have warranties on the electrical components lasting up to ten years.

    “Having no choice but to use the main dealer for a full decade shows just why independent workshops will have fewer vehicles coming through the doors in the years ahead.”

  • IMI launches new international EV training solution   

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    “Advances in new technology are creating hundreds of thousands of new jobs across the world, and individuals working in the industry should be adopting this new training to make themselves leaders in their area of expertise. It’s an exciting time for the motor industry and the IMI is committed to making sure we’re ready to embrace the changes that are set to transform the sector.”

  • technologies of electric and hybrid vehicles  

    In the previous two issues, we looked at the way batteries store energy. We could in fact compare a battery to a conventional fuel tank because the battery and the tank both store energy; but one big difference between a fuel tank and a battery is the process of storing the energy. Petrol and diesel fuel are pumped into the tank in liquid/chemical form and then stored in the same form. Meanwhile, a battery is charged using electrical energy that then has to be converted (within the battery) into a chemical form so that the energy can be stored.

    One of the big problems for many potential owners of pure electric vehicles is the relatively slow process of
    re-charging the batteries compared to the short time that it takes to re-fill a petrol or diesel fuel tank. If the battery is getting low on energy, the driver then has to find somewhere to re-charge the batteries, and this leads to what is now being termed ‘range anxiety’ for drivers.

    Whilst some vehicle owners might only travel short distances and then have the facility to re-charge batteries at home, not all drivers have convenient driveways and charging facilities. Therefore, batteries will have to be re-charged at remote charging points such as at fuel stations or motorway services; and this is especially true on longer journeys. The obvious solution is a hybrid vehicle where a petrol or diesel engine drives a generator to charge the batteries and power the electric motor, and for most hybrids the engine can also directly propel the vehicle. However, much of the driving will then still rely on using the internal combustion engine that uses fossil fuels and produces unwanted emissions. The pure electric vehicle therefore remains one long term solution for significantly reducing the use of fossil fuels and unwanted emission, but this then requires achieving more acceptable battery re-charging times.

    Charging process and fast charging
    Compared with just a few years ago, charging times have reduced considerably, but there are still some situations where fully re-charging a completely discharged electric vehicle battery pack can in take as long as 20 hours.  It is still not uncommon for re-charging using home based chargers or some remote chargers to take up to 10 hours or more.

    Although there are a few problems that slow down charging times, one critical problem is the heat that is created during charging, which is a problem more associated with the lithium type batteries used in nearly all modern pure electric vehicles (as well as in laptops, mobile phones and some modern aircraft). If too much electricity (too much current) is fed into the batteries too quickly during charging, it can cause the battery cells to overheat and even start fires. Although cooling systems (often liquid cooling systems) are used to help prevent overheating, it is essential to carefully control the charging current (or charging rate) using sophisticated charging control systems that form part of the vehicle’s ‘power electronics systems.’

    Importantly, the overheating problem does in fact become more critical as battery gets closer to being fully charged, so it is in fact possible to provide a relatively high current-fast charge in the earlier stages of charging; but this fast charging must then be slowed down quite considerably when the battery charge reaches around 70% or 80% of full charge. You will therefore see charging times quoted by vehicle manufacturers that typically indicate the time to charge to 80% rather than the time to fully charge. In fact, with careful charging control, many modern battery packs can achieve an 80% charge within 30 minutes or less; but to charge the remaining 20% can then take another 30 minutes or even longer.   

    Battery modules
    Many EV battery packs are constructed using a number of individual batteries that are referred to as battery modules because they actually contain their own individual electronic control systems. Each battery module can then typically contain in the region of four to 12 individual cells.  One example is the first generation Nissan Leaf battery pack that contained 48 battery modules that each contained four cells, thus totalling 192 cells; although at the other extreme, the Tesla Model S used a different arrangement where more the 7,000 individual small cells (roughly the size of AA batteries) where used to form a complete battery pack.

    The charging control systems can use what is effectively a master controller to provide overall charging control. In many cases  the electronics contained in each battery module then provides additional localised control. The localised control systems can make use of temperature sensors that monitor the temperature of the cells contained in each battery module. This then allows the localised controller to restrict the charging rate to the individual cells to prevent overheating. Additionally, the localised controller can also regulate the charging so that the voltages of all the cells in a battery module are the same or balanced.

    One other problem that affect battery charging times is the fact that a battery supplies and has to be charged with direct current (DC) whereas most charging stations (such as home based chargers and many of the remote charging stations) provide an alternating current (AC). Therefore the vehicle’s power electronics system contains a AC to DC converter that handles all of the charging current. However, passing high currents through the AC to DC converter also creates a lot of heat, and therefore liquid cooling systems are again used to reduce temperatures of the power electronics. Even with efficient cooling systems, rapid charging using very high charging currents would require more costly AC to DC converters; therefore, the on-board AC to DC converter can in fact be the limiting factor in how quickly a battery pack can be re-charged. Some models of electric vehicle are actually offered with options of charging control systems: a standard charging control system which provides relatively slow charging or an alternative higher cost system that can handle higher currents and provide more rapid charging.

    Home & Away
    One factor to consider with home based chargers is that a low cost charger could connect directly to the household 13-amp circuit, which would provide relatively slow charging of maybe 10 hours for a battery pack. However, higher power chargers are also available that connect to the 30-amp household circuits (in the same way as some cookers and some other appliances); and assuming that the vehicle’s AC to DC converter will allow higher currents, then the charging time could be reduced to maybe 4 hours operate (but note that all the quoted times will vary with different chargers and different vehicles).

    Finally, there are high powered chargers (often referred to as super-chargers) that are usually located at motorway services or other locations. These super-chargers all provide much higher charging currents to provide fast-charging (as long as the vehicle electronics and battery pack accept the high currents); but in a lot of cases, these super-chargers contain their own AC to DC converter, which allows direct current to be supplied to the vehicle charging port. In effect, the vehicle’s on-board AC to DC charger is by-passed during charging thus eliminating the overheating problem and the high current DC is then fed directly to the battery via the charging control system.

    In reality, the potential for re-charging a battery pack to 80% of its full charge in 30 minutes or less usually relies on using one of the super-chargers, but battery technology and charging systems are improving constantly, so we
    will without doubt see improving charges times for
    newer vehicles.  

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