Last reviewed 2 November 2021

With the government having announced in November 2020 that no more petrol or diesel cars and vans will be sold after 2030 and with it confirmed that the sale of petrol and diesel vehicles up to 26 tonnes will be banned by 2035, it is evident that some major changes are coming for the transport industry and the fleet of vehicles that keep the country running.

This article looks at some basic technical issues to be considered by operators intending to introduce battery electric vehicles (BEVs).

The key concerns

Perhaps the most fundamental consideration when assessing the suitability of BEVs for long-range transport is the distance that can be travelled on a fully charged battery and this can be combined with the availability of recharging points and the time taken to recharge. These are, of course, all interlinked issues and can to some extent be traded-off against each another. For instance, if the distance on a full charge allows nine hours of driving time it may not matter if it then takes 12 hours at the depot to recharge because the driver will be taking their daily rest. On the other hand, the distance that can be travelled may not seem so important if there are ample opportunities to recharge the battery and it can be completed in only a few minutes (ie the situation that currently exists with petrol or diesel). Unfortunately, the issue of battery durability is not that simple and here we will consider some of the limitations.

As a benchmark example we can then consider a current trunking operation with diesel vehicles and if we assume an average speed of 40mph then the vehicle can cover 9 x 40 = 360 miles in a nine-hour driving day. In that nine hours the driver must take at least 45 minutes break so a BEV with a range of 180 miles and a recharge time of 45 minutes should give us the same operating profile. A 30mph average speed gives us a required range of 135 miles with the same recharge time. However, there are a number of other issues that need to be considered.

Quoted range

The range of a conventional vehicle is calculated by dividing the fuel tank capacity by the rate of consumption per mile and the same is true for a BEV, except that battery capacity replaces tank capacity. The official “fuel consumption” of a BEV is determined in exactly the same way as with a petrol or diesel vehicle, ie for light vehicles by the WLTP test and for heavy duty engines by the VECTO computer simulation tool.

In both cases, it should be noted that the results are obtained from a standard test profile so that the result can be compared to the given standard and so that different vehicles can also be placed in an order of merit in terms of energy consumption to help potential buyers. Hence the results are purely for comparison purposes and what the tests cannot do, and are not intended to do, is give an accurate figure for the actual energy consumption of any particular vehicle. It is now well recognised that the WLTP figure for fuel economy with a conventional engine is seldom, if ever, achieved in practice and we can expect no better with a BEV.

Actual range

Most of the variables that contribute to the difference between test and actual figures are the same for conventional and battery vehicles, eg:

  • speed

  • acceleration and braking

  • gradient

  • all-up mass.

Whenever these are different from the test conditions the results will vary but one additional factor for BEVs is the ambient temperature. In theory, this also affects the power output, and hence the fuel economy, of a petrol or diesel engine but in normal use the effect is generally not noticeable. With a battery vehicle, however, low or high ambient temperature can result in a significant reduction in range.

The combination of these factors can result in a very significant variance between the quoted range and the real range, perhaps 25% or more.

It must further be considered that range is often quoted for a full battery charge (ie 100% down to 0%), which is an unrealistic range for the normal use of batteries.

Energy reserve

Running out of fuel with a diesel or petrol vehicle is problem enough but resolving the same scenario with a BEV likely means that the vehicle will have to be recovered to a charging station. Additionally, BEVs (and hybrids) usually cannot be towed and must be recovered on a flatbed truck, meaning complete failure of the delivery and the potentially significant costs of recovery.

Under the circumstances, it would therefore be most unwise to allow the battery charge to fall below 10%, and preferably not below 20%. The rate of battery discharge is not necessarily linear and, as people will be aware from experience with mobile phones, that last 20% can disappear awfully fast.

Battery capacity

The capacity of a fuel tank does not change with age but that of a battery certainly does. Battery capacity heavily influences the range of a BEV and it is that battery capacity that governs its useful life.

A battery is usually considered to have reached the end of its useful life when its capacity falls to 80% of the original and to prolong that life the battery must be charged and discharged in the correct manner.

Much is made of the ability of some vehicles to recharge the battery from near zero to at least 80% over a very short period but this requires rapid charging and, while the occasional rapid charge should do little harm, repeated rapid charges will reduce the battery capacity significantly. In an experiment by the University of California (2020) it was found that using the fast charging method found at motorway service stations, battery capacity was found to fall to the end-of-life level (80%) after only 25 consecutive cycles. Exceeding 80% and/or going below 20% on an occasional basis should do no harm but that should not be a regular occurrence.

However, continually recharging an only partially discharged battery (ie taking advantage of every opportunity to “top-up” the charge) can also reduce its capacity and the approved method of prolonging battery life is to wait until the battery charge level falls to 20% and then recharge (using a slow charge rate, ie overnight) to 80%.

An operational example

Our first benchmark vehicle above assumed a required range of 180 miles followed by a single recharge of 45 minutes. However, this does not mean that a quoted range of 180 miles on a 100% charge will be sufficient. If we observe the rule not to charge above 80% or discharge below 20% we will only be able to use 60% of the quoted maximum (100%) range. If we then factor in the potential effect of the other variables listed above (eg temperature, speed), we could lose as much as 25% of that remaining 60%, which could mean the actual range is reduced to 45% of the quoted maximum range.

This is a purely hypothetical example, and very much an extreme worst-case scenario. The actual range required will depend on the nature of the work which the vehicle undertakes and the operating conditions may be nowhere near as demanding as the example. There is very little experience of battery performance on real operations with large goods vehicles and all the evidence so far has come from cars, the great majority of which will have been used on short journeys with favourable conditions and most recharging at slow rate on a domestic charger. However, operators interested in switching to or incorporating BEVs into their fleets need to make these sorts of calculation before taking any decision and if the required range is not possible then the nature of the operation will have to change to reduce the daily distance or risks will be introduced.

Additionally, and as a final consideration, the above example also assumes that rapid charging will be conducted during the mandatory 45-minute break but doing so will potentially cause the early demise of the battery and could require a replacement battery (around £7,000 for a car and more for a heavy goods vehicle) potentially every 25 days.

Furthermore, best practice is to split the driver break in two (15 minutes after 2 hours driving; then 30 minutes after another 2.5 hours driving) but following that would mean topping-up an only partially discharged battery at a rapid charge rate on both occasions, neither of which is good for the battery.

Conclusion

It is clear that there are currently many questions to be answered about the durability of BEVs and their suitability for the role of goods and passenger transport, especially over long distances. Certainly, any mandated move to battery electric heavy goods or passenger transport vehicles in the shorter term will require significant improvements to be made to battery technology and some potentially more fundamental changes to operational practices if present supply chain fragility is not to get much worse.

Further information on alternatively fuelled vehicles is also available in the topic Alternatively Fuelled Vehicles.