13 mins read
As more fleets begin to electrify, more drivers are being exposed to electric vehicles for the first time. EVs can arguably be driven without giving any thought to how they could be managed differently from petrol and diesel vehicles, and still deliver good results and happy drivers. However, as a battery electrochemist, I’ve seen plenty of examples where fleets could extract even more performance and lifespan out of their EVs, and save valuable time and money in the process. Here are my top charging tips that even seasoned fleet managers and EV drivers may not have considered until now.
1) How extreme hot and cold temperatures affect EV batteries and their charging performance
The ideal temperature for an electric vehicle battery pack is a trade-off between performance and lifespan. Colder temperatures technically increase the lifespan of the battery by slowing down degradation reactions that take place very slowly over time; hence why most EV batteries have a liquid-based thermal management system that removes excess heat on hot days or during fast charging. However, you may have noticed that cold temperatures also result in your electric vehicle achieving less range in winter. This is because cold temperatures increase the viscosity of the electrolyte – the liquid in the cell that allows lithium ions to travel between the positive and negative electrodes. Increased electrolyte viscosity makes it more difficult for lithium ions to move within the cell, resulting in slower “kinetics.” This in turn increases the internal resistance of the cell, which means that the cell reaches its minimum voltage faster during discharge, or its maximum voltage faster during charging, than it would at warmer temperatures.
At very cold temperatures, the internal resistance of the cells is so high that charging them at too high a power could result in the growth of dendrites – branches of lithium that can eventually puncture the separator and internally short the cell, leading to thermal runaway. Consequently, when the battery is cold, EVs deliberately reduce the maximum charge power that they are willing to accept until the battery has warmed up enough to make it safe to charge at full power. So, during particularly chilly winters, some EVs may achieve less range than normal and take longer to fast charge.
If your fleet starts to struggle with cold weather performance, try timing their overnight charging sessions to finish just before the vehicle starts its next journey. This will help to warm up the battery, reducing its internal resistance and improving its range compared to a battery that has been left in freezing conditions overnight. Many EVs also let you schedule the preheating of the cabin so that the vehicle is defrosted and toasty for when you need it. This will result in further improvements in range by reducing the energy required to heat the cabin whilst driving, and allowing the cabin to be heated up using mains electricity while the vehicle is plugged in, thus saving the energy in the battery for driving.
2) Heat packs can be a game changer
A heat pack takes advantage of the electrochemistry outlined in 1) above. It allows an EV to heat its battery pack via the thermal management system’s coolant loop, recovering much of the range and performance that would otherwise be lost. Tesla was the first EV manufacturer to take full advantage of battery heating: when a supercharger is set as the destination on the car’s sat nav, a Tesla deliberately preheats its battery to 50 °C to minimise internal resistance, maximise the charge power that the battery can accept, and minimise charge times. Leaving the battery at a high temperature for a prolonged period of time would slowly damage it, so once charging is completed, the car immediately starts to cool the battery back to more ambient temperatures (below 30 °C).
Other EV manufacturers have adopted this strategy to maximise performance and minimise charge times on cold days. However, heat packs aren’t always standard specification, and the different in performance can be significant. For example, the original Hyundai IONIQ 5 was capable of blisteringly quick charging when the battery was warm, but did not include the heat pack as standard. On cold days, this could make the difference between achieving a charge power of 220 kW, or a mere 30 kW (until the battery gradually warmed up by itself), with an otherwise identical specification of vehicle. For fleets, those few hundred Euros spent purchasing an optional heat pack result in reduced downtime, increased productivity and increased residual values. Therefore, heat packs are excellent value for money, and strongly recommended for your fleet.
3) Fast charging to 100% is a waste of time and money
Fleets that do a lot of motorway mileage, or don’t have charging infrastructure at depots or staff homes, may be dependent on DC fast chargers. These are capable of charging many EVs to about 80% within the average dwell time at a motorway service station. However, some fleet drivers have been known to regularly remain on the fast charger until the battery is fully charged. For business owners, this is a bad use of time and money.
When charging, the voltage of the cells in the EV’s battery pack gradually rise as current flows into them. Eventually, the cells reach their maximum permissible voltage, beyond which they would become irreversibly damaged. Since power is equal to current multiplied by voltage, and the maximum voltage cannot be exceeded, the only option is to throttle back the current – and therefore power – being fed to the vehicle.
The point at which the charge power starts to taper off considerably varies from vehicle to vehicle, but it tends to be around 80 – 90%. Beyond this, the charge power quickly dwindles to a figure that is more comparable with home or destination (AC) charging. For some EVs, the time taken to charge from 80 – 100% is about the same as the time taken to charge from 0 – 80%. Therefore, for fleets, where time spent charging is time not spent on the job, it is a much more efficient use of time to stop fast charging at around 80% State of Charge, continue your journey, and plug in again later on if needs be. With over three quarters of modern EV makes and models having a real-world range of over 320 km per charge, the number of fast charging stops that would be required in a single day should be small; timing those stops to strategically coincide with the likes of teleconferences and lunch breaks that you were going to take anyway results in a net “downtime” of mere seconds, rather than tens of minutes. Plus, if a fast charge to 80% gives you enough range to finish your shift and get the vehicle back to your home or depot, you can avail of much cheaper electricity from your AC charge point(s) to top up the vehicle overnight.
4) Some EV batteries hate being routinely charged to 100%, but others quite like it
Some EVs may state in their instruction manuals that it is recommended not to charge the battery to 100% every time; indeed, some EVs point this out to you on a message on the dashboard if they’re charged to 100% several times in a row. This applies to EVs that have types of lithium-ion cell that contain cobalt and nickel, most notably “NMC,” and the reason why requires a quick electrochemistry lesson. The voltage, or “potential difference,” of a cell is the difference in potential between its positive and negative electrodes. When an NMC cell is fully charged, the positive electrode’s potential becomes so high that it crosses the threshold at which the electrolyte starts to very slowly degrade against it. This happens over many years and many thousands of kilometres, but does degrade the cells faster than if the battery had been routinely charged to a lower State of Charge. 80% is often recommended as the charge cut-off for daily use, but if you need the vehicle’s full range, then of course make full use of it and fully charge the battery. Just don’t leave the vehicle sitting at 100% State of Charge, undriven, for weeks on end.
An increasing number of EVs use a different form of lithium-ion cell called lithium iron phosphate, or LFP. This has numerous advantages, not least its lower cost and more ethical raw materials (e.g. no cobalt or nickel), but arguably its biggest selling point for fleets is its improved lifespan. This is in part because, when fully charged, the positive electrode doesn’t exceed the electrolyte degradation threshold like NMC, so the slow degradation reactions within the cell take place over a much longer period of time. One of LFP’s downsides is that, unlike NMC, its voltage barely changes between approximately 20 – 90% State of Charge, which makes it tricky for the vehicle’s Battery Management System to determine the State of Charge accurately from cell voltages alone. As a result, some EV manufacturers actively encourage drivers to routinely fully charge their LFP-equipped EVs, which helps to recalibrate the Battery Management System, and improve State of Charge and remaining range estimates on the dashboard, while not risking the increased degradation of the battery pack.
As a rule of thumb, NMC cells are typically found in “Long Range” EVs, with many “Standard Range” vehicles using LFP cells. This should give an indication of the ideal charging patterns for your current and prospective fleet vehicles. Note that some brands exclusively use a single cell chemistry across all of their EV makes and models, most notably BYD, which uses its LFP cells in all of its vehicles, from the Dolphin Surf city car to the Seal executive saloon.
It is incredible that many EV manufacturers and dealers still don’t tell their customers some of the invaluable top tips above. Some can be performed quickly and easily on your existing fleet, while some may completely change the outcome of your procurement decisions by highlighting vehicle specifications or alternative EV brands that are better equipped than what you’d been considering until now. However, all of them will result in minimised downtime and maximised value from your fleet. Fleet electrification is already rapidly becoming a no-brainer for fleet managers, but these tips will allow your business to gain a competitive edge and run even more smoothly than before.