Feature: How to avoid battery degradation in electric buses

Alexander Schabert, Senior Director of Global Bus at ChargePoint, explains why electric bus batteries degrade and how we can optimise their lifespans. 

Public transport fleets have been electrifying worldwide at an impressive speed as cities commit to achieving zero-emission targets in the coming decade. The UK has set 2040 as the preliminary date by which they aim to have all new vehicles on the road be zero-emission. Today, one-third of Belfast’s Translink fleet is zero-emission, with plans to decarbonise completely by 2030. In the Netherlands, 80% of new buses were already battery-electric in 2020.

This also means the electric bus market (eBus) is growing rapidly — a 48% increase was recorded between 2017 and 2018 in Western Europe and Poland alone. While the initial investment in sustainable vehicles can be high, the total cost of operation of an eBus is less expensive than that of a diesel bus over time.

With intelligent fleet monitoring and charge process management, these costs can be reduced even further. For example, driving style monitoring tools can help operators identify and reduce idling where possible by training drivers based on concrete action points. Smart charging lets operators use existing charging infrastructure more efficiently, reducing the need for expensive updates and further reducing charging costs through load balancing and peak shaving.

Almost 50% of the total cost of an eBus is its battery alone. In the EV world, it is customary to no longer use a battery when it reaches 80% of its original capacity, making it even more important to look at battery health. Focusing on prevention — as well as opportunities to reuse batteries — can allow fleet operators to fully maximize their uptime. Keeping eBus batteries healthy not only prolongs their life, but also protects the operators’ long-term investment. 

All batteries degrade over time. Nevertheless, this degradation depends highly on usage, and in particular on four main factors: temperature, high power, depth of discharge and the average state of charge. Let’s see what each of those means in practice.

Like people, batteries find temperatures between 15 and 30 degrees Celsius optimal. High temperatures tend to increase the speed of the ions within the battery, leading to chemical reactions that prevent the ions from doing their job – transporting energy around. But extreme low temperatures also affect ion movement and decrease their capacity to store energy. 

Secondly, unless batteries are optimized for fast charging, it’s usually best to avoid high power/current when charging or discharging them. High power makes the battery cells wear out faster and decreases the overall life expectancy. Therefore, it’s generally considered that lower current and power requests are better for eBus batteries.

The third aspect you should look at for making sure your batteries are as healthy as possible is the depth of discharge (DOD). Before we get into details, it’s important to understand what a full charging cycle means — namely, either going from empty (0% battery) to full (100% charged battery) to empty (0% battery), or from full (100%) to empty (0%) to full (100%). Both are considered full cycles. Based on this, we can say that going from 0% to 50% to 0% is half of a full cycle, at a DOD of 50%. 

Charging cycles are important because they directly impact the ageing of a battery. The lower the cycle height, the better it is for the battery. In other words, it’s best to get 1,000 charging cycles by charging the battery 2,000 times from 50% to 100% to 50%, rather than charging 1,000 times from empty to full to empty. 

The average state of charge has an impact on battery health as well. In general, having your batteries stalled or operated close to 100% or close to 0% state of charge is detrimental to the health of the battery. A best practice is to operate batteries at a 50% state of charge on average instead.

Now that we went through some of the main factors impacting battery degradation, how can operators reduce it and increase the lifetime of eBus batteries?  

First, consider power and current. We saw that the rate at which the battery is being charged and discharged affects battery health. Training your drivers to drive more smoothly can help with this. By avoiding harsh accelerations and braking, and using more regenerative braking, the battery would discharge at a slower rate, increasing the battery’s state of health (SOH). While improving someone’s driving style may initially seem like a tough job, there are advanced driving style monitoring tools out there that can support you, as well as help you reduce idling and optimize your overall operation. 

Furthermore, as batteries prefer slower current, it’s recommended to use overnight (slow) chargers whenever possible and reduce the use of opportunity chargers that can have a maximum capacity of 350kW.

Maintaining a vehicle temperature in the optimal range of 15-30 degrees Celsius can be difficult during operation. Therefore, taking advantage of parking can help you regulate this better. During extreme high temperatures, parking in the shade can have a significant benefit for your batteries. If an overhead shelter is not available, parking under a tree can also help. Similarly, on very cold days, parking in a garage can help minimize battery degradation.

We’ve also seen that batteries don’t like to be stalled and operated close to 100% or close to 0% SOC. In other words, if possible, you should avoid parking when the battery is at 100%. Also, using a smart charging solution can help keep the SOC in an optimal state, while benefiting your operation as well. 

But even a battery that’s well taken care of will become unusable over time. That does not mean it has to go to waste, though. There are several ways in which a battery can be reused. It is possible to resell batteries to third parties such as recyclers, to a secondary market, or to sell the battery back to the manufacturer. Remember, while a battery that loses 20% of its initial capacity is deemed unusable for public transport, there are still 80% healthy battery cells inside. There are also multiple in-house usages, such as for tram and train substations, or in combination with bus charging stations. Make sure to take this option into account when calculating the total cost of ownership of your electric bus.

Photo by Possessed Photography