dec 11

Top 4 Factors That Influence Battery Degradation In Electric Buses & How To avoid them

One of the biggest concerns for electric vehicle owners is battery degradation. It can have far-reaching consequences for owners and operators, eventually resulting in reduced energy capacity, efficiency, and a delay in the return on investment. 

The battery can make up 50% of the cost of an electric bus. Keeping it healthy means you’re protecting your investment.  

And with the electric bus market growing at a staggering speed, 48% increase between 2017 and 2018 in Western Europe and Poland alone, understanding the consequences of battery degradation is increasingly important. Moreover, a focus on prevention, as well as reusing opportunities for batteries can allow fleet operators to fully maximize their uptime. Operators can minimize the TCO of their electric bus fleets.

Lessons from the webinar: Maximize life & residual value of bus batteries 

A few weeks ago, ViriCiti co-hosted a webinar with Claudius Jehle of the Fraunhofer Institute for Transportation and Infrastructure Systems IVI – Europe’s largest application-oriented research organization. Since 2014, he manages the group »Energy Storage Systems« and is responsible for battery diagnosis systems development. 

During his part of the presentation, Claudius offered us insights into the relevance of battery degradation, the main factors that influence the battery health, and how to prevent battery degradation in an electric bus. 

Below you will find a summary of the key takeaways. 

Putting battery degradation into context

To understand battery degradation, it is helpful to think of a battery as a human heart. The heart is the core of the body, delivering blood, nutrients, and energy to the body, just as the battery is the core of the vehicle, delivering energy to the vehicle. 

Without a strong, healthy heart, the body cannot run optimally. In the same way, a vehicle cannot run efficiently without a healthy battery.  

All batteries will degrade inevitably with use. Just like the heart, a battery degrades depending on the usage. 

💡 In the EV world, it is generally considered that when a battery reaches 80% of its original capacity it is no longer good for use. So, for a bus with a 350kWh pack this means that that 280kWh would be its maximum capacity.

Factors that influence battery degradation in electric buses

As you can see from the summary in the above picture, the main factors that contribute to degradation are temperature, high power, depth of discharge, and the average state of charge. 

Claudius offered some useful guidelines for each factor. However, it is important to keep in mind that these are just rules of thumb, and not precise measurements. The actual parameters will differ per OEM – and how the battery is designed and manufactured.

Let’s have a closer look at each of these factors.

Temperature 

The optimal temperature range for a battery is approximately between 15 to 30 degrees Celsius. 

Very high temperatures are bad because the ions within the battery tend to look for other partners and move around more quickly. Chemical reactions are more likely to happen and they are not available for transporting energy anymore. Extremely low temperatures also hinder the moveability of these ions and are also bad for energy storage. Therefore, It is best to avoid temperatures that are too high or too low.

(High) Power

High power/current is not optimal for batteries – both when charging or discharging the battery. 

There can be exceptions to the rule, such as batteries that are optimized for high power applications like fast charging. But as a rule of thumb, the lower the current and the power requests, the better it is for your batteries. 

Depth of discharge (DOD)

To better understand the impact the DOD has on the SOH, let’s compare a battery with a glass of water, as we do in the image below. 

Here are some useful definitions: 

  • Capacity of the battery is the size of the glass 
  • State of Charge (SOC) is the filling level of the glass
  • A full cycle is either a) going from empty to full to empty OR b) from full to empty to full. Both options are considered full cycles. 
  • Full cycle equivalents – going from 0 to 50% to 0 is half of full-cycle at a DOD of 50%. 
  • 2 half-cycles are equivalent to 1 full cycle.  

💡 It is important to remember that the way you complete a full cycle will have a completely different impact on the State of Health of the battery. 

You can complete 1000 cycles either by: 

  1. charging 1000 times from 0% to 100% and back to 0%   or…
  2. charging 2000 times from 50% to 100% to 50%. Or from 0% to 50% to 0%. 

These different charging cycles have different impacts on the aging of the battery. A tendency is that the lower the cycle height (like the one in the second example), is better for the battery. 

💡 This means that as guiding principle is better to charge from let’s say 30% to 70% instead of from 0% to 100%. 

Furthermore, although it might be counter-intuitive, a battery that is only used between 50-70%  for example (which might seem overengineered as 80% of the energy content is effectively not used) can still be more economical in a Life Cycle Cost (LCC) investigation. Due to the strong positive effects of low DODs the lifetime is expected to more than double compared to a 100-0% usage. 

The average State of Charge 

Batteries usually don’t like to be stalled and operated close to 100% or close to 0% SOC. As a tendency they like to be operated at 50% state of charge on average, which can be a problem for opportunity charging. 

So how much will your battery be affected by these factors?  

It all depends on the individual susceptibility factors of the used cell type! This cannot be stressed enough. 

Tips to minimize battery degradation in electric buses

1. Think about the power/current

We’ve seen that the rate at which the battery is being charged or discharged will impact the SOH.

To minimize the effect of current on your battery it is important to train your drivers to use far-sighted driving. This means that they should accelerate and decelerate more smoothly. Why? 

Harsh accelerations means that the battery will discharge at a higher rate. So, by easing their way to the optimal cruising speed the battery will discharge at a slower rate, thus it will be better for the SOH. 

Changing the braking techniques can also help preserve the battery. A far-sighted driver will brake more smoothly, reducing power peaks, thus reducing ageing. Especially during recuperative braking, the vehicle controller usually takes care of controlling the power load correctly. Thus, regenerative braking puts much less stress on the battery.

Also, as another inherent general guideline, because batteries prefer slower current, overnight chargers with 50-70kW will be preferred to opportunity chargers that can go up to 350kW. 

2. Take Advantage of Parking

Due to passengers, doors opening and closing, temperature control is challenging during operation. When the bus is parked, however, you have more control. This is a great opportunity to ensure the vehicle is at the optimal temperature of 15-30 degrees Celsius. 

High heat and extreme cold put extra strain on the battery. It seems simple but is very important. So what can you do? 

💡 On a hot day, parking in the shade can have a big benefit for your batteries. Don’t have an overhead shelter? Find a nearby tree to park under. In the same way, when it’s cold, finding a spot in a parking garage rather than leaving the bus outside, can help minimize the effects of battery degradation.

3. Average SOC

We’ve seen that it isn’t good for the battery state of health to be stalled and operated close to 100% or close to 0% SOC. 

In practical terms, this means that you should avoid parking at 100% if you don’t have to. Also, using a smart charging solution can help keep the SOC in an area as optimal as possible, while benefiting your operation as well. 

Smart Charging allows you to monitor and manage each charging session. With a software back-end solution, real-time data can be brought from charging vehicles and charging events to the operator’s fingertips. 

💡 To learn more about Smart Charging you can check our Ultimate Guide on Smart Charging Electric Buses

4. Control the depth of discharge 

Keep in mind that a low DOD (Depth of Discharge) is better than a higher one. This means that you should not wait until your batteries reach 5% -10% to start charging them. 

How to manage the DOD of your electric bus? 

Managing the DOD proves to be very difficult. The standard monitoring software included by the manufacturer is usually limited. It does not allow for a detailed view of every single battery cell. 

💡 If you are looking for a powerful tool that gives you cell-level insights into your batteries, we recommend taking a look at ViriCiti. This system allows you to monitor things like temperature, SOC of every single cell in real-time. The data also is available in historic reports. 

Battery health factor reports

To better understand the state of health of your batteries you can also get a battery stress level report. This is a project created as a result of the collaboration between ViriCiti and Fraunhofer IVI.  

ViriCiti stores data on millisecond-level from batteries over a long timeframe. Then Fraunhofer analyses this data and evaluates the level of stress applied to the batteries.  Above you can see an example of such a battery stress report. 

💡 If you are curious to learn more about the battery stress evaluation you can do so here

Second life options for electric bus batteries


Even with optimal care, battery degradation over time is inevitable, but that does not mean the battery has to go to waste! There are several ways in which the battery can be reused. 

💡 It is possible to resell the battery to a third party such as recyclers, to a second market, or to sell the battery back to the manufacturer. 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 TCO of your electric bus.

One company which is already making use of batteries’ second life application is Heliox, one of ViriCiti’s partners. In collaboration with InvertedPower they are bringing to market a new generation of battery buffered high power charging stations

These charging stations provide energy to the vehicle that comes both from the grid and from an energy storage system, facilitating the high power “sprints” of charging power needed for opportunity charging. This new charger generation is for both electric buses and electric trucks. 

Another company that is making use of the second life of batteries is Irizar, an electric bus manufacturer. The OEM just announced a partnership with Ibi allowing the reuse of the batteries at the end of their useful life. Ibil will use second-life batteries to store energy at charging stations for electric cars implemented in Repsol stations.

Another project that aims to reuse second-life batteries comes from Volvo Bus.  In Gothenburg they work together with a few other parties to use the second-life batteries as electricity storage systems in apartment buildings. 

So, it safe to say you will definitely find a way to reuse your batteries once they are no longer fitted to operate your electric buses. 

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