Whats the best strategy for utilizing my battery? Whats the projected lifespan of my battery? Is my eBus suitable for this route? All these typical customer questions are not so simple when one is considering a Li-ion battery.

A typical Li-ion battery starts life with a certain capacity, an amount of energy it can hold. But this amount of energy doesn’t hold constant over the battery’s lifetime, it decreases. It decreases because there are several processes going on in the battery which degrade its ability to hold charge. These can be chemical processes whereby Li-ions are captured in a partially conducting layer and therefore no longer available to take part in the charging/discharging process. They can be mechanical processes such as fatigue and fracturing of the material within the battery cell. To make things more complicated these mechanical and electrochemical effects are interlinked. Mechanical fracturing can lead to the exposure of active material to electrochemical processes. For those of you who are interested in the deep dive details there is a rather nice talk from the Faraday Institute in the UK which goes into the detail of the processes and why degradation is important – https://www.youtube.com/watch?v=QoArmQYSREA.

Whatever the underlying reason, the bottom line is that as I use my battery its ability to deliver energy and power diminishes. The amount of degradation is dependent upon how I used my battery, how old my battery is, what is the ambient temperature. Furthermore, the dependence is not only multi-parameter but also highly non-linear. To make matters worse there is normally a rapid transition between when my battery is degrading in a slow and predictable manner and when it falls off a cliff. On an engineering level, we can characterize degradation as 2 orthogonal processes (simplistic but allows us to derive some helpful rules) namely calendar aging and cyclic aging.

Calendar aging happens all the time, even if I dont use the battery. The degradation is some function of time, typically square root, but this can vary from battery chemistry to battery chemistry. In addition, higher temperatures accelerate the aging with a Boltzmann type dependency. Finally, the state of charge (SOC) at which the battery also has an effect, with higher SOC accelerating the aging.

Cyclic aging happens when I use the battery. It is typically some function of the energy which has flowed through the battery. In addition, high temperatures will accelerate the degradation – similar to calendar aging. The rate at which the battery is charged and discharged will also affect the degradation. Higher charge/discharge rates will lead to increased degradation. In addition, the depth of discharge is important. This means that large swings in the SOC, e.g. from 90% – 10% will cause the battery to degrade faster than smaller swings e.g. from 60% to 50%. Finally, batteries dont like being charged at low temperatures (due to a process called lithium plating) and they dont like being drained to low levels of SOC (which is why mainly OEMs will insist that SOC is kept above 20%).

With some knowledge of how batteries behave it is possible to extend battery life by following some simple measures. Take temperature, basically batteries are pretty much like human, they are most comfortable around 18-22C. Keeping the batteries in this range will help prolong life. Not charging quickly will help reduce cyclic aging. Reducing time spent at high SOC will reduce calendar aging. It is also possible to include these rules when optimizing the SMART charging of batteries.

Some conclusions….

Battery degradation can be a significant issue for commercial customers and can affect the successful business case for an electric fleet. As such, uncontrolled charging can affect battery degradation. To manage this risk, it is possible to employ digital solutions to manage the charging. Siemens has a range of digital products to support fleets and depots. If you want to know more about charging management for fleets and depots then please come and talk to us. The easiest way to do this is to visit our website www.siemens.com/evdepot-digital. Here you can contact us to ask for more info or to book a slot for a demo or to talk to our experts.