15 June 2018

Battery Degradation

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 batterys 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 – 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. For those of you who are interested in the details there is a rather nice video from Prof David Howeys group at Oxford which shows these processes in one picture.

Whatever the underlying reason the bottom line is that as I use my battery its ability to deliver energy and power dimishes. The amount of degradation is dependant 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. This is shown in this idealised grapth.

This non-linear degradation with dependence upon multiple paramaters presents a significant risk for the customer. With this in mind Siemens has invested in a new Mindsphere Application Centre for Connected Batteries to develop a suite of digital services which will help the customer to manage their risks over the lifecycle of their battery project. These digital services will be offered vendor agnostic, not linked to a particualr EPC vendor. The Connected Batteries program draws upon the battery knowledge within Siemens through the Siemens Core Company Technology for Energy Storage. Batteries are used throughout the Siemens company in many applications e.g. aviation, Buses, marine applications, trains, grid-connected batteries……. The research across these businesses is co-ordinated through the CCT which means that there are common programs to develop the required models and algorithms required for battery operation.

The Connected batteries Mindsphere Application Centre leverages this battery knowledge and couples this with connectivity solutions to bring live battery data into Mindsphere. Within Mindsphere a suite of applications is developed that takes this raw data and calculates key battery metrics – degradation, efficiency, safety. These are the biomarkers for the battery, how healthy is the battery today. Although this information is obviously useful, it is even more useful to be able to say what happens in the future if I operate my battery in a certain manner. This requires predictive algorithms which predict how the battery will behave given a certain useage profile.

With this knowledge the useage of the battery can be optimised over its lifetime. At the beginning of the project the battery can be accurately sized based on the real application profile. The financial planning can be undertaken with a high fidelity model of the expected capacity reduction. As the project progresses the customer will be able to understand what effects their dcesions are making on the asset – changing from an FFR contract to the wholesale market increases the degradation. On an operational basis the effect of an individual trade can be evaluated and given a marginal cost. Real time data allows for predictive maintenance and the list goes on…..

To get an in-depth view of what the Mindsphere Application Centre for Connected Batteries is offering the next blog will be talking to Ian Lloyd within Siemens Energy Management – who is heading up this initiative.

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