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How fit is the battery of my electric car?

Reference measurement set-up for Li ion battery cells

PTB-News 3.2017
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For the practical use of battery-operated electric vehicles, measurement methods are required which provide fast and reliable information as to whether and to what extent the maximum charging capacity of a Li ion battery has already decreased. This is referred to as the “state of health”. To measure a battery's state of health, PTB has installed a reference measuring set-up that uses electrochemical impedance spectroscopy as well as a model designed to simulate aging.

Referenzmessplatz für den Gesundheitszustand von Li-Ionen- Batteriezellen für E-Fahrzeuge. Aufgrund der großen elektrochemisch gespeicherten Energiemenge müssen die Alterung und die Impedanzmessungen in klimatisierten Sicherheitsschränken durchgeführt werden.

A promising measurement method to quickly measure the health of a Li ion battery is electrochemical impedance spectroscopy. If the electrochemical processes in a battery cell change due to battery aging, then the impedances measured at different frequencies (AC resistances) – the socalled impedance spectrum – also change. As a result, these changes can be used as a measure of a battery cell's state of health.

The aging that to date has been quantified merely empirically by impedance spectroscopy is now additionally simulated with a battery model. It is based on charge and mass transport equations which describe the processes in the battery cell mathematically. If – in addition – equations are implemented for the description of the aging processes, the degradation process can also be simulated. The modeling, unlike the experiment, offers the possibility to vary all parameters independently of one another, for example diffusion coefficients, layer thicknesses or reaction rates, in order to analyze their influence on the impedance spectrum.

The simulations implemented by PTB together with the Institute of Energy and Process Systems Engineering of the Technical University (TU) Braunschweig can be used to describe the change of measured impedance spectra during aging by means of parameter variations. Meanwhile, initial model calculations have been carried out successfully. The aging model is now applied to identify parameters derived from the impedance spectra that change with the state of health of a battery cell.

After the measuring set-up has been validated by means of further lifetime tests, it will be included in PTB's range of services. This will provide industry with type-specific reference spectra for the measurement of the health of Li ion battery cells using electrochemical impedance spectroscopy.


Steffen Seitz
Department 3.4
Physical Chemistry
Phone: +49 (0)531 592-3019
Opens window for sending emailsteffen.seitz(at)ptb.de