Electrochemical impedance spectroscopy (EIS) is becoming increasingly important as a technique for dimensioning and characterizing batteries and other electrochemical systems. The advantages of this non-invasive measurement method can be exploited not only in modelling energy storage units such as batteries and fuel cells, but also in the fields of basic battery research and diagnostics. In many cases it is now possible to dispense with highly complex laboratory assemblies and measurement systems in favour of monolithic System-on-Chip solutions integrated into the battery system itself.
What was the motivation for this application?
The main point at issue is to ensure a permanent source of electric power whilst at the same time main functions are increasingly being realized by means of electronic equipment. Effective registration of the true state of a battery cell - in respect of performance, state of charge (SOC) and ageing (state of health, SOH) - as well as aspects such as cell monitoring, performance forecasting, operating strategy, thermal management, cell balancing, cell ageing and high-voltage charge management are decisive factors which determine how far a vehicle can travel per charge and how long the battery will last altogether.
However, these physical and/or electrochemical variables can only be defined in a usable way if the battery can be subjected to an exact mathematical definition as well as modelled and represented in the form of a complex equivalent circuit. EIS provides a valuable contribution towards achieving this end.
At present, lithium-ion batteries represent the most promising storage technology option available for use in the coming generation of hybrid vehicles. However, irrespective of the technology involved, the communication between cells and/or stacks represents one of the elementary necessities within the total system. In order to address this aspect, HAW Hamburg has developed system proposals (Fig. 1) for connecting cell sensor elements and cell balancing elements to the control device.
Fig. 1. Possible system integration in vehicle (Source: HAW Hamburg , Faculty of