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Electrical Characterisation

As the battery market continues to grow faster, batteries need to improve continuously. Batteries have to be lighter and smaller, but at the same time have more capacity, a longer life time and still be offered at lower prices.

The electrical characterization of battery cells forms the basis for system design, modeling, cell diagnosis and cell monitoring and thereby;

  • reduce dangers during operation by meeting the voltage, current, and temperature limits.
  • the capacity and the operation time is estimated more accurately, which increases the range of, for example, electric vehicles.
  • aged cells are replaced faster to extend the life of the battery pack and reduces costs.

The electrical characterization of test cells includes, among others, standard tests such as the measurement of the capacity, open circuit voltage measurement, differential voltage analysis, pulse testing, dynamic charge acceptance tests, electrochemical impedance spectroscopy, and other frequency-based excitations.

Capacity

Capacity Measurement with different Currents
Lupe

The capacity of a battery displays the maximum amount of electrical charge stored in a battery. To measure the capacity, a fully charged battery is discharged with a constant current to a defined cut off voltage. Since the capacity of a battery, depending on the technology, can be current-dependent, this is partially repeated for different currents. The capacity of a battery decreases with increasing discharge current. This effect is described by the Peukert equation. In addition, the removable capacity is affected by the limited speed of the electrochemical processes and the charge transport processes in the battery.

Open Circuit Voltage

Open Circuit Voltage
Lupe

The open circuit voltage (OCV) of a battery indicates the voltage, in the stationary state for each state of charge. Using the open circuit voltage, the state of charge of a battery can be estimated and is thereby contributing to the accuracy of operation forecasts. The open circuit characteristic is unique for every battery technology and material properties.

Differential Voltage Analysis

Differential Voltage Analysis
Lupe

If the open circuit voltage characteristic is derived from the electrical charge, the differential voltage analysis (DVA) can be performed. With this illustration method, plateaus of the open circuit voltage are shown as peaks. As a result, transitions between phases, for example, during the incorporation of lithium ions into the active material, can be shown. At any point of phase transition a structural change of the active material is performed, that has not only an effect on the voltage curve, but also significantly accelerates the aging process.

Dynamic Charge Acceptance

Dynamic Charge Acceptance with and without Carbon Additives
Lupe

Recuperation for start-stop engines is installed in most new car models. The energy that would normally be lost during braking is charged into a battery, when accelerating the stored energy can be used again. Through the fuel savings this new technology helps protecting the environment. But this simple-sounding process is an immense challenge for every battery, therefore charge acceptance tests are of increasing interest for car manufacturers. Only very few battery technologies have the ability to operate with very high charging currents efficiently. Furthermore, the dynamic charge acceptance strongly depends on the preconditioning of the battery; state of charge, previous charging or discharging step, and temperature.

Puls Test

Plus Tests and Simulation
Lupe

Pulse tests are used to identify the frequency-dependent electrical behavior of batteries. For this purpose, batteries are operated with current pulses with different amplitudes and frequencies and the voltage response is measured. This allows to model and simulate the electrical, and the dynamic behavior of the battery.

Electrochemical Impedance Spectroscopy

EIS at different state of charges
Lupe

Electrochemical impedance spectroscopy is used to map the frequency-dependent electrical behavior of batteries. For this purpose, a sinusoidal current with a defined frequency is embossed on the battery and the voltage response is measured. If the system is a linear, causal and time-invariant the output signal is also a sinusoidal signal with the same frequency. Using the current and the voltage response, the impedance Z (f, IDC) = VAC / IAC can be calculated. The entire spectrum consists of several measurements at different frequencies and therefore maps the entire dynamic behavior of the battery. For different battery types and technologies, the impedance spectrum changes with a variety of factors, such as superimposed currents, temperature, state of charge, and aging. Therefore, a variety of measurements must be made for a wide operating range.

Other Frequency-Based Excitations

Measurement and Results of frequency-based excitations
Lupe

The cell is excited with a test signal consisting of a combination of several frequencies. The amplitude of the test signal should be small enough to prevent changes in the state of charge. During cell operation, the cell status can be monitored by the frequency response of the output voltage. During the measurement, it is necessary to excite an appropriate combination of frequencies to obtain the desired information of the battery cell. An analysis of harmonic components due to non-linearity of cell properties may also be considered.

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Technische Universit├Ąt Berlin
Electrical Energy Storage Technology
of Energy and Automation Technology
IV
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D-10587 Berlin

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