Electrical Characterization

The capacity of a battery reflects the maximum amount of electrical charge stored within it. To measure the capacity, a fully charged battery is discharged with a constant current to a defined cutoff voltage. Because 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 as the discharge current increases. This effect is described by the Peukert equation and is amplified by the voltage drop across the internal resistance of the cell, which causes the output voltage to drop more quickly. In addition, the removable capacity is affected by the limited speed of the electrochemical processes and the charge transport processes in the battery.

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. This is significant to achieve accurate operation predictions. The open-circuit voltage characteristic is unique for all material properties and thus every battery technology.

Once the open-circuit voltage characteristic has been derived from the electrical charge, differential voltage analysis (DVA) can be performed. In this illustration method, plateaus of the open-circuit voltage are shown as peaks. As a result, transitions between phases can be shown, for example, when lithium ions are incorporated into the active material. A structural change of the active material occurs during phase transitions. This not only impacts the voltage curve but also significantly accelerates the aging process.

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 so that the stored energy can be used again when accelerating. This allows the consumer to save on fuel and thus protects the environment.

However, this simple-sounding process is an immense challenge for every battery. As a result, charge acceptance tests are becoming increasingly popular among car manufacturers. Only very few battery technologies have the ability to operate efficiently with very high charging currents. Furthermore, the dynamic charge acceptance strongly depends on the preconditioning of the battery, state of charge, previous charging or discharging, and temperature.

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 us to model and simulate the electrical and dynamic behavior of the battery.

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 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,I_DC)=V_AC/I_AC 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. As a result, a variety of measurements are needed across a broad operating range.

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 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 the cell properties may also be conducted.