Bode 100 - Application Note
Battery Impedance Measurement
深圳市迪福伦斯科技有限公司www.deffense.com.cn
Table of Contents
1Executive Summary (3)
2Measurement Task (3)
3Measurement Setup & Results (4)
3.1 Device Setup (5)
3.2 Calibration (6)
3.3 Measurement (7)
4Conclusion (8)
1 Executive Summary
Battery impedance includes information about the internal state of a battery. The impedance depends on many factors such as the chemical properties and mechanical design of the battery. Measuring the battery impedance over frequency helps to identify the characteristics of the battery.
The Bode 100 in conjunction with the Picotest J2111A Current Injector offers an easy way to measure the impedance of a battery in the frequency range from 1 Hz to 10 MHz.
This application note shows the connection setup and the device settings of the Bode 100 necessary to perform the impedance measurement.
2 Measurement Task
The impedance of an alkaline 9V block is measured in the frequency range from 1 Hz to 10 MHz. After discharging the battery to a no load voltage of V0=7.5 V the impedance spectrum is measured again and compared to the measurement performed on the full charged battery.
Furthermore, the impedance spectrum of a 3.7 V lithium ion battery is measured to demonstrate that even cells with low output resistance can be measured with the presented method.
3 Measurement Setup & Results
The impedance of a battery, Z i, can be measured by loading the battery with an AC1 current and measuring the resulting AC output voltage of the battery. Dividing the AC output voltage v out by the AC output current i out leads to the impedance of the battery.
Z i=v out out
The output current of the battery is modulated by the J2111A current injector, driven by the output signal of the Bode 100.
The output current is then measured by connecting CH1 of the Bode 100 to the current monitor output of the J2111A. The output voltage of the battery is measured directly using a 1:1 voltage probe connected to CH2.
The connection setup is shown in the figure below:
Figure 1: Connection Setup
Note: The maximum allowed battery voltage with this setup is 40 VDC !
1 Alternating Current (sine-waveform)
3.1 Device Setup
Current Injector J2111A:
The positive bias of the Current Injector must be switched on
(+bias) since the Bode 100 output voltage does not have an
offset. The positive bias provides a 25 mA offset current,
allowing the current injector to operate in class “A” mode. For
the best performance, the output wires from the J2111A
should be twisted or be coaxial.
Bode 100:
The battery impedance measurement can be performed directly with the Bode 100 using the external reference function. The Bode 100 is set up as follows:
Measurement Mode: Frequency Sweep Mode
Start Frequency: 1 Hz
Stop Frequency: 10 MHz
Sweep Mode: Logarithmic
Number of Points: 201 or more
Receiver Bandwidth: 100 Hz
Attenuator 1 &2: 0 dB
Level: 0 dBm
To switch on the external reference start the device configuration window
and click on the external reference switch symbol:
In addition, the input impedance of channel 1 must be set to 50Ω, while channel 2 need to remain in high impedance mode:
Trace 1 settings:
It is advisable to activate the Full Speed Mode to achieve a higher measurement speed since we are measuring over a low frequency range.
3.2 Calibration
To remove the influence of the voltage probe, we recommended calibrating the setup. To do this the voltage probe at CH2 is connected to the current monitor output of the current injector and a THRU calibration is performed.
Figure 2: Connection during THRU calibration
CH2
CH1
3.3 Measurement
For the measurement of the battery impedance, the battery under test is connected as shown in the picture below.
Figure 3: Measurement Setup Example
First, we measure impedance of the fully charged battery. Starting a single sweep leads to the
following impedance spectrum:
The Gain magnitude in this case equals the impedance magnitude in Ohm. At 1 kHz the impedance shows a value of 2.9 Ω.
Now the battery is discharged to a no load voltage of V0=7.5 V and a second sweep is performed.
The battery impedance at 1 kHz did increase to ≈3.8 Ω.
The same measurement setup can be used to measure all types of batteries. As an example we measured the impedance of a 1000 mAh, 3.7 V lithium ion cell. The impedance of this cell is shown in
At 1 kHz the lithium ion cell shows an impedance of 126 mΩ which is much lower than the impedance of the alkaline battery.
4 Conclusion
The Bode 100 in conjunction with the Picotest J2111A Current Injector offers a test set that enables simple and fast measurement of the battery impedance. The impedance of low and high impedance batteries can be evaluated over the frequency range from 1 Hz to 10 MHz.
深圳市迪福伦斯科技有限公司www.deffense.com.cn