Rechargeable batteries are the first elements in EVs, cellular gadgets, and power storage techniques. The batteries’ working situations, together with state of well being (SOH), state of cost (SOC), and temperature, are important to reliably and effectively function gadgets or tools. Predicting battery SOH and SOC is turning into a precedence with the intention to enhance their efficiency and security.
Bodily, you may signify the batteries as {an electrical} circuit mannequin, as proven in Determine 1. The resistors (Rs) and capacitors (Cs) within the mannequin have good correlations with battery states. Electrochemical impedance spectroscopy (EIS) applied sciences are essential to characterize the weather of the mannequin with the intention to get hold of the batteries’ working situations.
Determine 1 The equal circuit of a battery exhibiting Rs and Cs which have a very good correlation with battery states. Supply: Texas Devices
Rs and Cs change when the batteries are in numerous states, resulting in impedance adjustments. With EIS strategies, making use of AC alerts to the batteries and measuring their voltage and present response permits calculations of the impedance knowledge of the batteries within the frequency domains. By analyzing the impedance knowledge, you may know the battery’s SOC, inner temperature, and battery life. EV producers at the moment are researching how you can apply EIS strategies to a battery administration system (BMS).
Nyquist software
Making use of an AC voltage to a circuit excites the AC present. Equation 1 calculates the impedance, which varies as frequencies change if the circuit shouldn’t be a pure resistance load.
Determine 2 illustrates Ohm’s legislation for an AC voltage. You’ll be able to plot the impedance by making use of many frequencies. Sometimes, a battery is modeled as Rs and Cs together, as proven in Determine 1. Determine 3 illustrates the impedance plot utilizing a Nyquist software.
Determine 2 Ohm’s legislation in an AC circuit, impedance will be plotted by making use of many frequencies. Supply: Texas Devices
Determine 3 The plot of impedance utilizing the Nyquist software. Supply: Texas Devices
Strategies of excitation present era
You should use the EIS approach for one cell, a number of cells, modules, or a pack. Performing an EIS measurement requires the appliance of AC present to the batteries. For various battery system voltages, there are 4 totally different strategies to generate the excitation present. Let’s evaluate them.
Methodology #1: Resistor dissipation on the cell degree and module degree
In Determine 4, the ability change (S1), energy resistor (Rrestrict), sense resistor (Rsense), and a controller produce the excitation supply. The controller generates a sinusoidal pulse-width modulation (SPWM) sign for S1. One or a number of battery cells are linked in sequence with the excitation supply. Turning on S1 attracts the present from the batteries by way of Rrestrict. The power burns and dissipates. When the voltage is excessive, the ability dissipation is considerably massive.
Determine 4 EIS with a resistor load the place S1, Rsense, and the controller supply produce the excitation circuit. Supply: Texas Devices
You should use this technique on the cell degree and small module degree with low voltage, however it’s not a sensible answer for high-voltage batteries in EVs or hybrid EVs (HEVs) as a result of the ability dissipation is simply too nice.Â
Methodology #2: An remoted DC/DC converter on the pack degree
In an EV powertrain, high-voltage batteries cost low-voltage batteries by way of an remoted DC/DC converter (as proven in Determine 5), which you’ll be able to design to help bidirectional energy move. Throughout EIS excitation, energy transfers from high- to low-voltage batteries in the course of the constructive cycle; energy is then reversed from the low- to high-voltage aspect in the course of the unfavorable cycle. This technique makes use of present {hardware} with out including further prices. Nonetheless, the excitation supply is proscribed by the capability of the low-voltage batteries. It’s significantly difficult for 800V-12V battery techniques.
Determine 5 EV energy prepare with high-voltage batteries charging low-voltage batteries by way of an remoted DC/DC converter. Supply: Texas DevicesÂ
Methodology #3: A non-isolated DC/DC converter in stack mode for the pack
This technique makes use of a non-isolated DC/DC converter to generate excitation present between two battery modules. Throughout EIS excitation, the cost transfers from Vbat1 to Vbat2 in the course of the constructive cycle, and the cost transfers again to Vbat1 from Vbat2. In Determine 6, two battery modules are linked in stack mode. Two energetic half-bridges are linked in sequence, and their switching nodes are linked by way of an inductor and a capacitor.
There are a number of benefits to this technique: one is the usage of low-voltage score switches in a high-voltage system; the opposite is that the switches function below zero-voltage switching (ZVS) situations. Moreover, this technique permits the manufacturing of a bigger excitation present with out including stress.
Determine 6 A non-isolated DC/DC converter connecting two battery modules in stack mode. Supply: Texas Devices
Methodology #4: A non-isolated DC/DC converter in parallel mode for the pack
This technique connects two battery modules in parallel mode, as proven in Determine 7. Two modules share a standard floor. The fees are transferred to the inductor and capacitor from VBat1; then the costs saved within the inductor and capacitor are transferred to VBat2. The parallel mode and stack mode are swappable by correctly reconfiguring two modules to satisfy totally different charging stations or battery voltage techniques ,akin to 400 V or 800 V.
Determine 7 A non-isolated DC/DC converter in parallel mode for the pack. Supply: Texas Devices
EIS measurement
Determine 8 divides the battery pack into two modules. S2 and S3 are battery-management ICs. The BQ79826 measures the voltage of each cell by way of an analog entrance finish. Making use of the AC present to battery modules builds up the AC voltage of every cell, which the BMICs then measure. A present measurement IC is used to measure the excitation present sensed by a present shunt. A communication bridge IC connects all BMICs by way of a daisy-chain communication bus. The BQ79826 makes use of the EIS engine to calculate the impedance, which is transmitted to a microcontroller for the Nyquist plot. The Controller Space Community (CAN) protocol gives communication whereas MCU1 controls the era of excitation present.
Determine 8 Block diagram of an EIS measurement that divides the battery pack into two modules, every monitored by BMICs. Supply: Texas Devices
 In a simulation of a non-isolated stacked energetic bridge (SAB) based mostly excitation circuit, the situations have been VBat1 = VBat2 = 400 V, Fs =100 kHz, and present amplitude = 5 A. Determine 9 reveals the excitation present waveform from the simulation. The blue hint is VBat1 present, whereas the inexperienced hint is VBat2 present.
Determine 9 Excitation present generated by stacked energetic bridge, the blue hint is the VBat1 present, the inexperienced hint is the VBat2 present. Supply: Texas Devices
The frequency synchronization between the controller of the excitation supply and the BQ79826 is important to attenuate measurement errors. One answer is to take the SPWM sign generated by the BQ79826 because the reference of the excitation supply (Determine 10). The excitation supply and EIS engine of BQ79826 are mechanically synchronized.
Determine 10 Block diagram for system timing synchronization of the excitation supply and the BQ79826 with the intention to reduce measurement errors. Supply: Texas Devices
When constructing {hardware} to judge an EIS measurement, the excitation supply ought to have excessive effectivity to attenuate cost losses within the batteries. The full present harmonics also needs to be small with the intention to enhance the signal-to-noise ratio (SNR). Determine 11 reveals the effectivity measurement of the converter utilizing a DC voltage and a DC load. With a better excitation present, the ability effectivity is greater due to the bigger ZVS vary. Above a 1-A amplitude of excitation present, effectivity is >95%. All the ability dissipates within the conventional technique of utilizing a load resistor.
Determine 11 Effectivity measurement of a stacked energetic bridge-based energy stage. Supply: Texas Devices
A quick Fourier remodel (FFT) is a software to judge the SNR of the excitation present. Putting six 18650 batteries in sequence for one module, with two modules linked to the stacked energetic bridges, demonstrates the standard of excitation present. In Determine 12, two tones of 10 Hz and 100 Hz are generated concurrently to scale back the excitation time.
Determine 12 FFT of excitation present the place two 10 Hz and 100 Hz tones are generated concurrently. Supply: Texas Devices
Determine 13 is a Nyquist plot exhibiting the impedances of various cells utilizing two 200-V battery modules. At decrease excitation frequencies, the distinction between the measurements is small. There are extra discrepancies at greater excitation frequencies (proven on the left aspect of the graphs), however the impedances inside this vary should not essential.
Determine 13 Nyquist plot exhibiting the impedances of various cells utilizing two 200-V battery modules. Supply: Texas Devices
EIS approach
EIS is an evolutionary approach with functions for EV and HEV batteries. EIS strategies allow customers to acquire real-time details about the SOC, SOH, and temperature throughout battery system operation.
Attaining good EIS outcomes nonetheless requires resolving challenges akin to creating correct algorithms, using dependable excitation techniques, and minimizing noise sensitivity.
Sean Xu presently works as a system engineer in Texas Devices’ Energy Design Companies crew to develop energy options utilizing superior applied sciences for automotive functions. Beforehand, he was a system and software engineer engaged on digital management options for enterprise, knowledge middle, and telecom energy. He earned a Ph.D. diploma from North Dakota State College and a Grasp’s diploma from Beijing College of Know-how, respectively.
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