DCIR measures the ohmic series dc output resistance of the cell. To answer that question, let’s look at the meaning of DCIR. Now, if we’re applying a pulse to the cell, how wide should the pulse be? Also, if we’re measuring V_afterstep, when is the right time to make the measurement? It’s immediately after applying the pulse or toward the end of the pulse before the cell is returned to its “before step” state (which is typically resting state, as mentioned above). In either case, it isn’t desirable to change the state of charge (SoC) of the cell, so the current is usually applied as a short pulse. With high currents, you can’t leave the high current to be applied indefinitely or the cell will heat up and charge (if the current pulse is positive) or discharge (if the current pulse is negative). For a 50-Ah cell, that’s 1000 A, so DCIR equipment can be large and expensive. I’ve seen requests for current steps as high at 20 C. In fact, you may want to measure the DCIR in both directions and compare or average the results.Īs to the size of the step of current, this is normally a large current step because the very low resistance of the cell will need a large current step to create a measurable response in voltage. The applied step change in current can be a step up in current, which is a charge pulse, or it can be a step down in current, which is a discharge pulse. Normally, the first measurement (before step) is made when the cell is at rest, so V_beforestep = cell open circuit voltage (OCV) and I_beforestep = 0 amperes. Instead, a step change or pulse is used: DCIR = (V_beforestep – V_afterstep) / (I_beforestep – I_afterstep). In this case, because it’s DCIR, we’re making a true dc resistance measurement-there’s no impedance and no ac signal generation. As with ACIR, to measure resistance, you apply a change in current and measure the voltage response. However, ACIR has become a very standard way of assessing the cell’s resistance.ĭCIR, on the other hand, has no such standardization.ĭCIR looks to measure the dc resistance characteristic of the cell. Therefore, this measurement of ACIR doesn’t reveal much about how the cell will behave in a real-world application. It’s unlikely that any real-world application of the cell will present a 1000-Hz sinusoidal current load on the cell. So, to keep it simple, the real part of the impedance Vac/Iac is called the ACIR. There can be phase shift between the Iac and Vac as we measure impedance. The impedance at 1000 Hz is calculated as Vac/Iac. The ac current is usually around 100 mA and the frequency is 1000 Hz. To measure ACIR, an ac signal, typically an ac current (Iac) is passed through the cell and the voltage response (Vac) of the cell is measured. DCIR is also sometimes interchangeably called pulse testing, which can lead to further confusion.įirst, let’s take a quick look at ACIR. A LiFePO 4 battery is tested in the test station for 37 weeks to verify the validation of the proposed method.While ac internal resistance (ACIR) measurements are quite common and somewhat standardized when measuring lithium-ion cells, dc internal resistance (DCIR) measurements are nonstandard and generally misunderstood. Afterwards, SVM is able to establish the optimal SOH estimator on the basis of the optimal feature combination and the battery SOH. In order to find the most effective feature for SOH estimation, all the possible combinations of the features are investigated and compared. After applying the short term current pulse test (few seconds), the keen points and the slopes in the voltage response curve are selected as the potential candidate features. The benefit of the proposed method is that the features come from the short-term test, which is much convenient to be obtained in real applications. Since the terminal voltage measured at the same condition varies with the battery aging process, the features for SOH estimation are extracted from the voltage response under a specific current pulse test. Utilizing the features from the terminal voltage response of the Li-ion battery under current pulse test, a new method is proposed in this paper by using the Support Vector Machine (SVM) technique for accurately estimating the battery SOH. State of Health (SOH) of Lithium-ion (Li-ion) battery plays a pivotal role in the reliability and safety of the Battery Energy Storage System (BESS) in the power system.
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