Enhancing EV Battery Design with Advanced Testing

Figure 1. Example of a battery test lab. (Image: Keysight)

As the demand for EVs grows, it will be necessary to innovate batteries that achieve durability, power density, safety, lower cost, increased range, and faster recharge time using a fast, cost-effective, and energy-efficient process.

One important aspect of battery design is performance testing to ensure new batteries meet their design goals. EV battery testing can be expensive and time-consuming without the latest systems and methodologies. Using best practices and state-of-the-art technologies throughout the battery test process can help you quickly and easily resolve battery design challenges.

This article will explore how advanced testing using end-to-end EV battery test systems can improve the quality and performance of EV battery designs.

Identify Performance and Safety Issues

It is essential to consider the impact of poor performance. Omitting non-compulsory testing can lead to performance or safety issues that go undiscovered. Recalling a product further down the production process is costly. The time it takes to rectify the issue significantly affects the timeline for bringing a fully operational version to market. In a market evolving at such a quick pace, these delays are not justifiable.

Omitting testing from the earlier design and production stages may feel like a shortcut. In reality, it is a high-risk strategy that could result in extensive time-to-market delays if an issue remains undetected.

Lower Operational Costs

A well-designed test laboratory can drive tangible operational cost savings for those working in the EV battery R&D sector.

High-performance, state-of-the-art battery test systems can provide up to 96 percent energy efficiency while regenerating discharging battery power back to the AC grid. This can lead to significant savings in operating costs over the lifetime of the EV Battery Test Lab (Figure 1).

The technology minimizes the costs of a busy R&D lab in two ways: (1) upfront by optimizing the installation of cooling infrastructure and (2) on an ongoing basis via a notable reduction in energy costs.

Improving Lab Operations

Thorough testing requires efficiently managing and evaluating vast amounts of data. One way to manage high volumes of test data is to choose lab operations software that offers data integrity and traceability functionality. Software applications can also offer data analysis tools alongside workflow management functions that streamline your test lab for optimal efficiency.

Complex System Testing

Test scenarios for EV batteries and battery management systems include the following:

Functional, aging, environment, and performance tests.
Standard and standards-compliant tests (ISO, DIN, EN, SAE).
Resistance (internal), charge, energy, capacity, efficiency, cyclic, calendrical durability, temperature behavior, and mechanical resistance.
Durability, range, and efficiency analysis.
Electrochemical impedance measurement and cyclic voltammetry.

Example Test - DC Internal Resistance Measurement (DCIR)

DCIR measures the DC resistance characteristic of a battery cell. We will touch on DCIR as it is an important measurement in the automotive industry due to the high peak currents seen by EV batteries. Engineers must understand how the battery pack will respond to these high peak currents, so knowing the DC resistance is critical.

Figure 2. Expected voltage and current waveforms from DCIR measurement using +100 Ampere Charge Pulse. (Image: Keysight)

To measure resistance, you apply a change in current and measure the voltage response. In this case, because it’s DCIR, we’re making a true DC resistance measurement. As shown in Figures 2 and 3, a step change is used and DCIR is calculated as DCIR = (V_beforestep – V_afterstep) / (I_beforestep – I_afterstep).

Figure 3. Expected voltage and current waveforms from DCIR measurement using -100 Ampere Discharge Pulse. (Image: Keysight)

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. 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. In fact, you may want to measure the DCIR in both directions and compare or average the results. See Figure 4.

Figure 4. Expected voltage and current waveforms from DCIR measurement using +/-100 Ampere Charge, then Discharge Pulse. (Image: Keysight)

As to the size of the current step, it is normally large because the cell’s low resistance will need a large current step to create a measurable response in voltage. Requests for current steps can be as high as 20 °C. For a 50-Ah cell, that’s 1000 A, so DCIR equipment can be large and expensive. 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). In either case, changing the cell’s state of charge (SoC) isn’t desirable, so the current is usually applied as a short pulse.

Now, how wide should the pulse be if we’re applying one to the cell? 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 returns to its “before step” state (typically a resting state, as mentioned above).

Digging into DCIR

To answer the question of pulse width, let’s look at the meaning of DCIR. DCIR measures the ohmic series DC output resistance of the cell. The cell’s ohmic resistance comes from the current collectors, the active materials of the electrodes, the ionic conductivity of the electrolyte, and other connections.

For DCIR, we only care about the non-time varying DC ohmic resistances. The voltage change due to these ohmic resistances will appear instantaneously upon application of the current pulse. Thus, to measure DC ohmic resistances, you must immediately measure the voltage response upon application of the current pulse. This means the pulse length does not matter, and the pulse need not be any longer than the measurement time of the cell’s voltage response. In fact, you want that pulse to be as short as possible to avoid self-heating and any unnecessary change in SoC caused by the charging or discharging of the cell during the pulse.

Engineers and scientists often request DCIR pulses that are 1, 10, or 30 seconds wide and measure the cell’s voltage response V_afterstep at the end of these pulses. This is not a DCIR measurement but instead a DC pulse measurement.

If measured at the end of the pulse, V_afterstep will certainly include the effects of the DC ohmic resistance. However, V_afterstep will include some AC electrochemical effects and, most significantly, will include a change in voltage due to charging or discharging the cell during the pulse. As the pulse length becomes longer and the pulse amplitude gets larger (remember, this test could be run at 20 °C), this charging or discharging effect on OCV can be quite large compared to the minimal voltage change caused by a 20 °C current flowing through a few milliohms of true cell ohmic resistance.

Test Setup

Measuring DCIR using the test setup in Figure 5 requires two instrumentation characteristics:

The device applying the current pulse needs a rise time of a few milliseconds or faster. If the edge is slow, the time it takes to make the transition from I_beforestep to I_afterstep will allow for non-DC, fast electrochemical effects to occur, such that the measurement of the voltage response will include both DC ohmic and some AC electrochemical voltage components..
The voltage response V_afterstep must be measured fast and immediately after the applied current step is completed. If the measurement is slow or delayed, the V_afterstep will include non-DC, fast electrochemical effects. Taken to the extreme, if the V_afterstep is measured too slowly after the transition, the DCIR measurement becomes a DC pulse measurement.

Conclusion

Investing in EV battery testing is not merely a technical necessity but a strategic imperative for the future of transportation. The integration of advanced testing methodologies is crucial for enhancing EV batteries’ safety, efficiency, and longevity, thereby supporting the EV market’s rapid growth.

This article was written by Bob Zollo, Solution Architect, Battery Testing for Energy and Automotive Solutions and Brian Whitaker, Product Marketing Manager, both at Keysight Technologies (Santa Rosa, CA). For more information, visit here .