DC Resistance

General Description

Internal resistance or DC resistance (DCR) is a powerful battery KPI, and refers to the opposition to the flow of electrical current within the battery as it charges or discharges. It indicates how much voltage needs to be applied to maintain the same level of current.

DCR leads to energy loss in the form of heat, which reduces the battery's overall efficiency and speeds up battery degradation. After State of Health (SoH), it is the second most important battery degradation KPI used to identify battery aging. Factors such as temperature, age, and state of charge (SoC) can affect a battery's internal resistance, but the overall battery design and materials used also play a role.

For large-format cells typically used in stationary storage applications, the DCR often slightly decreases at the beginning of life, then increases as the battery ages. When plotted over time, DCR shows a hockey stick growth.

The TWAICE DCR calculation

How does TWAICE calculate DCR?

TWAICE DCR is the time-dependent resistance of the battery and varies based on SoC, temperature and length of the pulse (spike in current). It is calculated by dividing voltage change by current change when a current spike event begins. The time constant is 2 seconds or the minimum raw data sampling rate for current and voltage signal.

The timeseries is filtered to take current pulses above a certain size, within a temperature and SoC range. Therefore, the impact of temperature and SoC on the evolution of the DCR is filtered and the DCR can be compared across time and components. As a standard, the TWAICE DCR is given for a middle SoC and temperature range, but a multitude of DCR is possible to be utilized and shared with the user as the TWAICE platform tracks every single DCR estimate with its respective meta information, like SoC, temperature, current change etc.

As for any other estimated signal, sensor limitations can introduce noise into the signal. To counteract these effects, a rolling average or rolling median method is used to reduce the noise.

The advantages of the TWAICE DCR calculation

A single DCR value would not help to identify BESS issues or underperformance. This is one of the advantages of TWAICE's DCR - it can be analyzed on multiple levels, including inverter and string, therefore provides a variety of options for statistical comparison and helps customers to identify anomalies and trends. The requirements for calculating DCR are voltage, current and temperature measurements for a specific level. For example, to calculate DCR on the string level, measurements for voltage, current and temperature for the string must be provided.

The TWAICE approach means that DCR can be estimated without having to induce current spikes and interrupting operations. During normal operation, the DCR updates regularly and is therefore a fast indicator of degradation and internal issues.

Impact of operation on outcome

The TWAICE DCR is dependent on dynamic operation patterns which span across the SoC range within a more or less controlled temperature window. If the operation is using only very shallow cycling with low power requirements, updates of the DCR values will take more time. On the other hand, the DCR values update more quickly with highly dynamic operations such as frequency containment.

It is important to be aware of the different ways a DCR can be defined. Temperature, SoC and current or power pulse duration impact the calculated DCR value. Hence, to be able to compare one DCR with another, it is important to clarify at which temperature and initial SoC it was measured and how long the pulse was applied.

What you can do with the TWAICE DCR

Identify faulty and defect components

The TWAICE DCR can be used to identify components which experience quality issues due to internal manufacturing problems or overall lower quality compared to other components of the same type. In particular, the DCR is a powerful KPI that can be used to detect safety-relevant trends early on. 

Detect underperforming components

As mentioned, the TWAICE DCR can be determined on various system hierarchies. If the signals are available, it can be also used to determine cell-individual DCR values. A higher DCR compared to other components on the same hierarchy is usually a sign for weak or inefficient components. These components can lead to interruptions in planned power patterns and hence limit achievable revenues.

Spot connection problems

The TWAICE DCR can also be used to spot connection problems, such as cable connections of modules and other components. This is especially useful for inspections during the commissioning phase or during maintenance check ups. At these moments, the on-site teams are supported by additional analytics insights and can correct malfunctioning cables or connectors before they could lead to inefficiencies, overheating or more severe safety incidents during operations.

Deep dive into temperature anomalies for root cause analysis

The TWAICE DCR can be used to evaluate temperature patterns arising in your operation. Root causes can be identified and issues separated into HVAC problems, connection problems or degradation problems.

Reliable power during operation

High resistance can cause issues during operation if a power requirement, e.g. during frequency containment service cannot be met due to violations of voltage, current or temperature demand. Constant monitoring of the DCR helps to mitigate this risk.

Quality control

Each battery cell will have a different DCR, mainly because it is very much influenced by the materials used, the manufacturing process and the assembly into a module and string. With this, the TWAICE DCR enables the user to identify the homogeneity of the components which are installed within the BESS. Some systems show a variation of more than 30% difference in TWAICE DCR which is driven by the quality differences of battery cells. Hence, the TWAICE DCR provides a deep insight into the quality control of the batteries used.

Impact of data on KPI

The TWAICE DCR benefits of a dynamic operation across the entire SoC range and a high time and data resolution. Current and voltage jumps with different time duration and current or power dimensions provide a rich data set to derive the displayed TWAICE DCR. This way, the impact of different operation patterns and temperature on the calculated TWAICE DCR can be minimized.

If the time and data resolution are only following the minimum data requirements, the operation pattern and temperature can impact the calculated TWAICE DCR more significantly. This means, operational changes or bigger temperature variations can impact the displayed TWAICE DCR.

Conclusion

DCR is one of the most powerful battery KPIs. As well as being an important indicator of battery aging, it helps to identify safety risks early on and to detect battery issues or underperforming components. With the TWAICE DCR, users gain vital DCR insights without having to interrupt BESS operations.