Importance & Challenges

During the usage of batteries in all applications users often encounter different problems. These problems can have performance, mechanical, or electrical nature. All these problems in most cases lead to module replacement, what requires not just replacing the module but also to ensure that the replaced module has as close as possible grade to other modules in the system.

Grading of batteries starts from the cell level, since cells produced even in the same batch can have differences in performance Figure 1(b) and expand further to module and pack level leading to performance imbalances even on new batteries as shown on Figure 1.

a. SOC imbalance on new batteries

b. DCIR imbalance on new batteries

Figure 1 Imbalances at the beginning of life

In real time operations these imbalances can get even higher due to many things that cause it, including but not limited to temperature variations and charging behavior.

By grouping cells/modules with similar capacity, internal resistance, and SOC battery packs perform more uniformly and age more slowly. Before deciding on which module shall replace the faulty one it is necessary to perform a grading assessment. Grading typically involves checking several key parameters such as:

• Charge/Discharge Capacity – It represents amount of Ah or Wh battery can deliver under certain C-rate when called upon. To test it service engineering teams need instruments capable of controlling charging/discharging with constant current, power and voltage as well as to cover several module voltage and power ranges.
• DC Internal Resistance – It represents the state of health for high-power applications. Tests are usually conducted according to IEC 61960-2330 standard, where the battery is discharged with 0.2C for 10 seconds with recording of V1 and I1, and then discharged with 1C with recording of V2 and I2. Tests can be conducted according to ISO 12405 standard or by using customized test methods.
• Open Circuit Voltage (OCV) – The open-circuit voltage (OCV) curve is the voltage of a battery as a function of the state of charge when no external current is flowing and all chemical reactions inside the battery are relaxed. Each battery chemistry and cell type have an individual OCV curve based on its inner state, which is why the OCV curve can be compared to a fingerprint.
• State of Charge (SOC) – represents available energy in the battery compared to nominal energy, it is a highly important parameter in energy storage systems (ESS) but also in other applications due to its usage in energy management algorithms. All types of battery ESS use LFP cell technology which is characterized by a flat OCV cure in a SOC range from 35% up to 95% as shown in Figure 2

Figure 2 LFP battery OCV curve

This makes relying on OCV as a proxy for SOC remarkably unreliable. Therefore, to balance SOC levels with the modules it is recommended to charge/discharge a module from 0% to 100% SOC to determine the amount of available Ah in it and then discharge a certain percentage of it to ensure proper balance within the system.

How to use DV Power devices to assess a battery grade and calibrate SOC?

To mitigate the grading concerns and ensure balance within the battery system after replacing the module usage of the instruments with charge/discharge capability is essential. DV Power Battery Load Unit – BLU Series as well as Battery Charge Units BAC Series provides charging and discharging functionalities on different voltage ranges and for different power requirements.

In addition to charging/discharging capabilities all instruments within the portfolio enable users to define customized profiles for DCIR measurements as well as integration with Battery Voltage Supervisors BVS series for high accuracy voltage measurement on several modules simultaneously.

If you’re a company that provides battery aftermarket or recycling services, contact us for a consultation. We can help you choose the right battery discharger and charger to meet your needs and ensure compliance with regulations.

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December 11, 2024