1) Operating voltage and open circuit voltage

We call electrical voltage, the electrical charge that circulates inside an electrical circuit. It is measured between two electrodes using a voltmeter and is expressed in volts. The electrical voltage of a cell differs according to its chemistry and the ranges of voltages of use are included between :

• LFP: 2.5V and 3.65V (3.2V nominal)
• NMC: 2.5V and 4.2V (3.6V to 3.7V nominal)

The operating voltage measured in a closed electrical circuit differs from the open-circuit voltage (OCV), which is obtained when no current flows between the circuit terminals. In the same initial state (SoC or SoH), the voltage is different if the circuit is closed (battery in operation, charge or discharge), or if it is open (battery at rest).

2) Calculation of the state of charge by the OCV method

The open circuit voltage or "OCV", evolves according to the state of charge of the battery called "SoC": the higher the charge, the higher the voltage and the lower the charge the lower the voltage. The application of this ratio and the knowledge of the chemistry of the battery allow to evaluate the state of charge of the battery. This is known as the OCV method: it involves measuring the open circuit voltage at a time T, then using the OCV/SoC relationship to determine the battery's state of charge.

If the minimum and maximum thresholds are provided by the manufacturers, the OCV/SoC relation specific to each cell is obtained by tests carried out beforehand by our engineers. This method takes into account that the internal resistance influences the voltage in closed circuit and imposes a sufficiently long rest time of the cell in open circuit before being able to measure the OCV. The results are then programmed into the BMS to allow it to instantly inform the user of the battery's state of charge.

The discharge curve of LFP cells is relatively flat, especially between 20% and 80% state of charge. The OCV method is very reliable when an OCV/SoC relationship is observed on a graphical curve with many representative points. The more points the BMS has to interpret on the curve, the more accurate it can provide a SoC.

3) The coulombmeter for LFP cells

This process, complementary to the OCV method, consists in a parameterization of the BMS from the capacity and the state of charge of the cell. The coulombmeter measures the energy entering or leaving the cell, this result allows it to define the SoC with precision. The coulomb counter method is all the more efficient as it can be used in closed circuit, i.e. when the cell is in operation, therefore it requires less time than the OCV method.

4) Tests by our Laboratory

The BMS plays a decisive role in the synergistic interaction of the battery cells. Some BMS on the market coordinate both methods for even better results. To support you in this integration project, our Laboratory offers cell characterization tests that will provide you with the essential data for the best setting of your BMS. These results combined with those of the OCV/SoC tests, guarantee the efficiency and the accuracy of the information relayed by the BMS.

Our team is at your disposal to advise you on the appropriate tests and optimize your programming.