Differential voltage in power battery system – the causes and improvements

Battery cell production process

From the 446 voltage difference warning samples, 3 samples were selected whose self-discharge amount was higher than the battery cell specification requirements and the charge end voltage difference still increased by ≥20 mV/month after manual equalization.

The reason for the high voltage is that there are particles in the core that break through the separator. The particles are composed of iron, chromium, and stainless steel. Therefore, the battery cell production process is one of the reasons for the voltage difference problem in the battery system. Statistical analysis was conducted on the production batches of battery cells in the abnormal samples, and it was found that the battery cells produced in the 13th and 24th weeks had the highest frequency of warnings.

While comparing with the factory voltage difference of the normal batch of battery cells, no abnormality was found, as shown in the figure below, the overlap between the two is relatively high; comparing with the self-discharge test results of the normal batch of battery cells, no abnormality was found. The excellence rate values of each batch are not much different. Therefore, the production batch is not the cause of the cell voltage difference problem.
Comparison of cell voltage differences between different batches:

BMS balancing strategy

Differences in the self-discharge rate (self-discharge capacity/rated capacity) of different cells in each sample are unavoidable. The balancing function must be used to make the cell voltages consistent and make up for the self-discharge rate difference between cells.

The estimated equation for the required equilibrium time is:

The number of balancing hours required per day = cell capacity × self-discharge rate difference of monthly cells / (average balancing current × proportion of balancing on time × 30)

In the formula:

• The cell capacity is the rated capacity of the cell;
• The monthly cell self-discharge rate difference (the difference in cell self-discharge rate between the highest voltage cell and the lowest voltage cell in the sample) is obtained by measurement;
• The average balance current is the rated voltage/balanced resistance of the battery cell;
• The proportion of balanced on time depends on the hardware capability, and the balanced on time of different hardware is different;
• A month is calculated as 30 days.

Based on a monthly cell self-discharge rate difference of 2.5%, this model of battery system requires 4.5 hours of balancing hours per day. According to big data statistics, the daily use time of private electric vehicles is 2 hours, which is less than 4.5 hours. Therefore, the voltage difference of the battery system will gradually increase during use.

Hardware failure

Some vehicles have the problem that the cell voltage in concentrated areas is lower than other string numbers, as shown in figure below. Voltage differences caused by battery cell problems are often discretely distributed. The cell sampling controller is powered by the module. The concentrated and low voltage is mostly caused by BMS hardware abnormalities.

From the picture below we can see that one group of cells has a low voltage:

As shown in the figure below, the PIN 17 is directly connected to the optocoupler PIN 4, which is the way in which ESD (Electro-Static Discharge) is introduced. ESD will occur if electrostatic protection is improper during the assembly of the sampling controller or the battery system. When the optocoupler is normal and the low voltage is not powered on, the optocoupler will not turn on and will not power subsequent chips.

When the optocoupler is abnormal, the photocoupler will not turn on due to the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) damage, causing leakage of the optocoupler PIN4 to PIN3, and the subsequent chip operates due to power supply, resulting in abnormal leakage current.

When perform thermal imaging analysis on the failed optocoupler, as shown in the figure below, we can see the failure location. Therefore, the failure of the optocoupler is one of the reasons for the increase in voltage difference.

Optocoupler failure position 1:

Optocoupler failure position 2:

Usage habits

Different usage habits will affect the charge and discharge rate, and charge and discharge depth of the battery system. A single sample has various usage scenarios and has no analytical value. Comparing the data indicators of abnormal samples and control samples within one year, including charging time, charging start and end SOC (State of Charge), single lithium battery state of charge ΔSOC, as shown in the figure below, the difference between the two groups of samples under each indicator is small, indicating that the usage habits are basically the same.

Therefore, the usage habits have basically no impact on the increase in the voltage difference of the battery system.

Improvement of battery voltage difference

• Optimize the battery production process and establish a market maintenance mechanism:

At the production end, the management of the production environment should be strengthened, the dust removal process should be optimized, so that the possibility of dust mixing can be reduced.

Establish a maintenance mechanism on the market side. For battery systems with a charging end voltage difference ≥50 mV, balanced charging is required, and the balance qualification standard is a charging end voltage difference ≤30 mV. For balanced cells, when the monthly charging end voltage difference increases ≥ 20.72 mV/month, the voltage difference problem needs to be improved through manual balancing maintenance or module replacement.

• Optimize balancing strategy:

The battery cell capacity, monthly battery cell self-discharge rate difference and balancing current are all fixed values. By increasing the balancing on-time, the balancing efficiency can be improved, thereby reducing the problem of voltage difference increase. However, this will increase the BMS temperature rise and over-temperature. High temperature will cause device damage and to harm to lithium battery safety. Therefore, replacing the BMS shell with a metal shell increases heat dissipation and reduces heat accumulation.

• ESD protection and optocoupler replacement:

Check the ESD protection in the production environment and make improvements from the aspects of personnel, equipment, materials, etc.: replace the electrostatic gloves worn by personnel with surface impedance exceeding the standard, replace the electrostatic leather and seats with excessive impedance, and replace the non-anti-static materials. Defective products should be placed in boxes to improve the production environment.

Since the optocoupler cannot improve its ESD resistance in the short term, the optocoupler can only be replaced at present. The area of the replaced chip has increased to 8.18 times that of the previous one, the withstand voltage has increased by 20 V compared with before, and the distance between the MOSFET peripheral protection rings has increased. After that, the vehicle was tracked, and it was found that the voltage difference of the vehicle was within the normal range, and the voltage difference problem was greatly improved.