Battery over discharge – how influence on voltage, internal resistance and capacity

Experiments were carried out with NMC battery. Eleven NMC batteries were over discharged to different DODs (106%, 107%, 108%, 110%, 115%, 118%, 120%, 125%, 130%, 135%, and 140%). These over-discharged batteries were then left for 72 hours. The voltage distribution of these batteries over the first 12 hours is shown in the figure below. The following conclusions can be drawn:

● Batteries with different degrees of battery over discharge can recover to a certain extent after rest. When the DOD is greater than 110%, it is difficult for the voltage to recover above 2v.

● The greater the degree of battery over discharge, the smaller the degree of voltage recovery, which means that the greater the degree of battery over discharge, the greater the internal damage and the more serious the ISC.

Analyze the voltage change of batteries with different DOD, after battery over discharging, re-charging, and then standing still.

● For batteries with DOD less than 110%, the voltage and SOC changes little during the static stage, indicating that the degree of internal short circuit is small or does not exist.

● For batteries with a DOD greater than 115%, the voltage and SOC drop significantly during the rest phase, indicating the presence of an internal short circuit.

Effect of battery over discharge on battery capacity

Analyze the capacity loss of batteries with different DODs after battery over discharge.

The capacities of over-discharged batteries with different DODs were tested respectively. The test results were compared with the initial capacity of each battery before battery over discharge. Due to the small difference in the initial capacity of each battery, and an overdischarged battery with a high DOD may not be fully charged. The differences in discharge capacity and coulombic efficiency between batteries with different DODs and corresponding new batteries were compared. The following conclusions can be drawn:

The differences in discharge capacity and coulombic efficiency increase with increasing DOD, especially when DOD is greater than 110%. This also shows that the internal short-circuit current of the battery increases with the increase of DOD, especially the battery with higher DOD.

Since batteries with a DOD greater than 120% cannot be fully recharged, the capacity of these batteries cannot be determined. Therefore, the narrow range of DOD from 118% to 120% is defined as the critical point of the battery over discharge characteristic.

Analyze the battery with different DOD, after battery over discharging, re-charging, and then the capacity loss during the standing period. The following conclusions can be drawn:

● Batteries with a DOD of less than 110% had very little capacity loss, and there was little difference between losses.

● The power loss of a battery with 118% DOD varies greatly with time, which is similar to the electrical characteristics of a linear resistor.

Effect of battery over discharge on battery internal resistance

Analyze the battery with different DOD, after battery over discharge, re-full charge, and then the change of internal short-circuit current during the rest period. The following conclusions can be drawn:

● The internal short-circuit current of the over-discharged battery decreases rapidly at the beginning of the rest period. After a period of time, the internal short-circuit current tends to be stable. This phenomenon indicates that the internal short circuit caused by battery over discharge was partially repaired at the beginning of the resting period.

● The internal short-circuit current of all batteries is very small.

Analyze the internal short-circuit resistance of batteries with different DODs, after battery over discharging, re-charging, and then standing still.

The average internal short-circuit current (IISC) is used to describe the magnitude of the internal short-circuit current. The internal short-circuit current tends to be stable after standing for 20 hours. Therefore, the average value of the internal short-circuit current after standing for 20 hours is obtained to obtain IISC, and the IISC value of the unit whose DOD is less than 120% (before the critical point) is obtained. Both IISC and its rate of change increase with the increase of overdischarge, especially near the critical point.

According to the voltage and internal short-circuit current curve before the critical point of the static stage, the value of the internal short-circuit resistance can be obtained by dividing the voltage by the internal short-circuit current. After standing for 20 hours, average the average internal short-circuit resistance (RISC) of each over-discharged battery.

Batteries with a DOD greater than 120% cannot be fully charged at 1/3C current; therefore, RISC is estimated by dividing the regulated voltage by the charging current, since the charging current sufficiently passes through the RISC. According to RISC characteristics, the battery over discharge process can be divided into 3 stages:

● In stage I (100% < DOD < 118%), the RISC decreased sharply, indicating that the degree of internal short circuit induced by battery over discharge increased rapidly with the increase of DOD.

● In stage II (118% < DOD < 120%), the degree of internal short circuit increases the most, and the degree of internal short circuit is severe.

● In stage Ⅲ (120% < DOD < 40%), the degree of internal short circuit increases slowly. However, RISC has become very small, indicating that the internal short circuit has become very serious.

The changes of the battery after a long time of standing still after battery over discharge

Analyze the capacity and energy loss of the discharged battery after standing for a long time (100 d).

The over-discharged battery is left to stand for 100 days. These batteries are then tested for capacity and their energy calculated. Comparing the obtained results with the capacity and energy before standing for a long time, the following conclusions can be drawn:

The capacity and energy of the over-discharged battery are greatly reduced during the long-term rest, indicating that the capacity of the over-charged battery is greatly reduced during the long-term rest.

Cells with a DOD greater than 120% that could not be fully charged previously could be fully charged after a long rest. The significant increase in coulombic efficiency indicates self-healing of the internal short circuit during prolonged rest.

The capacity loss and internal short-circuit current changes of the battery over discharged were analyzed after being left for a long time (100 d), recharged, and then left for 72 h.

The capacity loss of different DODs batteries during standing for 72 hours is shown in the figure (a) below. It can be seen that the battery with a DOD of 125% exhibits an obvious linear resistance characteristic (power loss varies linearly with time). The capacity loss of a battery with a DOD of 125% is 1.5 Ah after 20 cycles (about 200 h), that is, the capacity loss rate is about 7.5 mAh/h.

However, during the 72-hour rest period, the power loss reached 2.7 Ah (about 37.5 mAh/h). This indicates that the internal short-circuit current is the main side reaction of the overdischarged battery.

Therefore, the change in internal short-circuit current during the remaining process can be approximated by dividing the capacity loss by the time used, as shown in the figure (b) below. It can be seen that the internal short-circuit current of the battery with a DOD of 125% is basically stable during the static process, while the internal short-circuit current of the batteries with a DOD of 110%, 115%, 118% and 120% gradually decreases during the static process. This result reflects the self-healing properties of battery over discharged.

Estimate IISC values for cells with a DOD of 110%, 115%, 118%, 120%, and 125%, respectively. The results showed that the IISC value of the cells after standing still was smaller than that of the cells before a long rest, indicating that the self-repair phenomenon occurred during the standing process.

Conclusion

Through the above experiments, the following conclusions can be drawn:

Battery over discharge can cause internal short circuit of NMC battery. The possibility of internal short circuit increases linearly with the increase of overdischarge degree. In addition, with the increase of DOD, the short-circuit current increases, the average short-circuit resistance decreases, and the rate of internal short-circuit increases.

For batteries with a DOD greater than 120%, the recovery of the open circuit voltage during the standing process does not exceed 2V, and an irreversible internal short circuit occurs. The electrical properties of this battery resembled a linear resistance, and self-healing could not be observed after a short period of rest (72 h).

For batteries with a DOD of less than 120%, the open circuit voltage could recover to above 2 V during the resting process, and a certain degree of self-repair occurred after a short resting period (72 h).

After a long period of standing (100 d), batteries with a DOD of less than 125% experienced internal short-circuit self-healing. However, these batteries with a DOD exceeding 130% could not be charged normally even after standing for a long time, which indicated that an irreversible internal short circuit had occurred. For batteries with DOD greater than 120%, the capacity drop is very obvious.