Overdischarge refers to excessive discharge.When the battery is discharged, the stored electric energy is gradually released and the voltage drops slowly.When the voltage drops to a specified value, the discharge shall be stopped and the battery shall be recharged to restore the energy storage state of the battery.If the discharge continues below the specified value, it is over discharge. Over discharge may cause damage to the active substance of the electrode, loss of reactivity, and shorten the life of the battery.
Chinese name
Over discharge
Foreign name
overdischarge
Alias
Over discharge
Object
Battery
Consequences
Damage or even failure of electrode active material
Considering the single battery in the group, there must be a relative over discharge.In the later stage of discharge, the voltage is close to the horsetail curve, and the single capacity of the group is normally distributed. The voltage distribution is very complex, and the voltage drop of the single unit with the smallest capacity is the earliest and fastest. If the voltage drop of other batteries is not obvious at this time, the voltage drop of the single unit with small capacity is covered up and has been over discharged.
Observe the over discharge of the single body and enter the horsetail curve. If the current continues to be high, the voltage rapidly decreases and "reverse polarity" occurs soon. At this time, the battery is charged in the reverse direction, or called passive discharge, and the active substance structure is destroyed. Another side reaction occurs quickly. In a short time, the active material of the battery is nearly lost, equivalent to a passive resistance, and the voltage is negative,The value is equal to the voltage drop generated by the reverse charging current on the equivalent resistance. After the discharge is stopped, the electromotive force of the original battery disappears and the voltage cannot be recovered. Therefore, one reverse charging is enough to make the battery scrapped.
Therefore, during the battery discharge process, the battery management system must be used to detect the voltage of each battery in the battery pack in real time, output the battery voltage information to the vehicle controller, slow down the vehicle when the single battery voltage is close to the alarm voltage, and stop the vehicle when the single battery voltage drops to the cut-off voltage, so as to prevent the battery from over discharging.
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If the battery is over charged and over discharged without protection, the positive and negative poles of the lithium battery will be permanently damaged.From the molecular level, it can be intuitively understood that excessive discharge will lead to the excessive release of lithium ions from the negative electrode carbon and collapse of its lamellar structure. Overcharging will force too many lithium ions into the negative electrode carbon structure, making some lithium ions can no longer be released. Excessive discharge will cause copper plating on the negative electrode plate to the positive electrode, destroying the microstructure of the positive electrode.As shown in the figure "Overdischarge cathode damage of lithium battery", it is a photo of the cathode damaged by overdischarge. From the photo, it can be seen that dark red copper is filled between black active substances.
Overdischarge cathode of lithium battery is damaged
Generally, the 2V voltage is controlled as the lower limit of discharge, and the pulse discharge is limited to 5C current within 30s.3C current is used for constant current discharge, and it is cut off at 2V.
The discharge operation is limited to - 20~+60 ℃, and the charging current is limited to 0~45 ℃.
Lithium batteries are better thanLead acid battery"Delicate", reasonable working conditions are strict, which is why lithium batteries are usually equipped with a supporting battery management system（BMS）Reason for.[1]
And overcharge reaction
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Take nickel battery as an example.In case of over discharge, the NiOOH in the positive active material has been consumed (the positive limiting capacity is adopted in the design of the battery), at this time, water molecules on the positive electrode will be reduced to hydrogen and OH-Ions.On the negative pole, due toHydrogen storage alloyThe catalytic action of OH-Ions react with hydrogen to form water.
During overcharging, oxygen will precipitate on the positive electrode, and then diffuse to the negative electrode for depolarization reaction to generate OH-Ions.In the process of battery overcharge and overdischarge, the reaction on the positive and negative electrodes can be expressed by the following formula:
Positive pole: oxygen 4OH released from overcharge-—→HtwoO+Otwo+4e
Hydrogen 2H evolved from over dischargetwoO+2e —→Htwo+2OH-
Negative pole: oxygen consumption 2H during overchargingtwoO+Otwo+4e —→4OH-
Hydrogen consumption by over discharge Htwo+2OH-—→2HtwoO+2e
It can be seen that the hydrogen storage alloy not only plays the role of hydrogen storage, but also acts as a catalyst. When the battery is overcharged and over discharged, it can eliminate the O generated by the positive electrodetwoAnd HtwoSo that the battery can withstand overcharge and overdischarge.However, with the charge discharge cycle going on, the catalytic ability of hydrogen storage alloy gradually degrades, and the internal pressure of the battery will rise, eventually leading to the battery leakage failure.If the hydrogen storage alloy used is of poor quality, the result will be the same.[2]
General protection principle
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According to the analysis of the discharge characteristics of the battery, when the discharge reaches the voltage equivalent to the terminal discharge, the battery has been discharged.According to this principle, the voltage measurement and voltage comparison circuit is set in the controller. By monitoring the terminal discharge voltage value, it can determine whether the battery should end discharging.For open type fixed lead-acid battery, the final discharge voltage (terminal discharge point voltage) under standard state (25 ℃, 0.1C discharge rate) is about 1.75~1.8V.aboutValve regulated sealed lead-acid batteryThe final discharge voltage under standard state (25 ℃, 0.1C discharge rate) is about 1.78~1.82V.The terminal discharge voltage set by the comparator in the controller is called "threshold voltage" or "voltage threshold".
Impact on battery
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Influence of over discharge on battery performance
When the battery is discharged to a voltage lower than the specified termination voltage, it is called overdischarge.It should be pointed out that the terminal voltage value specified in the battery standard is the voltage value reached when the battery is discharged continuously;However, the actual use process is mostly intermittent discharge, so even if the discharge reaches the specified termination voltage, there will often be over discharge.
In addition, when the battery is discharged to the termination voltage, the battery voltage will automatically rise when it is left standing for a few minutes to half an hour.This misleads users to believe that the battery can continue to discharge, resulting in over discharge of the battery.
In areas with frequent power outages (even three days in a week), outdoor batteries can not be charged in time even when they are discharged to 0V, resulting in severe sulfation of batteries, which is extremely detrimental to battery life.[3]
Influence of over discharge on battery life
Periodic treatment charging and discharging of the battery can improve the battery capacity compared with the previous one, but continuous excessive deep discharge can not further activate the active substances not participating in the reaction, and on the contrary, it will cause positive grid corrosion and some α - PbOtwoTo β - PbOtwoAs a result, the cycle life of the battery will inevitably be shortened.It can be seen from the data in Table 1-1 that the discharge capacity of the battery has decreased from time to time after reaching the highest value in the second cycle.The deeper the discharge depth is, the faster the battery capacity decreases, the more obvious the side effects caused by over discharge and periodic treatment, and the shorter the cycle life of the battery.
It is also measured that the cycle life of 6DZM10 battery discharged to 10.5V is 396 times, while the cycle life of 6DZM10 battery discharged to 0V is only 260 times.[3]
Resilience
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The battery after multiple over discharge tests in the first cycle shall be fully charged again, and then the 5A discharge and over discharge in the second cycle shall be carried out. See Table 1-1 for the change of the first discharge capacity in each cycle.As can be seen,Battery packThe first discharge capacity of the second cycle is higher than that of the first cycle, which is due to the reactivation of the part of the active substance that was not completely formed. It is natural that the discharge capacity has been improved.[3]
Table 1-1 Change of Discharge Capacity of 6DZM10 Storage Battery
