Battery discharge plateau and battery quality knowledge popularization



1. What is a battery discharge plateau

The battery discharge plateau is specifically for rechargeable batteries and indicates the time for stable discharge of rechargeable batteries. The relevant regulations for nickel metal hydride battery discharge plateau and lifepo4 battery discharge plateau are different.


First of all, let's talk about the battery discharge plateau, which refers to the voltage change state of the battery when the fully charged lithium battery is discharged. Simply put, when the batteries are continuously discharged, the voltage changes are reduced, stabilized, and reduced.

One of the stabilization phases is the discharge plateau. When the lithium battery is discharged from a full voltage of 4.2V to 3.7V, the time is very long. But once the 3.7V is over, the discharge is fast, and this phenomenon is related to the battery discharge plateau. This article collects and sorts out some information about lithium batteries to give you a summary.

2. Lithium battery discharge process

Battery constant current discharge, battery voltage to go through three processes, namely fall, stability, and then decline. Of the three processes, the stabilization period is the longest. The longer the stabilization time, the higher the battery discharge plateau.

The level of the discharge plateau is closely related to the battery manufacturing process. It is because the market positioning of each lithium battery company is different, the technical process means are different, the battery discharge plateau controlled by it is different, and the quality is very different.

In general, an 18650 battery with a full voltage of 4.2V, when discharged with 1C current discharge to 3.7V, is placed for 60 minutes. Then we say that the battery's used capacity is 2200mAh. During this time, according to the characteristics of the rechargeable batteries, for an 18650 lithium battery with a capacity of 2200mAh, it takes 1 hour to discharge 1C to 3.7V.

Lithium battery discharge process


Then the problem comes. For a better lithium battery, when testing the product, the voltage drops quickly after 3.7V, so there is very little power to put in a short time. On the contrary, when a bad battery is discharged from 4.2V to 3.7V, the voltage drops rapidly. After 3.7V, the voltage drops very slowly, and the general capacity of this battery is not good, and the general capacity is also very low.

Then the discharge plateau of a good lithium battery is 3.7V. In general, under constant voltage conditions, the charging voltage is 4.2V, and charging is stopped when the current is less than 0.01C. Then leave for 10 minutes. At any discharge current rate, when discharged to 3.7V, the length of time experienced by battery discharge is an important indicator to measure battery quality.

3. Nimh battery discharge process

The battery discharge plateau of nickel metal hydride rechargeable batteries usually refers to the voltage range in which the working voltage of the battery is relatively stable when the battery is discharged under a certain discharge system. Its value is related to the discharge current, and the larger the current, the lower its value.

The discharge plateau of lithium-ion batteries is generally charged at a constant voltage to a voltage of 4.2V and the current is less than 0.01C, stop charging, then sit for 10 minutes, and discharge to 3.6V under any discharge current. It is an important standard for measuring battery quality.

For nickel metal hydride batteries, the discharge plateau of nickel metal hydride batteries usually refers to the fact that after the batteries are fully charged, the voltage change of the battery is not the same slope reduction. Usually, when the discharge is started, the voltage drops relatively quickly. After a while, as the discharge progresses, the battery voltage hardly changes.

Nimh battery discharge process


In other words, the change is small, and this segment accounts for a large part of the overall discharge time, which is the discharge voltage stage. When the power is almost discharged, the battery voltage decreases faster until the power off voltage of the discharge is completed.

The battery discharge plateau is for rechargeable batteries and is to characterize the time for the smooth discharge of rechargeable batteries. The regulations for nickel metal hydride battery discharge plateau and lithium battery discharge plateau are different, so what does the working voltage mean?

4. What does operating voltage mean

Also known as terminal voltage, it usually refers to the potential difference between the cathode and anode of the battery under operating efficiency. That is, there is an outdated current in the circuit.

Under the effective discharge efficiency of the batteries, when the current flows through the inside of the battery, there is no need to overcome the resistance caused by the internal resistance of the batteries. Therefore, the operating voltage is often less than that of an open-circuit battery, and the opposite is true when charging. The discharge working voltage of lithium batteries is about 3.6V.

5. The relationship between battery capacity and battery discharge plateau

It can better understand the rechargeable battery capacity and the truth of the battery discharge plateau, and it can also be said to measure the working time of the battery's high power. The same two battery capacities are the same, and it is assumed that they will be put from 4.2V to 3.7V at the same time after a full charge, but one time is long and the other time is short. It is that the battery discharge plateau is high for a long time, and the working time of high voltage is long.

For example, both batteries are used on the mobile phone, and the standby time is the same. But two mobile phones call together, the battery notification time will be long when the battery discharge plateau is high and the battery notification time will be long, and the battery talk time will be short when the battery discharge plateau is high and short.

The relationship between battery capacity and battery discharge plateau


Another meaning of the high battery discharge plateau is also meaningful for understanding the monitoring of lithium battery power management. For example, at present, there are generally two methods for monitoring the power of rechargeable batteries. Method for measuring battery voltage. When the charger detects the battery voltage while charging, it is considered full when the voltage reaches the specified voltage value.

For example, when the lithium battery voltage is charged to 4.2V, it is considered full. The accuracy of the voltmeter to detect the voltage should reach an accuracy of plus or minus 1%. It will be damaged due to overcharging of the lithium battery. If you want to monitor the charging and discharging of the battery, calculate the load resistance according to the capacity and discharge it with the standard discharge current.

At this time, the voltage is then detected, as long as it can reach or approach the nominal discharge time. This method of measuring battery voltage has many disadvantages. For example, the top 10 lithium battery companies in the world produce different batteries and the relationship between open circuit voltage and capacity is different. The advantage is that the design cost is relatively low.

Another common way to monitor residual capacity is to estimate battery life by knowing the remaining battery capacity in places where more accurate battery capacity is required, and to estimate residual battery capacity by measuring the net charge flowing in/out of the batteries.

Integrating the total current in/out of the cell, that is, finding the area under the curve in the figure, the resulting net charge number is the remaining capacity. The battery capacity calculation method is currently considered to be a more accurate method of calculating battery power. Of course, the design cost is also relatively high.

6. Specific analysis of battery discharge plateau

The flat plateau in the charge-discharge curve is the characteristics of the cathode and anode materials themselves reflected in the voltage-capacity curve, representing most of the batteries can complete the charging and discharging process at a certain voltage. When the batteries go through a long cycle, the internal resistance of the battery increases.

Battery charge curve


During the charging process, each unit of current provided externally, in a unit of time, needs to do more work to achieve the migration of Li ions. Because part of the work will be used to overcome internal resistance, it is reflected in the rise of the charging plateau.

Conversely, during the discharge process, a unit of current provided inside the lithium-ion battery, in a unit time, the work done needs to be divided into parts to overcome internal resistance. External work will be reduced, which is reflected in the decline in the discharge plateau.

As the number of cycles increases, the charge-discharge curve shows the following trend: In the charging curve, the flat part has risen overall, but the increase is not large.

The inflection point of rapid voltage change is significantly advanced, because with the increase of the number of cycles, the content of active Li decreases, which is reflected in the decrease of capacity. If in the state after 1000 cycles, the batteries can be supplemented by active Li, its inflection point can be moved back again.

Battery discharge curve


And the capacity is increased. In the discharge curve, the flat part is reduced overall, and again, the reduction is not large. The inflection point where the voltage changes rapidly is significantly advanced. This is also because as the number of cycles increases, the active Li content decreases, which is reflected in the decrease in capacity.

7. Conclusion

However, do not blindly pursue a high battery discharge plateau, sometimes the platform voltage is high, but the capacity is reduced. Because, under different rate conditions, the battery discharge plateau is different. Therefore, the issue of the platform should be considered from many sides. It must have a high capacity and a long duration of the specified voltage to be good batteries.


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