Why doesn't the e-bike endurance in winter?
 Feb 22, 2020|View:342

In the winter, the temperature drops, and various electric products also start to suffer from battery life problems. Even when they are fully charged, battery life is much lower than before. Consumers who do not know the truth may suspect that they have been pitted, and only bought for a few months, the battery life has shrunk so much, and many businesses have called injustice. So why is this happening? In fact, this is a common problem of all batteries.


Temperature effect

First of all, most of the electric mobility tools use a lithium battery of type 18650, where 18 indicates a diameter of 18mm, 65 indicates a length of 65mm, and 0 indicates a cylindrical battery.

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18650 lithium battery


The internal structure of a lithium battery is mainly composed of a positive electrode, a negative electrode, and an electrolyte solution between the positive and negative electrodes. The principle of battery discharge is that the negative electrode precipitates lithium ions through a chemical reaction and moves to the positive electrode through the dielectric. At this time, the positive electrode is in a lithium-rich state, and the negative electrode is in a lithium-depleted state. In the process of lithium ion activity, the generated electron movement generates a current.

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When the temperature decreases, the speed of this charging and discharging process will become slower. Therefore, the discharge current becomes smaller, and the capacity of the battery becomes smaller. Lithium batteries have a marked operating temperature of 0-40 ° C, but in fact, when the temperature is lower than 25 ° C, a certain degree of performance degradation occurs: the battery capacity decreases, the discharge rate becomes slower, and the battery output power becomes smaller.


The following figure is the temperature capacity curve of lead-acid battery and lithium battery. It can be seen that the capacity of lithium battery can reach 100% at 25 ° C, and the capacity curve shows a downward trend below 25 ° C. Since the lithium battery can reach the maximum capacity value at 25 ° C, this is also the reason why the temperature is set to 25 ° C as the experimental temperature in various mileage tests.


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Of course, the above table is only for certain types of batteries and is for reference only. Actually, the data will be different due to the differences in materials and processes of lithium batteries. However, there is a relatively simple and crude calculation method online:


With 25 ° C as the standard, for every 1 ° C drop in temperature, the capacitance decreases by about 1%. That is to say, in the summer (25 ~ 28 °), you can run a car of 25 kilometers. Under the conditions of 5 ° in winter, considering only the battery factor, you may only be able to run up to 20 kilometers.


The decrease in battery life caused by temperature is reversible. When the temperature rises, the battery life of the car will resume again. This shows that electronics, like people, don't want to move in winter.


Excessive temperature has an impact on the lithium battery itself. When the temperature is higher than 45 ° C, the chemical balance in the battery will be disrupted, harmful side reactions will occur, and the life of the lithium battery will be reduced. So in normal use environment, the biggest factor affecting the operation of lithium battery is temperature. Similarly, a mobile phone using a lithium battery will use its power faster when it is cold, which is also due to the decrease in battery capacity due to the influence of temperature.

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Impact of charge and discharge times

Of course, in addition to temperature, one of the factors affecting the capacity of a lithium battery is the number of charge and discharge cycles. According to the experimental data found on the Internet, the 18650 lithium battery completed 50 cycles of charge and discharge at 25 ° C, and the discharge capacity remained at 96.6%. Taking the 18650 model as an example, after the number of cycles reaches 400 to 500 times (one cycle is equal to the process of the battery being fully charged to fully discharged), the battery capacity will generally decrease by 10% -20%.


For example, if you have an electric vehicle using a lithium battery, the battery life can reach 25km, and the battery is fully charged and used up once a day, then the battery life is only about 20km. However, this must be calculated based on actual use. If only 50% of the electricity is used to charge each day, it is equivalent to a cycle every 2 days, and it may be that the follow-up flight will be reduced by 20% after 3 years.


The reason for the loss of lithium battery capacity is that the electrodes in the battery gradually wear out with the charging and discharging process, which is different from the temporary change in battery capacity due to temperature drop. This is an irreversible process.


In addition to the electrode loss caused by charge and discharge, the overcharge will also cause the electrode to peel off. However, most lithium batteries now come with a charge protection control, so even if the charging plug is always plugged in, the charging connection will be disconnected intelligently, so the situation of overcharging can basically be avoided.


Maintenance of electric vehicle batteries

1. When storing electronic devices using lithium batteries in winter, it is best to move them indoors to reduce natural electrical losses. Since the charging temperature of the lithium battery is slightly higher than the discharging temperature, it is best to place it in a room with a temperature of 0-25 degrees Celsius during charging, and the charging efficiency is high.


2. Do not leave the lithium battery in high temperature environment for a long time in summer, it will reduce the service life.


3. If the lithium battery is not used for a long time, it should be charged regularly. It is best to keep the lithium battery's power at about 30%.


4. Lithium battery is suitable for charging and using, and charging once ≠ one charging and discharging cycle, so it is not recommended to charge completely without power, which will cause relatively large damage to the internal electrodes of the lithium battery.


5. Use the rated voltage for charging. Although an excessively high charging voltage can speed up the charging speed, it will also damage the life of the lithium battery.


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