1. What is the self discharge rate of the battery?
Self discharge, also known as charge retention capability, refers to the ability of a battery to maintain its stored energy in an open circuit state under certain environmental conditions. Generally speaking, self discharge is mainly affected by manufacturing processes, materials, and storage conditions. Self discharge is one of the main parameters to measure battery performance. The lower the storage temperature of the battery, the lower the self-discharge rate. However, it is also important to note that too low or too high a temperature may cause damage to the battery and render it unusable. After the battery is fully charged for a period of time, a certain degree of self discharge is a normal phenomenon.
The IEC standard stipulates that NiCd and NiMH batteries are fully charged, and the discharge time at 0.2C is greater than 3 hours and 3 hours and 15 minutes respectively after 28 days of open circuit operation at a temperature of 20 degrees Celsius and a humidity of 65% Compared to other rechargeable battery systems, the self discharge rate of liquid electrolyte solar cells is significantly lower, at about 10% per month. The self-discharge rate of Hongyi's lithium iron phosphate battery pack can be as low as 3%.
2. What is the internal resistance of a battery and how to measure it?
The internal resistance of a battery is the resistance of the current flowing through the battery during operation, generally divided into AC internal resistance and DC internal resistance. Due to the small internal resistance of rechargeable batteries, when measuring the DC internal resistance, due to the polarization of the electrode capacity, there will be a polarized internal resistance, so its true value cannot be measured. The AC internal resistance can be measured to avoid the impact of polarization internal resistance and obtain a true internal resistance value. The AC internal resistance test method uses the equivalent active resistance characteristics of the battery to give the battery a constant current of 1000 HZ and 50 mA,
3. What is the internal pressure of the battery, and what is the normal internal pressure of the battery?
The internal pressure of a battery is the pressure formed by the gas generated during charging and discharging. During the use process, it is mainly affected by factors such as the manufacturing process and structure of battery materials. Generally, the internal pressure of the battery is maintained at a normal level, but in the case of overcharge or over discharge, the internal pressure of the battery may increase: If the rate of composite reaction is lower than the rate of decomposition, the generated gas will not be consumed in a timely manner, resulting in an increase in the internal pressure of the battery.
4. What is internal pressure testing?
The internal pressure test for lithium batteries is to simulate the battery at an altitude of 15240m (low pressure 11.6kPa) (UL standard) to test whether the battery leaks or bulges. Specific steps: Charge the battery 1C at a constant current and constant voltage to 4.2V, cut off the current of 10mA, and then place it in a low-pressure box with a pressure of 11.6Kpa and a temperature of (20 ± 3) for 6 hours. The battery will not explode, catch fire, break, or leak.
5. How does ambient temperature affect battery performance?
The electrochemical reaction at the electrode/electrolyte interface is related to ambient temperature, and the electrode/electrolyte interface is considered the heart of the battery. If the temperature decreases, the reaction rate of the electrode will also decrease. Assuming that the battery voltage remains unchanged and the discharge current decreases, the battery power output also decreases. If the temperature increases, the battery output power increases. Temperature also affects the transfer rate of electrolyte, and an increase in temperature will accelerate the transfer rate of electrolyte. If the temperature drops, the transmission will slow down, affecting the battery charging and discharging performance.
6. What are the methods for controlling overcharge?
To prevent overcharging of the battery, it is necessary to control the charging endpoint. When the battery is fully charged, there will be some special information that can be used to determine whether the charging end point has been reached. There are generally six methods to prevent overcharging of batteries.
Peak voltage control: Determine the charging end point by detecting the peak voltage of the battery.
DT/dt control: Determine the charging end point by detecting the rate of change of the battery peak temperature.
T control: When the battery is fully charged, the difference between the temperature and the ambient temperature will reach the maximum value.
-V control: When the battery is fully charged and reaches the peak voltage, the voltage will drop by a certain value.
Timing control: Control the end of charging by setting a certain charging time, which is generally set to the control time required to charge to 130% of the nominal capacity.
TCO control: Considering the safety and characteristics of the battery, charging at high temperatures (excluding high-temperature batteries) should be avoided, so when the battery temperature rises to 60 ℃, charging should be stopped.
7. What is overcharge and how does it affect battery performance?
Overcharging is the behavior of a battery that continues to charge after being fully charged through a certain charging process. As the negative capacity is designed to be higher than the positive capacity, the gas generated by the positive electrode is compounded with the cadmium generated by the negative electrode through a membrane paper. Therefore, in general, the internal pressure of the battery does not increase significantly. However, if the charging current is too large or the charging time is too long, the generated oxygen will not be consumed in a timely manner, which may cause adverse phenomena such as increased internal pressure, battery deformation, and liquid leakage. At the same time, its electrical performance will also be significantly reduced.
8. What is over discharge and how does it affect battery performance?
"After the battery has discharged the internal stored energy and the voltage reaches a certain value, continuing to discharge will cause excessive discharge. The discharge cutoff voltage is usually determined based on the discharge current.".
0.2C-2C discharge is generally set at 1.0V/branch, and 3C or higher, such as 5C or 10C discharge, is set at 0.8V/branch. Overdischarge of the battery may bring catastrophic consequences to the battery, especially if the high current or repeated over-discharge has a significant impact on the battery. Generally speaking, over discharge will increase the internal pressure of the battery, damaging the reversibility of positive and negative active substances. Even if charging can only be partially restored, there will be a significant decrease in capacity.
9. What problems can arise when batteries of different capacities are used together?
If different capacities or new and old batteries are mixed, there is a possibility of leakage and zero voltage. This is due to differences in capacity during charging, resulting in some batteries being overcharged while others are not fully charged. During discharge, some batteries with high capacity do not discharge, while those with low capacity are over discharged. And the battery may be damaged and leaking or have a low (zero) voltage.
What is a battery explosion? How to prevent a battery explosion? Any part of the solid material in the battery will instantly discharge and be pushed to a distance of more than 25 cm from the battery, which is called an explosion. To determine whether the battery has exploded, use the following conditions for experiments. A net will cover the experimental battery, with the battery in the middle, 25 cm from either side of the net. The mesh density is 6-7 pieces/cm, and the mesh is made of soft aluminum wire with a diameter of 0.25mm. If no solid part passes through the mesh in the experiment, it proves that the battery has not exploded.
10. Lithium ion battery series connection problem.
As a battery in the production process, there are many processes to go through from the beginning of coating to becoming a finished product. Even though the voltage, resistance, and capacity of each group of power supplies are the same through strict testing procedures, there will still be some differences over time. Like twins born to the same mother, they may look exactly the same at first birth, and their mothers are difficult to distinguish. However, as the two children continue to grow, there will be differences in one way or another. The same is true for lithium batteries. After using the difference for a period of time, it is difficult to apply the overall voltage control method to lithium power batteries, such as a 36V battery pack that must be connected in series with 10 cells. The overall charge control voltage is 42V, and the discharge control voltage is 26V. Using the overall voltage control method, due to the particularly good battery consistency, there may be no problems during the initial use stage. After a period of use, the internal resistance and voltage of the battery fluctuate, forming an inconsistent state. (Inconsistency is absolute, and consistency is relative) At this time, it is impossible to achieve the goal by still using overall voltage control. For example, 10 batteries are discharged, including 2 2.8V batteries, 4 3.2V batteries, and 4 3.4V batteries. At this time, the overall voltage is 32V, and we will continue to discharge it to 26V. Make two 2.8V batteries lower than 2.6V in the over discharge state. Repeated over discharge of lithium ion batteries is equivalent to scrapping. Conversely, using the overall voltage control charging method to charge can also cause overcharging. For example, the voltage state of the above 10 batteries during charging. When the overall voltage reaches 42V, two 2.8V batteries are in a "hungry" state, rapidly absorbing electricity, which can exceed 4.2V. Batteries exceeding 4.2V are overcharged, not only because of high voltage scrap, but also because of danger. This is the characteristic of lithium power batteries.
