Why Is the 18650 Battery Pack Not Working Properly?
When you assemble various 18650 battery packs in your garage, do you often encounter various problems that make the circuit unconnected and feel distressed?
Following the previous article that made some guiding suggestions for DIY lithium battery packs, in this article our professional engineers will analyze the problem that the finished 18650 battery pack cannot be used normally. You can eliminate the influencing factors one by one according to the following reasons, and finally find the correct adjustment option.
In fact, the reason why the new 18650 battery pack cannot be used normally is very simple. That is, there are inconsistencies within the battery pack that prevent the whole from working in harmony with each other.
Now, let’s take a look at why is the battery pack inconsistent?
1. Parameter differences between single cells
The state difference between single cells mainly includes the initial difference of single cells and the parameter difference generated during use. There are many uncontrollable factors in the process of battery design, manufacture, storage and use, which will affect the consistency of the battery.
Improving the consistency of single cells is a prerequisite for improving the performance of battery packs. The interaction of single battery parameters, the current parameter state is affected by the initial state and time accumulation.
Therefore, before assembling the battery pack, the parameters of the single battery must be measured to ensure consistency.
1.1 Battery capacity
The inconsistent battery capacity will cause the discharge depth of each single battery in the battery pack to be inconsistent.
A 18650 battery with a small capacity and poor performance will reach the full charge state ahead of time, causing a battery with a large capacity and good performance to fail to reach the full charge state.
The inconsistency of battery voltage will cause the single cells in the parallel battery pack to charge each other. And the battery with a higher voltage will charge the battery with a lower voltage. That will accelerate the attenuation of battery performance and consume the energy of the entire battery pack.
1.3 Self-discharge rate
A battery with a high self-discharge rate will lose a lot of capacity. And the inconsistency of the self-discharge rate of the battery will lead to differences in the state of charge and voltage of the battery, affecting the performance of the battery pack.
1.4 Battery internal resistance
In a series system, the difference in the internal resistance of the individual batteries will lead to inconsistent charging voltages of each battery. The battery with a large internal resistance reaches the upper voltage limit ahead of time. While other batteries may not be fully charged at this time. A battery with a large internal resistance loses a lot of energy, generates high heat, and the temperature difference further increases the difference in internal resistance, leading to a vicious circle.
In a parallel system, the difference in internal resistance will lead to inconsistencies in the current of each battery. And the voltage of a battery with a large current changes rapidly, making the charging and discharging depth of each single battery inconsistent. And that also makes it difficult for the actual capacity of the system to reach the design value. If the working current of the battery is different, then its performance will vary during use, which will eventually affect the life of the entire battery pack.
2. Charge and discharge
The charging method affects the charging efficiency and charging state of the lithium battery pack. Both overcharging and overdischarging will damage the battery. And the battery pack will show inconsistency after multiple charging and discharging.
At present, there are several charging methods for lithium-ion batteries, but the common ones are segmental constant-current charging and constant-current constant-voltage charging.
- Constant-current charging is an ideal method, which can carry out safe and effective full charging.
- Constant-current and constant-voltage charging effectively combines the advantages of constant current charging and constant voltage charging. Itsolves the problem that the general constant current charging method is difficult to charge accurately. Besides, it avoids the impact of the excessive current on the battery caused by the constant voltage charging method at the initial stage of charging. The operation is simple and convenient.
The performance of lithium batteries will be significantly attenuated under high temperature and high discharge rate.
This is because when the lithium-ion battery is used under high temperature and high current, it will cause the decomposition of the positive electrode active material and the electrolyte. This is an exothermic process.
The short-term release of heat can cause the temperature of the battery itself to rise further, and the temperature rise accelerates the decomposition phenomenon, forming a vicious circle. What’s more, it accelerates the decomposition and further reduce the performance of the battery. Therefore, if the thermal management of the battery pack is improper, it will cause irreversible performance loss.
4. Battery external circuit
4.1 Connection method
In a large-scale energy storage system, the batteries will be combined in series and parallel, so there will be many connection circuits and control elements between the batteries and the modules.
Due to the different performance and aging speed of each structural part or component, and the inconsistent energy consumption of each connection point, different components have different effects on the battery, resulting in inconsistencies in the battery pack system. Inconsistent battery decay rates in parallel circuits can accelerate system degradation.
The impedance of the connection piece will also affect the inconsistency of the battery pack. The resistance value of the connection pieces is not the same, and the resistance value from the pole to each single battery branch is different.
The battery far away from the pole has a larger resistance value due to the longer connecting piece, and the current is relatively small. The connecting piece will make the single battery connected to the pole reach the cut-off voltage first, resulting in a decrease in energy utilization and affecting battery performance. The premature aging of the single battery will lead to overcharging of the battery connected to it, causing potential safety hazards.
As the number of battery cycles increases, the ohmic internal resistance will increase. Besides, the capacity will decay, and the ratio of ohmic internal resistance to the resistance of the connecting piece will change. In order to ensure the safety of the system, the influence of the resistance value of the connecting piece must be considered.
4.2 BMS input circuit
The battery management system (BMS) is the guarantee for the normal operation of the battery pack, but the BMS input circuit will adversely affect the consistency of the battery.
The battery voltage monitoring methods include precision resistor voltage division, integrated chip sampling, etc. These methods cannot avoid the external load leakage current of the sampling line due to the existence of the resistance and the circuit board path.
Impedance of the battery management system voltage sampling input will increase the non-uniformity of the battery state of charge (SOC), affecting the performance of the battery pack.
5. SOC estimation error
The reason for the inconsistent SOC:
(1) The initial nominal capacity of the single battery is inconsistent.
(2) The nominal capacity decay speed of the single battery is inconsistent during work.
For a parallel circuit, the difference in internal resistance of the single cells will cause uneven current distribution, which in turn will lead to inconsistent SOC. SOC algorithms include ampere-hour integral method, open circuit voltage method, Kalman filter method, neural network method, fuzzy logic method, discharge test method, etc.
The ampere-hour integration method has better accuracy when the initial state of charge SOC0 is relatively accurate.
But the coulombic efficiency is greatly affected by the battery state of charge, temperature and current, and it is difficult to measure accurately. Therefore, the ampere-hour integration method is difficult to meet the accuracy requirements of state of charge estimation.
After standing for a long time, the open circuit voltage of the battery has a definite functional relationship with the SOC, and the estimated value of the SOC is obtained by measuring the terminal voltage. The open circuit voltage method has the advantage of high estimation accuracy, but the disadvantage of long rest time also limits its application range.