In the field of battery industry, the charge-discharge rate is usually used to describe the relationship between charging speed and current size. When we customize lithium battery, charge-discharge rate is a important factor to consider. For example, the rate of 1 hour full battery is called 1C, the rate of only 30 minutes is called 2C, and so on, more than 1C can be called fast charge. Nowadays, the charging rate of lithium-ion batteries can generally achieve 1C-3C, and the highest can probably go to 5C, but compared with the discharge rate of 10C, naturally there is still a long way.
In addition to the bottleneck of the maximum charging rate, there are also differences in the charging rate that the battery can withstand under different SOC(State of Charge). In general, the rate of charging will follow a slow – fast – slow rhythm. Generally, when the SOC reaches more than 90%, the internal resistance of the battery will rise significantly, making the charging rate slow down.
So if you’re an electric car battery user and you want to save as much time as possible on charging, try not to use less than 10% of the charge. It doesn’t have to be full when charging, but more than 90%, or enough miles to cover your next trip. In addition to the bottleneck of the battery itself, the peripheral charging device has its own limitations.
In theory, the charging speed could indeed be increased by increasing the current. However, if the current is too large, the diffusion rate of lithium ions in the battery can not keep up with the diffusion rate of electrons, which will lead to the disconnection of electron-ion transport, affecting the battery performance, reducing the charging capacity that can be achieved, and the life of the battery is even worse, and there will even be the risk of fire and explosion.
Therefore, in general, when not in a hurry, we suggest using slow charging as much as possible, which is conducive to extending the battery life, and it is safer to use slow charging for lithium batteries. SmartPropel lithium batteries have built-in high quality BMS with multiple protections, we suggest you to buy the original charger together to ensure the charging safety.
During charging, the diffusion rate of lithium ions inside a lithium battery is closely related to temperature, cathode material and structure.
The first is the temperature. Generally speaking, the higher the temperature is, the faster the diffusion rate is. However, if the temperature is too high, it will also lead to reduced battery life and decreased charging safety. Too low temperature is no good either. At too low a temperature, the lithium metal in the battery will deposit, which can cause a short circuit inside the battery, especially the lithium iron phosphate battery. Generally at 0℃, the capacity of lithium iron phosphate battery is about 80%, and at -20℃, it is only a poor 20 ~ 40%. So in the cold northern winter, electric cars must be able to heat up the battery module, and thus drain power faster.
Secondly, materials, different materials diffusion ability gap is very large, such as lithium cobalt acid, lithium manganese acid, lithium iron phosphate, NCM, NCA and other cathode materials with very good performance, and the latter two are the two materials with the best performance and high application popularity at the moment. This is a big reason why today’s lithium-ion batteries are named after the cathode material.
The basic working principle and structure of lithium-ion battery
The basic principle of the battery: the positive electrode is reduced to gain electrons; The negative electrode oxidizes and loses electrons. Electrons pass through the load, passing from the negative to the positive, forming a current in the direction of the positive to negative.
There is always something to hold the lithium ions in the positive terminal and the lithium atoms in the negative terminal, just as there are always shelves for goods. So the shelf for lithium ions is cobalt-ate ions, which together with lithium ions form the positive electrode; The lithium atom of the negative electrode is made of graphite and other materials with holes, so that the negative electrode reaction is gone. Between the positive and negative electrodes are an electrolyte and a membrane, which is used both to flow lithium ions and to separate the positive and negative electrodes, preventing internal short-circuiting.
Why talk about the basic working principle and structure of lithium-ion batteries? Later on, the cut-off voltage of lithium battery charge and discharge and the harm of overcharge and over-discharge will be used.
Lithium ion battery characteristics
Users are most concerned about the characteristics of lithium-ion batteries is the capacity, such as the often said 2000mAh, refers to the lithium battery in the normal operation of the number of charges can be released. Let’s take a look at a lithium ion battery specification:
Some important parameters for this battery:
Charging cutoff voltage: 4.2V
Discharge cutoff voltage: 2.5V
Maximum charging current: 4000mA
Maximum discharge current: 20000mA
In short, it is all around the battery capacity and charge and discharge to consider. The capacity of a battery depends on how many electrons can be released by the negative electrode and how many electrons can be absorbed by the positive electrode.
Why is there a charging cutoff voltage? In other words, what is the problem after over-voltage charging? In the previous description of the lithium ion battery structure, it is mentioned that the negative electrode is composed of graphite and lithium atoms. In fact, lithium does not exist in the form of an atom, but in the form of lithium ion and graphite coexist. After over-voltage charging, lithium ions will precipitate into crystalline lithium, unable to participate in charge and discharge, resulting in reduced battery capacity.
Why is there a discharge cutoff voltage, in other words, what can happen when the discharge is over? After excessive discharge, a large number of lithium ions in the negative electrode flow to the positive electrode, resulting in empty graphite, part of the area collapsed, can no longer store lithium ions, will also lead to a reduction in battery capacity.
Specific to a lithium battery, its capacity is different in different discharge current and temperature, and decreases with the increase of the number of charge and discharge cycles.
What limits the charging speed of a lithium battery? It comes down to materials and technology. As mentioned earlier, lithium ions do not exist as atoms and need to coexist with graphite. The process of full charge of lithium battery is that lithium ions migrate between positive and negative electrodes and carry and release electric ions to realize the storage function of lithium battery. These need a certain reaction time, too fast charging will make the lithium battery abnormal reaction and crystallization, and if the charging speed exceeds the battery tolerance, it will increase the internal resistance of the lithium battery, so that the battery will produce too high temperature risk.
SmartPropel Original Writer: Michelle