Lithium Battery Knowledge

Generally, a 3.7V lithium battery needs to have a "protection circuit board" for overcharging and overdischarging. If the battery does not have a protection circuit board, it can only be charged with a voltage of about 4.2V, because the ideal full charge voltage of a lithium battery is 4.2V. If the voltage is exceed 4.2V, the battery may is damaged, it is necessary to monitor the condition of the battery at the same time when charging with this method.

The main components of lithium-ion batteries include positive electrode, negative electrode, electrolyte, separator, positive electrode lead, negative electrode lead, safety device and battery casing, etc. Most of the materials are combustible materials. The distance between the positive and negative electrodes of a lithium-ion battery is extremely short. Any small defect on the separator between the positive and negative electrodes may form a short circuit, causing local high temperature, and causing the separation of the positive and negative electrodes in nearby locations to decompose and chain The reaction will eventually cause the battery to catch fire or explode.

The nature of the ignition of lithium-ion batteries is that the heat in the battery fails to be released according to the design intent, causing internal and external combustion materials to ignite after ignition. The main reasons for the fire are external short circuit, external high temperature and internal short circuit.

Different types of batteries have different internal resistances. The same type of battery has different internal resistance due to the inconsistency of internal chemical characteristics. The internal resistance of the battery is very small, we generally use the unit of milliohm to mean it. Internal resistance is an important technical indicator to measure battery performance. Under normal circumstances, a battery with a small internal resistance has a strong high-current discharge capacity, and a battery with a large internal resistance has a weak discharge capacity. In terms of the schematic diagram of the discharge circuit, we can consider the battery and the internal resistance separately, and divide it into a power supply with no internal resistance at all and connect it with a small resistance in series. At this time, if the external load is light, then the voltage distributed to this small resistor is small. On the contrary, if the external load is very heavy, then the voltage distributed to this small resistor is relatively large, and part of the power will be consumed in this Internal resistance (may be converted into heating, or some complex reverse electrochemical reaction). The internal resistance of a rechargeable battery when it leaves the factory is relatively small, but after long-term use, due to the exhaustion of the battery’s internal electrolyte and the decrease in the activity of the internal chemical substances in the battery, this internal resistance will gradually increase until the internal resistance is large enough. The power inside the battery cannot be released normally, and the battery is dead at this time. The vast majority of aging batteries are useless due to excessive internal resistance and have to be scrapped. Therefore, we should pay more attention to the discharged capacity of the battery rather than the charged capacity.

At present, major lithium battery manufacturers have designed various types of lithium battery protection chips for different types of lithium-ion battery overcharge, overdischarge, and overcurrent protection requirements to ensure battery safety performance and avoid deterioration of battery characteristics. Most of these lithium battery protection chips are suitable for lithium-ion batteries with 1-4 cells in series, and very few new products, such as Texas Instruments' BQ77PL900 chip, are suitable for lithium-ion batteries with 5-10 cells in series, and their protection functions Perfect and widely used in many lithium battery protection circuits. However, for many serial numbers, such as battery packs with more than 10 strings of lithium batteries in series or the number of protection chips are different from the actual number of lithium battery packs in series, if the current integrated circuit chips on the market are used to make the protection circuit, there will be no The disadvantage of not being flexible enough to achieve protection or use.

Low-temperature lithium-ion batteries are classified according to their discharge performance: energy storage low-temperature lithium-ion batteries, rate-type low-temperature lithium-ion batteries.

Lithium-ion batteries often have certain failure phenomena during use or storage, including capacity decay, internal resistance increase, rate performance reduction, gas production, liquid leakage, short circuit, deformation, thermal runaway, lithium evolution, etc., which seriously reduce lithium ion The performance, reliability and safety of the battery. These failure phenomena are caused by the interaction of a series of complex chemical and physical mechanisms inside the battery.

Lithium-ion battery has a high energy density, its capacity is 1.5 to 2 times that of a nickel-hydrogen battery of the same weight, and it has a very low self-discharge rate. In addition, lithium-ion batteries have almost no "memory effect" and do not contain toxic substances. These advantages of lithium-ion batteries make it widely used in the field of new energy vehicles.

Lithium-ion batteries and lead-acid batteries are currently the two most widely used batteries. So what is the difference between these two batteries and which one is better? The following will focus on the difference between lithium-ion batteries and lead-acid batteries.

For lithium batteries, our focus is often on capacity and energy density. These data can usually directly reflect the length of the battery's cruising range. But what we should note is that most lithium batteries are the products of electrochemistry. Since they are related to chemistry, their performance has a great relationship with temperature.