Lithium battery protection circuit scheme

    Among all kinds of batteries used at present, lithium battery (also known as lithium ion secondary battery or lithium ion battery) is a new type of power supply developed in recent ten years. Lithium battery is different from the general chemical power supply. Its charging and discharging process is realized through the embedding and de embedding of lithium ions in the positive and negative electrodes of the battery. The cathode of lithium battery is carbon material, such as graphite; The positive electrode is a lithium containing transition metal oxide, such as lithium cobalt oxide (LiC002). In addition, the positive and negative electrode materials of lithium battery use lithium ion intercalation compounds with layered structure that can be freely embedded and separated from lithium ions. The lithium ions in the interlayer will undergo electrochemical reaction in appropriate electrolyte. During charging, lithium ions are separated from the positive electrode lattice driven by the external electric field, and embedded into the negative electrode lattice through the electrolyte. The discharge process is just the opposite. Lithium ions return to the positive electrode, and electrons reach the positive electrode through the external circuit to compound with lithium ions.

    Compared with commonly used nickel cadmium and nickel hydrogen batteries, lithium batteries have many advantages, mainly in the following aspects:

    (1) The power supply voltage of lithium battery is high, generally 3.6 V, which is about 3 times that of nickel cadmium battery and nickel hydrogen battery. For electronic equipment with high power supply voltage requirements, the number of batteries in series required by the battery pack can also be greatly reduced. Therefore, the lithium battery used in combination is easy to obtain higher voltage.

    (2) High specific energy, that is, lithium batteries with the same weight provide higher energy than other batteries. The specific energy of lithium battery is generally 2-3 times that of nickel cadmium battery and nickel hydrogen battery. Therefore, it is conducive to small and lightweight portable electronic equipment.

    (3) No memory effect. Both nickel cadmium battery and nickel hydrogen battery have memory effect, so they must be discharged regularly, otherwise, the battery will be invalid due to memory effect. The lithium battery has no memory effect and can be charged directly without paying attention to the amount of residual electricity. In this way, the efficiency of lithium battery can be brought into full play.

    (4) Long service life. The lithium battery uses a carbon negative electrode, which will not generate metal lithium during charging and discharging, so as to avoid battery damage due to internal metal lithium short circuit. At present, the cycle life of lithium battery can reach more than 5000 times, which is far higher than other types of batteries.

    (5) The working environment has a wide temperature range, and it can generally work between - 30 ℃ and 0 ℃, with excellent high and low temperature discharge performance.

    (6) Low self discharge rate. Self discharge rate, also known as charge retention rate, refers to the amount of automatic discharge when the battery is not in use. The self discharge rate of lithium battery is 2% ～ 5%, that of nickel cadmium battery is 25% ～ 30%, and that of nickel hydrogen battery is 30% ～ 35%. Therefore, the lithium battery has the longest charge retention time under the same environment.

    (7) Lithium batteries do not contain any mercury, cadmium and other toxic elements, and are truly green and environmentally friendly.

    Based on the above advantages, lithium batteries are widely used in portable electronic devices. On the other hand, due to its high energy density, it is difficult to ensure the safety of lithium batteries. Specifically, in the overcharged state, the electrolyte will be decomposed, causing the temperature and pressure inside the battery to rise; In the over discharge state, the electrolytic material copper in the cathode will melt and cause internal short circuit, which will increase the temperature: when the external circuit is short circuited or the discharge current in Chapter 1, Introduction 3 flows too much, due to the characteristics of high internal resistance, the internal power consumption of the battery will increase, and the temperature will also rise, which may cause the oxidation or decomposition of the electrolyte, leading to the shortening of the life of the lithium battery. In addition, if the lithium battery is discharged excessively, the electrolyte in the battery will change, and the number of rechargeable cycles will be reduced, thus affecting the service life of the lithium battery.

    Due to the shortcomings of lithium battery analyzed above, protection circuit must be added in the application of lithium battery. The basic functions of the protection circuit should also correspond to the above shortcomings, so we require the lithium battery power supply protection chip to realize the following basic functions: overcharge protection, over discharge protection, over-current protection and short circuit protection. It can be seen from the above application requirements of lithium battery that in order to improve the service life of lithium battery and ensure the safe use of battery, the lithium battery protection circuit needs to have the following functions:

    (1) If the charging voltage exceeds the maximum value allowed by the battery, the battery discharge circuit can be provided.

    (2) If the discharge voltage is lower than the minimum value allowed by the battery, the battery charging circuit can be provided. Then disconnect the battery from the external circuit, and disconnect the battery from the external circuit, and

    (3) If the charging and discharging current of the battery is greater than the limit value, disconnect the battery from the external circuit.

    (4) When the battery returns to the normal state, the protection circuit shall be able to release the protection state accordingly, so that the battery can continue to work normally.