18650 battery 4800mah.Lithium battery charging protection circuit and discharge protection circuit requirements

　　Lithium battery charging protection circuit and discharge protection circuit requirements. The design of the lithium battery charging circuit and discharge protection circuit is very important. Due to the high energy density of lithium batteries, in the overcharged state, the energy will be excess when the battery temperature rises, so the electrolyte decomposes and generates gas, which can easily cause the internal pressure to rise and cause spontaneous combustion or spontaneous combustion. There is a risk of rupture, so the protection of lithium batteries is very important.

　　The principle of lithium battery protection circuit

　　The reason why lithium battery (rechargeable type) needs protection is determined by its own characteristics. Since the material of the lithium battery itself determines that it cannot be overcharged, overdischarged, overcurrent, short-circuited, or charged and discharged at ultra-high temperatures, lithium battery components always appear with a delicate protective board and a current fuse. The protection function of lithium batteries is usually completed by a protection circuit board and current devices such as PTC. The protection board is composed of electronic circuits and can accurately monitor the voltage and charge return circuit of the battery core at all times in an environment of -40°C to +85°C. Current, timely control the on and off of the current loop; PTC prevents severe damage to the battery in high temperature environments.

　　Ordinary lithium battery protection boards usually include control IC, MOS switches, resistors, capacitors and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. Among them, the control IC controls the MOS switch to turn on when everything is normal, so that the battery core is connected to the external circuit. When the battery core voltage or loop current exceeds the specified value, it immediately controls the MOS switch to turn off to protect the battery core. Safety.

　　Lithium battery charge protection circuit and discharge protection circuit

　　1.Charging of lithium battery

　　The maximum charge termination voltage of a single lithium battery is 4.2V. It cannot be overcharged, otherwise the battery will be scrapped due to too much loss of lithium ions in the positive electrode. When charging lithium batteries, a dedicated constant current and constant voltage charger should be used. First, constant current charge until the voltage across the lithium battery is 4.2V, then switch to constant voltage charging mode; when the constant voltage charging current drops to 100mA, Charging should be stopped.

　　The charging current (mA) can be 0.1~1.5 times the battery capacity. For example: for a 1350mAh lithium battery, the charging current can be controlled between 135mA~2025mA. The conventional charging current can be selected to be around 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.

　　2. Discharge of lithium battery

　　Due to the internal structure of the lithium battery, all lithium ions cannot move to the positive electrode during discharge. A part of the lithium ions must be retained at the negative electrode to ensure that the lithium ions can be inserted into the channel smoothly during the next charge. Otherwise, battery life will be shortened. In order to ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited, which means that the lithium battery cannot be over-discharged.

　　The discharge termination voltage of a single-cell lithium battery is usually 3.0V, and the minimum cannot be lower than 2.5V. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (hours) = battery capacity/discharge current, and the lithium battery discharge current (mA) should not exceed 3 times the battery capacity. For example: for a 1000mAh lithium battery, the discharge current should be strictly controlled within 3A, otherwise it will cause The battery is damaged.

　　The design of lithium battery protection circuit is very important. We require lithium battery power protection chips to be able to achieve the following most basic functions: overcharge protection, over-discharge protection, over-current protection and short-circuit protection. From the above application requirements of lithium batteries, it can be seen that in order to improve the service life of lithium batteries and ensure the safe use of batteries.

　　Lithium battery protection circuit needs to have the following functions

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

　　(2) If the discharge voltage is lower than the minimum allowed value of the battery, a battery charging circuit can be provided. The battery is disconnected from the external circuit, and the battery is disconnected from the external circuit.

　　(3) If the charge and discharge current of the lithium battery is greater than the limit value, cut off the connection between the battery and the external circuit.

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

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