51.2v solar energy storage battery pack.Design of lithium-ion battery protection circuit

　　Lithium batteries must consider safety during charging and discharging to prevent deterioration of characteristics. However, the high energy density of lithium-ion batteries makes it difficult to ensure the safety of the battery. In an overcharged state, the energy will be excess after the battery temperature rises, so the electrolyte decomposes to produce gas, which can easily increase the internal pressure and cause the risk of spontaneous combustion or rupture. On the contrary, in an over-discharge state, the electrolyte decomposes, resulting in deterioration of battery characteristics and durability, and reducing the number of rechargeable batteries. Therefore, overcharge, over-discharge, over-current and short-circuit protection of lithium batteries are very important, so protection circuits are usually designed in the battery pack to protect lithium batteries. 1 Circuit Design 1.1 Circuit Overview The lithium-ion battery protection circuit includes overcharge protection, overcurrent/short circuit protection and overdischarge protection. This circuit is to ensure safety in such overcharge and discharge states and prevent characteristic degradation. It is mainly composed of integrated protection circuit IC, chip resistor, chip capacitor, field effect transistor (MOSFET), and some thermistor (NTC), identification resistor (ID), fuse (FUSE), etc. The circuit diagram is shown in Figure 1. Among them, the integrated protection circuit IC is used to detect the current voltage, current, time and other parameters of the protection circuit to control the switching state of the field effect transistor; the field effect transistor (MOSFET) controls whether the circuit needs to be turned on or off based on the protection IC. ; The chip resistor is used to limit the current; the chip capacitor is used to filter and adjust the delay time; the thermistor is used to detect the ambient temperature in the battery block; the fuse prevents excessive current flowing through the battery and cuts off the current loop. 1.2 Circuit principle and parameter determination 1.2.1 Overcharge protection When the charger overcharges the lithium battery, the internal pressure of the lithium battery will rise due to the temperature rise, and the current charging state needs to be terminated. At this time, the integrated protection circuit IC needs to detect the battery voltage. When it reaches 4.25V (assuming the critical point of battery overcharge voltage is 4.25V), the overcharge protection is activated and the power MOS is turned from on to off, thereby stopping charging. In addition, in order to prevent overcharging caused by noise from being misjudged as overcharge protection, a delay time needs to be set, and the delay time cannot be shorter than the duration of the noise to avoid misjudgement. The calculation formula of overcharge protection delay time tvdet1 is: tvdet1={C3×(Vdd-0.7)}/(0.48×10-6) (1) where: Vdd is the overcharge detection voltage value of protection N1. Simple calculation of delay time: t=C3/0.01×77(ms) (2) If the capacitance value of C3 is 0.22F, the delay value is: 0.22/0.01×77=1694(ms) 1.2.2 Over-discharge protection is In the case of excessive discharge, the electrolyte decomposes, causing battery characteristics to deteriorate and reducing the number of charges. Over-discharge protection IC principle: In order to prevent the over-discharge state of the lithium battery, assuming that the lithium battery is connected to a load, when the lithium battery voltage is lower than its over-discharge voltage detection point (assumed to be 2.3V), the over-discharge protection will be activated, causing the power MOSFET to It switches from on to cut off to cut off discharge to avoid over-discharge of the battery and keep the battery in a low quiescent current standby mode. The current at this time is only 0.1μA. When the lithium battery is connected to the charger and the voltage of the lithium battery is higher than the over-discharge voltage, the over-discharge protection function can be released. In addition, considering the situation of pulse discharge, the over-discharge detection circuit is equipped with a delay time to avoid malfunction. 1.2.3 Overcurrent and short-circuit current protection Overcurrent or short-circuit occurs due to unknown reasons (during discharge or the positive and negative poles are accidentally touched by metal objects). To ensure safety, the discharge must be stopped immediately. The principle of overcurrent protection IC is that when the discharge current is too large or a short circuit occurs, the protection IC will activate overcurrent (short circuit) protection. At this time, the detection of overcurrent is to monitor the Rds(on) of the power MOSFET as an inductive impedance. If the voltage drops, if it is higher than the set overcurrent detection voltage, the discharge will stop. The calculation formula is: V_=I×Rds(on)×2 (V_ is the overcurrent detection voltage, I is the discharge current) (3 ) Assuming V_=0.2V, Rds(on)=25mΩ, then the protection current is I=4A. Similarly, overcurrent detection must also have a delay time to prevent malfunction when a sudden current flows in. Usually after an overcurrent occurs, if the overcurrent factor can be removed (for example, immediately disconnected from the load), it will return to its normal state and normal charging and discharging operations can be resumed. 2 Conclusion When designing a protection circuit, users are very concerned about charging the battery to a full state. At the same time, safety issues are taken into consideration, so the charging state needs to be cut off when the allowable voltage is reached. To meet these two conditions at the same time, a high-precision detector is necessary. The current precision of the detector is 25mV. In addition, issues such as integrated protection circuit IC power consumption and high voltage resistance must also be taken into consideration. In addition, in order to effectively apply the Rds(on) of the power MOSFET during charging current and discharging current, the impedance value needs to be kept as low as possible. Currently, the impedance is about 20~30mΩ, so that the overcurrent detection voltage can be lower.

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