rechargeable battery 18650 3.7v

　　The impact of voltage detection circuit on rechargeable battery 18650 3.7v

　　There are many influencing factors, but the influence of the circuit cannot be ignored. The author of this article uses a practical... In the management system of rechargeable battery 18650 3.7v, it is necessary to detect the voltage of the single cells in the battery pack. Nowadays, the direct sampling method is widely used. Not only is the method complicated, but it also affects the consistency of the battery pack in practical applications. There are many influencing factors, but the influence of the circuit cannot be ignored. The author of this article uses a practical voltage detection circuit to analyze the impact of the circuit on the performance of the battery pack, uses the battery model to analyze the causes of this impact, and proposes methods to reduce the impact. Difficulties in experimental measurement The difficulty of the voltage measurement system in the battery pack is: how to detect the voltage of each single cell in the series battery pack, and these voltages must be relative to a common reference ground, so as to facilitate the sampling of voltage signals. The safe voltage of lithium-ion batteries is generally 2.7~4.2V, so for A/D conversion with a reference voltage of 5V, the single cell voltage is just within this range. If the reference ground is not the system ground, then the voltage of the nth battery will be the sum of the voltages of the previous n batteries, which has exceeded the reference voltage range of the A/D conversion, so it is converted to the same reference through the voltage measurement system. Land is crucial. Experimental design: 10 identical lithium-ion batteries were used in the experiment, with a rated capacity of 3Ah. The battery pack was in a series structure. The impact of the dynamic process on the rechargeable battery 18650 3.7v was examined by conducting cycle experiments on the battery pack. Before conducting the battery pack experiment, the single battery was cycled three times to stabilize the capacity. The charging and discharging system of the battery pack is: charge with a constant current of 0.50C to 42V, then charge with a constant voltage of 42V until the current decreases to 0.05C; discharge the battery pack with a constant current of 0.50C to 30V. During the experiment, the protection of the single battery was: overcharge protection 4.35V, overdischarge protection 2.50V. The author of this article has designed a practical voltage measurement circuit for battery packs, as shown in Figure 1. It is a subtraction circuit composed of operational amplifiers. Each battery voltage Vn′ corresponds to a subtraction circuit. Vn+ is the voltage of the positive electrode of the nth battery relative to the earth, and Vn- is the voltage of the negative electrode of the nth battery relative to the earth.

　　Because the maximum voltage of the 10 rechargeable battery 18650 3.7v is about 42V, which has exceeded the maximum voltage for the op amp, so the method of voltage attenuation and then amplification is used in the subtraction circuit. Because the amplification part uses the same amplification factor, it can be placed behind the analog switch. In this way, the entire detection circuit only needs one amplification circuit, which can save costs. Results and Discussion During the actual use of the battery pack cycle life experiment, in order to ensure the accuracy of 10 times the voltage attenuation, the resistor R was 33kΩ (1% accuracy). Because high-precision large resistors need to be specially customized and are expensive. During the experiment, the performance of the battery pack declined significantly, as shown in Figure 2. In the first 52 cycles (0.5C charge and discharge), there was a big difference in the discharge cut-off voltage of the single cells in the battery pack. As the cycle progressed, the phenomenon of "queuing up" in order was more obvious, and in the first After about 30 cycles, before the battery pack is discharged to 30V, the voltage of the first battery is already in the over-discharge protection state, which leads to the termination of the battery pack's discharge and the sudden reduction in battery pack capacity. In order to verify that it was caused by the leakage current on the detection line, after 52 cycles, the first and tenth batteries in the battery pack were exchanged, and the cycle experiment was continued. It was found that the discharge of the tenth battery was terminated. The voltage dropped slightly, but the discharge end voltage of the first battery increased greatly; because the original first battery was no longer in the over-discharge protection state, the second battery was in the over-discharge protection state. The above experiment shows that there is a certain leakage current in the voltage detection circuit, which was measured.

　　The experimental analysis is based on the designed circuit: In formula (3): In is the leakage current on the nth detection line. When the resistance R is fixed, the larger Vn+ and Vn- are, the larger In is, so the leakage current of the detection line closer to the positive electrode of the battery pack is larger. During the charging process, the charging current in the loop passes through each detection line, and a part of the current is diverted by the voltage detection circuit. The closer the battery is to the negative electrode of the battery pack, the more it is diverted. In this way, during the charging process, the more current is diverted. The battery charging current that is closer to the positive terminal of the battery pack will be greater. The same is true during the discharge process. The closer the battery is to the positive electrode of the battery pack, the smaller the discharge current. During the entire cycle, the battery close to the positive electrode of the battery pack is charged with more power and releases less power; while the battery closer to the negative pole of the battery pack is charged with less power and releases more power, which causes the inconsistency of the battery pack. s reason. As the cycle proceeds, the inconsistency intensifies. Even if the initial state of the battery is consistent, due to the cumulative effect, the battery pack will be inconsistent due to leakage current during the cycle. Although the leakage current is relatively small, multiple detection lines and Long-term accumulation will affect the consistency of the battery pack. In order to reduce the leakage current in the improved experiment, the input impedance was increased during the experiment, that is, the resistance R was changed to 200kΩ (1% accuracy). Because the accuracy deviation of large resistance resistors is relatively large, the experimental results show that the leakage current is significantly reduced. Small. Perform cycle experiments on the modified voltage detection circuit according to the above battery pack experimental system. The experimental results are compared with Figure 2. The impact of the voltage detection circuit on the discharge end voltage of the single cells in the battery pack is much smaller. However, the discharge end voltage of the single cells in the battery pack will still "queue" in order. . Conclusion In circuit design, the battery pack inconsistency caused by the circuit should be minimized. The methods to reduce the leakage current of the voltage detection circuit are: ① Increase the input impedance of the detection circuit to solve the matching problem of resistor pairs, which is more suitable for batteries with a small number of series. and a battery pack with a relatively large capacity; ② Add a control switch to the detection loop to reduce the impact of leakage current, but this increases the complexity and cost of the circuit, and is suitable for battery packs with a large number of series; ③ In the process of battery combination , the capacity of the battery close to the negative pole of the battery pack can be slightly larger than the capacity of the battery close to the positive pole of the battery pack, which will help reduce the inconsistency of the battery pack caused by the voltage detection circuit.

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