New energy and hybrid power are inseparable from power battery packs. One of the core technologies of new energy is power battery packs. Starting from this article, we will share a series of articles with you to systematically introduce the electrical measurement technology of lithium-ion batteries. I hope that through this article, front-line technicians who love learning can have a comprehensive understanding of the systematic measurement of lithium-ion batteries.
The second part
1. Battery monomer assembly
1.1 Resistance value inspection of terminal/polar ear welding part, resistance measurement of terminal/polar ear welding part.
In order to make full use of high-performance batteries, the welding quality of battery energy in and out of terminals (polar ears) is very important (Figure 8). Especially in the case of electric vehicles, it is hoped to minimize output losses and control the heat to a minimum. Therefore, the ideal state is to ensure that the resistance of the welding part is close to 0Ω. Generally, the resistance of the welded part is less than 0.1mΩ as the judgment standard for good and bad products. It is necessary to select a resistance meter with a resolution below 1µΩ, which is suitable for low resistance measurement.
1. Here are some points to pay attention to low resistance measurement.
- Measure the current First of all, for the measured object (in this case, the welding part), the voltage generated by its resistance component is measured by the output current through a constant current source. According to Ohm’s law, the resistance value can be calculated. The tester using this resistance measurement method is called a resistance meter. Generally speaking, low resistance measurement can only be correctly measured under the condition of a large measurement current. If the resistance value of the measuring object is below 1mΩ, the recommended resistance meter current is not less than 100mA, preferably above 1A.
- 4 Terminal Resistance Measurement When measuring low resistance, the influence of the wiring resistance of the measuring pen and the contact resistance at the front end of the pens is very large and cannot be ignored. In particular, it is possible to measure the contact resistance of the contact part of the pen from a few Ω to dozens of Ω according to the environmental conditions. When measured by the terminal method, the measured current I not only flows through the measuring object R0 but also flows through the distribution resistance and contact resistance r1 and r2. The measured voltage value E is E = I (R0+r1+r2). The resistance value calculated by this formula through Ohm’s law is R0+r1+r2 (Figure 9). In order to solve this problem, the 4-terminal method can be used. 4 The terminal flowing out of the terminal method to measure the current is separated from the terminal measuring the voltage. The measured current I flows through the resistance R0, but does not flow through the voltage to measure the terminals r3 and r4. That is to say, the r3 and r4 parts do not generate voltage. The result is that the test voltage E of the voltmeter is equal to the actual voltage E0 generated at both ends of the measuring object, and the correct resistance value that is not affected by r1, r2, r3, and r4 can be measured (Figure 10). For the above reasons, the resistance meter of the 4-terminal method is required for low resistance measurement of mΩ.
- Influence of temperature difference on electromotive force difference Temperature difference electromotive force difference refers to the phenomenon that different metals will produce a potential difference in the contact part. The potential difference caused by the contact between the metal part of the measuring object and the test pen is an important factor in the error caused by resistance measurement. Especially when the resistance value of the measured object is very small, the measured current IM flow detected through the RX IM is also very small, and the influence of the temperature difference electromotive force difference VEMF will become very large (Figure 11). In order to eliminate the influence of temperature difference and electromotive force difference, the influence of the temperature difference and electromotive force difference can be removed by inverting the current measurement method and testing in the positive and negative directions twice. The voltage detected when the positive direction flows through the measured current, minus the voltage detected when the negative direction flows through the measured current, the resistance value after removing the influence of the temperature difference electromotive force difference can be obtained. (Figure 12, formula (1)). The resistor RM3545 is equipped with the OVC function (OVC: Offset Voltage Compensation) to remove the influence of temperature difference electromotive force difference.
2. Insulation resistance inspection between electrodes and electrodes-shells before injection.
For lithium-ion batteries, the parts that should be insulated are not fully insulated, or if the insulation resistance is insufficient, there is a risk of low battery life or a fire accident. The main reason for the lower insulation resistance is the mixing of metal foreign bodies or the destruction of the diaphragm. The main parts that should be insulated are between the electrode and the electrode, between the electrode and the housing, Note 3.
In order to confirm whether the insulation resistance is sufficient, the insulation resistance of the battery monomer must be checked before injecting the electrolyte. Insulation resistance inspection requires the use of insulation resistance meters. An insulation resistance meter is a kind of resistance meter, which is specially used to measure high resistance values 4).
The insulation resistor meter calculates the resistance value by applying a high voltage to the insulator and detecting the small current flowing. It is equipped with a high-sensitivity ammeter that can correctly detect small currents at the PA (Pian) and fA (Fan) levels. Because the signal measured during insulation resistance measurement is very small, external interference and leakage current have a great impact on the measurement value. A good measurement environment is important for obtaining stable measurements. IT IS CUSTOMARY TO REFER TO DEVICES THAT CAN MEASURE 10GΩ AS INSULATION RESISTORS. An insulating resistor (typical TΩ = more than 1012 Ω) that can measure higher resistance values is called an ultra-high resistance meter. To show the difference 4, this manual will give a general description of the two measuring instruments, “insulation resistance meter” and “ultra-high resistance meter”. The cost of these two types of products is very different, and a suitable insulation resistor needs to be selected according to the good or negative judgment benchmark of the insulation state.
Notes: 3 – Especially in the case of poor insulation between negative electrodes and shells, it must be accurately judged as defective products. This will be detailed in the following “side voltage measurement”. 4 Ultra-high resistance meter, also known as a Pian meter.
If you don’t know how to choose the right measuring instrument, you can use the following key parameters as a reference point.
(1) Measurement range of insulation resistance you need to choose an insulation resistance tester that can measure whether the insulation is qualified and the insulation resistance value exceeds the threshold qualification range. In particular, if the threshold is above the number GΩ, it is necessary to ensure that the range is sufficient
- Criminal the threshold between a few MΩ and several GΩ, and the accuracy requires a few%: ST5520
- The threshold is between a few kΩ and a few hundred TΩ, and the accuracy is higher: SM7110, SM7120
(2) Voltage output performance Select the appropriate product according to the voltage applied during insulation resistance measurement.
- Below 1000V: ST5520, SM7110
- 2000V below: SM7120
At the same time, you need to pay attention to the electrostatic capacity of the battery monomer. The battery monomer has an electrostatic capacity of about a few hundred pF to a few hundred µFs, and the external voltage may be overshoot when measuring this battery monomer. If overshoot occurs, it will take some time for the output voltage to stabilize to the set test voltage.
This will have a negative impact on the intervals to be mentioned later. Because overshoot will apply a higher than set voltage to the measured object, the measured object may be damaged.
Excellent insulation resistance testers and high resistance meters can also suppress the overshoot of output voltage for capacitive loads, so they can be said to be very suitable for insulation resistance detection of batteries.
Current capacity (current limit)
Current capacity refers to the boundary value of the current that can be applied in the measurement circuit. In the product specifications of the tester, it may also be called “charge current”, “current limit”, “current limit”, “measure current”, “rated current”, etc. When measuring a capacitive object such as a battery monomer, the smaller the current capacity, the longer it takes from the beginning of the measurement voltage to the test. Select the tester with sufficient current capacity according to the required interval. The measurement of insulation resistance is to apply a constant voltage to the measured object. After applying the voltage, the insulating layer between the battery monomer electrode and the electrode before injecting the liquid, and the insulation layer between the electrode and the housing will accumulate a charge 5. At this time, the charge charging time of the capacitive subject is expressed in the following formula.
The capacity C and test voltage V of the measured object is determined by the process. Therefore, the time required for the insulation resistance to start measuring is inversely proportional to the current output by the tester. That is to say, if the time from the external measurement voltage is to the official start of measurement, an insulating resistor with a sufficient current capacity needs to be used.
For example, the current capacity of the insulation resistor of HIOKI is as follows.
ST5520: Up to 1.8 mA SM7110,
SM7120: Up to 50 mA (limit current variable)
Discharge function (charge absorption function)
The discharge function refers to the function of releasing the charge accumulated on the insulation layer of the measured object in the insulation resistance experiment. Inadequate discharge may lead to electrical shocks caused by residual charges or failures of the measured object. Therefore, after the measurement, the charge of the measuring object needs to be released through the discharge function. Most insulation resistors are equipped with discharge functions, including resistance discharge and constant current. For example, the capacitive measured object of the battery monomer can be discharged faster using the constant current method.
tt: Discharge time CC: Capacity of the measured object RR: Discharge resistance VV0: Interterminal voltage of the charged object under charge. <
VV1: Interterminal Voltage of the Discharge State Measured II: Discharge Current.<
HIOKI’S INSULATION RESISTANCE TESTERS ST5520 AND HIGH RESISTANCE METERS SM7110.
AND SM7120 IS EQUIPPED WITH CONSTANT CURRENT EDUCATION FUNCTIONS.
ST5520:10 mA constant current discharge
SM7110, SM7120: Constant current discharge according to the current limit (maximum 50mA).
Contact check function.
If there is no electrical contact between the measuring pen and the measuring object, of course, the insulation resistance value of the measured object cannot be measured, but the representation value of the insulation resistor is the maximum value in the open circuit state, and the expression value is no different from the good product (Figure 14). In this way, poorly insulated batteries may also be misjudged as good products. Therefore, it is important to confirm whether the pen has correctly touched the tested object before measurement. The function of confirming whether the pen has contact with the measuring object is called the contact check function. In order to prevent the flow of defective products into the market, an insulation resistor equipped with a contact inspection function should be selected.
Micro-short circuit inspection function
The small-area conduction state formed by metal foreign bodies and burrs is called micro short 6. When a battery with a micro-short circuit inside is measured for insulation resistance, the external application of high voltage 7 will generate a centralized electric field in the micro-short circuit area and burn the micro-short circuit part. In this case, because the measurement has “improved” the micro-short circuit state, the result will be judged as a good product (Figure 15). However, this kind of micro-short-circuit battery will have the risk of large-scale short circuits and must be judged as a defective product. In order to avoid this situation, a low voltage (about a few Vs) that will not improve the short circuit state needs to be applied before measuring the insulation resistance to detect the micro-short circuit state. It is recommended to choose an insulating resistor with this function.
3. Edge voltage measurement (layered lithium-ion batteries)
Even if the insulation before the electrolyte is confirmed, it may lead to poor insulation due to the formation of a new electrical circuit after injection. Therefore, it is very important to confirm the insulation state after injection. However, if the insulation resistance is measured at high voltage as before liquid injection, the abnormal situation that has no effect on the characteristics of the battery will be judged as “dead goods”. At the same time, the application of high voltage may also cause the decomposition of the electrolyte. For example, on the left of Figure 17, it is assumed that there is poor insulation between the negative electrode and the aluminum housing. And as shown on the right side of Figure 17, the resin film of the aluminum shell has cracked, and the electrolyte and the aluminum shell are channeled through. The current path is generated as shown in the right inner arrow in Figure 17. Because aluminum has a higher potential than anode material, Li-Al alloy is formed by a reduction reaction at the aluminum shell-electrolyte interface. This Li-Al alloy is very brittle, so it will lead to small holes in the aluminum shell. If moisture enters the battery through the pores, the lifespan of the lithium-ion battery will be significantly reduced (Figure 18). That is to say, in order to make high-quality batteries, poor insulation between the negative and aluminum housings must be detected. On the other hand, it is assumed that the insulation between the positive electrode shell is poor, and the electrolyte shell is short-circuited. At this time, due to the low potential of aluminum, the oxidation reaction occurs, and Li-Al alloy cannot be formed (Figure 19). Because it does not affect the life of lithium-ion batteries, it can be judged as a “good product.
When measuring the edge voltage, you need to pay attention to the following three points.
- The input resistance of the voltage tester
If the input resistance of the tester is too low, it will generate voltage division with the measured voltage, making it difficult to judge bad and good products. That is to say, in order to better check the results, it is recommended to use a voltage meter with high input resistance. FOR EXAMPLE, THE INSULATION RESISTANCE IS 10MΩ, AND IN THE CASE OF INPUT RESISTANCE RIN 1OMΩ, HALF OF THE SIDE VOLTAGE WILL BE DISCONNECTED. Therefore, it is necessary to use a voltmeter with higher input resistance, and it is recommended to use instruments with an input resistance of more than 10GΩ. However, most testers do not have such a high input resistance. The input resistance of general voltmeters or digital multimeters is mostly about 10MΩ, which should be noted when selecting.
- Reliable probes
Side voltage measurement is to determine whether the battery is good or not by whether the side voltage is close to 0V. On the other hand, as shown in Figure 21, when the pen does not touch the measuring object, the voltage meter will also measure 0V because of the input resistance RIN. This kind of bad contact will misjudge bad products as good products, leading to the entry of bad products into the market. Especially because there is an insulating coating on the side of the aluminum shell, it is prone to poor contact. HIOKI’s voltage meter DM7275/DM7276 has a contact check function, which can be measured after confirming the contact status. It is important to select a tester with a contact checking function to measure it after confirming the contact status.
- Interference countermeasures
When measuring the edge voltage, the output resistance of the measurement point is usually higher. Excessive external interference in the environment will lead to greater measurement deviation and the possibility of misjudgment. As a countermeasure, the shielded line is selected for the test line, so that it is not easily affected by external interference. In addition, in order to reduce the impact of power frequency noise of commercial power supply, it is also very important to do a good job in the integration time of the voltmeter and the synchronization of the power supply frequency.
Li-ion batteries are able to be recharged hundreds of times and are more stable. They tend to have a higher energy density, voltage capacity, and lower self-discharge rate than other rechargeable batteries. This makes for better power efficiency as a single cell has longer charge retention than other battery types.
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