Abstract: This article introduces battery online monitoring and condition evaluation , analyzes battery internal resistance and its influencing factors. It introduces the traditional DC discharge method and AC injection method for measuring the internal resistance of the battery, and analyzes the principle and characteristics of AC discharge method for measuring the battery internal resistance .
- Introduction
As the core component of DC power supply system, Battery plays a key role in storing electric energy , dealing with grid abnormalities and special working conditions, and maintaining the normal operation . It is the last secuite guard of high reliable power security. At present, the online monitoring and condition evaluation of storage battery are gradually taken seriously by people, And Battery is widely used in electric power, communication and other industries. However, the internal resistance measurement technology used in battery online monitoring and condition evaluation, which is not known by people, and there is still a vague understanding.
- Online Evalutaion for Battery State
At present, there are two methods for online evaluation of disable battery state: floating- charge voltage monitor method and battery internal resistance monitor method. The floating charge voltage monitoring method was widely used in the past, but theoretical analysis and a large number of experiments have proved that the floating charge voltage has nothing with evaluting disabled battery state and battery service life . So this method is no longer used to evaluate the state of the battery. Disable battery state and battery expected service life are closely related to its internal resistance after exeriments and theoretical analysis. Battery internal resistance is as the main indicator for battery monitor equipment and battery test system so far . Combining the change rate and historical data of battery internal resistance, a data system is established to conduct online evaluation of disable battery state and battery evaluated service life.
Large capacity batteries are often used in power station. Their internal resistance is extremely small from tens to hundreds of microohms. Even the firm tightness of the connector will not affect the measurement results. Moreover, the online operation of the battery is due to the ripple interference generated by the charging device. So the traditional resistance measurement technology cannot meet the requirements, Battery internal resistance can be measured to reflect the micro ohm level of battery internal resistance
- Battery Internal Resistance Factors
There are below factors affecting battery internal resistance as belows.
- Battery Backward : With battery service time, battery capacity is reduced and battery internal resistance is gradually large due to some factors such as battery water loss, electrode plate and connecting bar corrosion, electrode plate sulfation, electrode plate deformation and active substances shedding.
- Charge capacity of the battery: The same battery has different discharge degrees. Battery internal resistance varies greatly due to the obvious differences in the depth of the battery electrolyte, the thickness of the reaction material on the electrode surface, and the porosity of the electrode surface. The deeper the discharge, the greater the internal resistance of the battery.
Therefore, when evaluating the backward state of the battery, the battery of the same manufacturer and model shall establish a unified internal resistance standard. The internal resistance shall be measured after the battery is fully charged. The backward state of the battery shall be evaluated against the standard. If the battery is not fully charged, the evaluation is meaningless because it is impossible to distinguish whether the internal resistance increases due to the backward state of the battery or the battery state of charge.
- Temperature: With the increase of temperature, the diffusion of reactants, charge transfer, electrode dynamics and material transfer are easier to carry out, thus reducing the internal resistance of the battery.
- Battery model: Different manufacturers, different types and different models of batteries have different internal resistance due to different electrode, electrolyte and diaphragm material formulas, different battery structures and different assembly processes.
- Measuring signal frequency: At present, many battery internal resistance measurements actually measure the impedance of the battery, and the battery is not a pure resistance characteristic, but also contains capacitance, so its impedance is related to the measured signal frequency, making the measurement results of the battery internal resistance non objective. In order to make the internal resistance measurement result of the battery objective, the influence of the battery capacitance on the measurement result should be removed analytically according to the phase relationship between the measured signal current and voltage, so that the measurement result is independent of the measured signal frequency, that is, the internal resistance measurement result is unique at any measured signal frequency.
Measuring time and measuring current: When a large measuring current is used, at the moment when the measuring signal is applied and the measuring signal is closed, because polarization and establishment and stability are a changing process, different measuring current, different measuring time and polarization are different, so the internal resistance measurement results of the battery are not objective. In order to make the internal resistance measurement result of the battery objective, the internal resistance should be measured with a small signal current as far as possible. According to the experiment, the measured current should be ≤ 0.05C10, where C10 is the capacity of the battery at the 10 hour discharge rate. The internal resistance measurement result is independent of the measurement time and current, and the internal resistance measurement result is unique.
- Battery Internal Resistance Measurement Technology— AC Discharge Metho
AC discharge battery measurement technology is a further development on the basis of AC injection battery internal resistance measurement technology. This method combines the advantages of AC injection method and DC discharge method.
Characteristics of AC discharge battery internal resistance measurement:
- Safe and reliable. The main working circuit and circuit of the battery are not connected to any device. The measuring circuit is designed with a 10K Ω current limiting resistor and a fuse. The measuring circuit is designed with high resistance. The working circuit and measuring circuit of the battery are safe and independent from each other. The battery monitoring equipment can be replaced when the battery is working online.
- The discharge current is small, which will not damage the battery. The discharge current is 0.01C10-0.05C10, which will not impact the battery, cause grid plate deformation and active substance shedding, and have no impact on the battery life.
3) It has strong anti-interference ability and is suitable for real-time online monitoring of working batteries. The programmable band-pass filter is used for filtering, and the digital signal processing technology is used for signal processing, which effectively eliminates the influence of DC charging device ripple on the measurement. It has good interference performance and is suitable for real-time online monitoring of working batteries.
4)High measurement accuracy, accurate state evaluation and ordered prediction. Band pass filter+multi-stage high-precision operational amplifier+digital signal processing make the internal resistance test precision of battery much higher than that of traditional DC discharge method and AC injection method. Accurate online assessment and life prediction of battery state require the reflection of such small changes as micro ohm level of internal resistance of battery. The online test accuracy of internal resistance of battery should be within 2%, and the repetition accuracy should be within 1%. At present, the traditional DC discharge method and AC injection method cannot achieve this
The test result is that the true internal resistance of the battery is independent of the measurement time, signal frequency and test current, which is objective and convenient for horizontal comparison of data.
