Usability Evaluation Method for VRLA Battery Measuring Equipment

,


Introduction
Lead-acid batteries provide backup power and have been widely used in DC power systems and uninterruptible power supply system.As the last defense of safe, whether the battery can work well is very important.Battery experts in the field have widely recognized that battery resistance directly affects the actual capacity of the battery and the use of performance, the battery resistance, especially the increase in resistance is an important indicator of battery performance degradation.Therefore, it's very important to monitor the changes of the battery internal resistance after using the battery and ensure the performance of the battery.
Although the IEC60896-2 standard specifies the resistance level of the lead-acid battery and define a standard secondary discharge method to measure the internal resistance of the battery, the secondary discharge method requires a smooth control of very short and high current and measurement (for example, to test 800AH battery, we should control 5 seconds and 1600A current discharge), this measurement method is currently only applicable to the laboratory, commercial battery testing equipment will generally use their own research and development technology, which led to the bad situation that there is no uniform standard for internal resistance measurements, and the relevant national certification laboratories do not provide standard actual measurements.Which led to dilemma, resistance value measured by different measurement equipment is different and we have no idea that which test value is correct.
However, in the current application environment, even if the test results have some differences (of course, the reference standard should not deviate too much, such as the theoretical value of 300 micro-ohms, measured values of 250 micro-ohm to 350 micro-Ohms are acceptable), but if the test equipment has good stability, repeatability and sensitivity, then through the internal resistance change can also infer the battery performance changes, such as the internal resistance from 300 micro-ohm gradually increased to 400 micro-Ohms, the device can accurately measure its trend, will be able to judge the performance of the battery to provide a reliable reference.
In this paper, the methods and mathematical models for evaluating the internal resistance test equipment of lead-acid battery are considered, also we consider the change rate of internal resistance, the change rate of micro-incremental resistance, and the allowable repetition test precision of the equipment itself.This data model validates the measurement stability, repeatability and sensitivity of the battery resistance test equipment to infer the availability of the equipment.
The mathematical model is also suitable for other battery measurement equipment, such as lithium batteries and super capacitors, etc., but in the actual battery maintenance process, monitoring the internal resistance changes for lead-acid batteries is more important because of its electrochemical properties and very small internal resistance value, it is difficult to measure the actual value, we do not have the official reference value of the case, the method mentioned in this paper is particularly useful.Relative to the resistance of lead-acid batteries (usually tens of micro-ohms to several hundreds micro-ohms), lithium battery resistance is relatively large (usually tens of milliohms), it is easy to measure, so this method has little significance used for evaluation of lithium battery measurement equipment.

Design Principle
The resistance value of the VRLA battery varies from tens of micro-ohms to several thousand micro-ohms depending on the capacity, and its regression trend is related to its internal resistance increment.According to the current large number of tested data, if the battery's internal resistance base value gradually increased by about 50%, its capacity will degenerate to about 80%, once the battery degeneration to 80% capacity will quickly degenerate, so 80% of the capacity of the battery reliability of the demarcation point.The use of internal resistance trends to determine the battery performance changes require measurement equipment can be very accurate measurement of its trend changes.As mentioned earlier, in the case of actual resistance value without reference standard, it is very important to be able to monitor the small change of internal resistance.Assume that for the same battery, A device measures its internal resistance base value of 100 micro-ohms, B device measures the internal resistance of the value of 120 micro-ohms.We re-measure it 5 years later, A device measures resistance value of 150 micro-Ohms, B device measures resistance value of 180 micro-Ohms, the change percent is 50%, so the trend provided by A or B equipment has the same effect.
So we can series resistance with a certain resistance in the preparation of the accuracy, error and other related parameters in the battery test circuit and use an evaluation mathematical model.Resistance to simulate the incremental change in the internal resistance, so that the entire test circuit both with the electrochemical characteristics of the battery, but also to achieve increased analog resistance.Through the series resistance of different resistance, and taking into account the internal resistance of the measured value, incremental size and equipment to repeat the measurement error.This method is achieved by calibrating a series of power micro resistors (typically a shunt with a high thermal stability which is very important for micro additions) that is known in series with a pole in the measured battery.As shown in Figure 1, with the battery resistance test equipment were tested r0 and r1, and then calculate the measured incremental value:

Simulated Lead-Acid Battery Resistance Increase in the Implementation of Micro-Increase
△ r = r1 -r0 (1) Compared with the known R value, calculate the measurement error δ: (4) By the σ can be identified by the battery resistance test equipment to verify the value of the battery resistance of the measurement error average square deviation.

Incremental Test Steps and Test Data
The test is carried out as follows: 1) Use the BMMS2000 battery on-line monitoring device manufactured by eTocsin company with repetitive accuracy of ± 2% to measure the internal resistance of a battery 10 times and calculate the average value; 2) 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% of the average value of the shunt are calibrated according to the average.
3) Respectively series shunt and respectively measure its resistance value.
Considering the influence of the repeatability of the measuring device itself and the magnitude of the internal resistance microinstruction (ie, the absolute range of the error precision of the device repeatability and the variability of the micro-increment size), we will divide the increment into two groups to define The average standard deviation of the measurement error of the incremental value is calculated from 20% to 50% of the reference standard of 50% ~ 100% The following is the measured data: The average value of the error and the average variance of the error are: + σ = 6.77% + 2.01% = 8.78%.The average value of the error and the average variance of the error are: + σ = 3.58% + 0.57% = 4.15%.
From the above test results, we can draw the following conclusions: The internal resistance increment test of BMMS2000 with online monitoring device of eTocsin with repetition accuracy of ± 2% was used.The incremental error and incremental error variance of the two sets of data were A) group A ( 20%, 30%, 40%, 50% ) is 8.78% B) group B ( 50%, 60%, 70%, 80%, 90%, 100% ) is 4.15% Therefore, the value 15% of group A (20%, 30%, 40%, 50% increase in the incremental increment + incremental error variance) and the value 5% of group B(50%, 60%, 70%, 80% %, 100% increase in the incremental increment + incremental error variance) are able to effectively determine the on-line monitoring device measurement accuracy and stability.

Results
This article describes the verification of lead-acid battery internal resistance test equipment to measure the accuracy and stability of the increment is easy to implement, fast and accurate features.In the development of incremental error + incremental error variance evaluation criteria should take full account of the theoretical value of the battery resistance, incremental size, test equipment, measurement accuracy and repeat the error and other parameters to finalize a reasonable evaluation criteria for the user to select high-availability measurement equipment to provide technical verification averages to solve the current problem of non-standard.

References
Bullock, K. R. ( 2013).Lead acid battery systems and technology for sustainable energy.In Batteries for .Springer New York.

Figure
Figure 1.Connection method batteries with different number of hours and different volts.And then calculate the calculated difference and the error value of the average square error σ: 100% (2)By replacing the different calibrated series of power trace resistors (typically 10%, 20%, 30% of the internal resistance values measured by the battery resistance test method specified in IEC 60896-2 and GB/ T 19638.2-2005,(e.g., 2V,  6V, 12), and a series of internal resistance increment measurement error values δ1, δ2, ... ... δn are obtained by replacing the