Professional Tool for Health Diagnosis of Backup Power Supplies：Intelligent Battery Internal Resistance Tester

Professional Tool for Health Diagnosis of Backup Power Supplies:Intelligent Battery Internal Resistance Tester

I. Battery Internal Resistance Testing: The "Stethoscope" for Backup Power Supply Systems

In critical applications such as power systems, data centers, communication base stations, and rail transit systems, battery banks serve as both DC operating power supplies and backup emergency power sources, acting as the final line of defense for ensuring the safe operation of these systems. However, since battery banks are often maintained in a float-charging state over extended periods, the degradation of individual cells is frequently insidious—good external appearance does not necessarily indicate sufficient capacity; by the time faulty cells are identified during actual discharge events, serious consequences may have already occurred.

The battery internal resistance, also known as internal impedance, is a key indicator reflecting the battery's health status. Research and practical experience have shown a significant correlation between the degradation of battery capacity and the increase in internal resistance. When the battery's internal resistance exceeds 1.25 times its initial value, the battery can no longer pass performance tests; when the internal resistance doubles the initial value, the battery's actual capacity falls below 80% of its rated capacity – indicating that the battery has deteriorated and must be replaced. Therefore, regularly measuring the battery's internal resistance is the most effective method for predicting battery health trends, proactively identifying faulty individual cells, and ensuring reliable power supply for the entire battery pack.

The MY-254 Intelligent Battery Internal Resistance Tester is a professional testing instrument developed by Guodian Zhongxing to meet the daily patrol inspection and health diagnosis requirements for valve-regulated lead-acid batteries. Utilizing the mature instantaneous discharge testing principle, this device can simultaneously measure battery voltage and internal resistance, estimate remaining capacity, and monitor ambient temperature; it also supports automatic identification and switching between batteries operating at different voltage levels (2V,6V, and 12V). Its handheld, portable design, combined with a 1,999-entry high-capacity data storage capacity and USB flash drive data export functionality, provides a comprehensive solution for on-site battery inspection and data analysis.

II. Core Technical Principles: Instantaneous Discharge Method and Thevenin Equivalent Model

2.1 Lead-acid battery internal resistance composition

The internal resistance of a storage battery is not a single resistor but is instead composed of multiple impedance components. From an equivalent circuit perspective, the battery's internal resistance consists primarily of two parts: first, the metallic ohmic resistance Rm, which includes the resistance of metallic components such as electrode materials, busbars, and terminal posts; this resistance changes gradually as a result of processes such as metal corrosion, creep, or sulfidation; second, the electrochemical resistance Re, which comprises both the electrochemical reaction resistance and the particle concentration polarization resistance; the value of this resistance varies in real time according to the battery's state of charge, but it is often masked by the capacitive reactance of the parallel-connected capacitor during AC testing scenarios.

During traditional AC injection testing, due to the large equivalent capacitance (C) of the battery, the majority of the AC current is bypassed through the capacitor branch, while only a negligible amount of current flows through the electrochemical resistance (Re). As a result, the measured impedance corresponds to that of a series combination of Rm and C; since Re is neglected, the test results cannot fully reflect the actual health state of the battery.

2.2 Technical Advantages of the Instantaneous Discharge Method

The MY-254 employs the instantaneous DC discharge method, which theoretically avoids the capacitor current-sharing issue. During testing, the instrument applies a instantaneous high-current load to the battery and simultaneously measures the voltage drop ΔU at the battery terminal as well as the load current I; based on the Thevenin equivalent model, the battery internal resistance is calculated using the formula R = ΔU / I.

To eliminate errors induced by the voltage and current transients at the moment of closing the circuit, the instrument uniformly employs the voltage recovery value at the moment of load disconnection for calculations; by this point, the current has already reached steady state, making the test data more stable and reliable. The internal resistance measured using this DC discharge method encompasses both ohmic resistance and electrochemical polarization resistance, thereby providing a more accurate representation of the battery's actual discharge capability and exhibiting a stronger correlation with battery capacity; this approach is widely recognized within the industry as a high-precision testing methodology.

2.3 Relationship between internal resistance and capacitance

It is important to note that batteries from different manufacturers, with varying models or manufacturing processes, may exhibit differences in their initial internal resistance even when they have the same capacity. Therefore, when assessing battery condition, one should not rely solely on the absolute value of the internal resistance; instead, greater attention should be paid to the trend of the internal resistance relative to its initial baseline value. The MY-254 features a built-in baseline setting function, allowing users to input the baseline internal resistance value for batteries of various models; the instrument then automatically compares this value to calculate the capacity percentage, enabling a more scientific and accurate interpretation. The industry-standard criterion is as follows: an increase in internal resistance of ≤25% indicates a healthy battery condition; an increase between 25% and 100% indicates a warning condition; and an increase exceeding 100% (i.e., the internal resistance doubling) indicates a degraded or报废 condition.

III. Comprehensive Product Functionality Overview: Eleven-Core Capability Matrix

IV. Comprehensive Technical Specifications and Parameters

V. Standard Testing Procedure: Six-Step Inspection Method

Step 1: Pre-test Preparation

Reliably connect the test cable to the main unit of the internal resistance meter using a dedicated plug. Verify that the instrument's built-in lithium battery has sufficient charge; if the battery level is low, charge it in advance. Prepare the technical documentation for the battery pack to be tested – including the battery model, rated capacity, and initial internal resistance baseline – to facilitate entering the corresponding baseline value into the system settings. For the first test, it is recommended to record the battery's factory date and commissioning date to establish an initial reference record.

Step 2: Visual Safety Inspection

Prior to testing, the battery under test must undergo an eight-item visual safety inspection to eliminate any obvious potential faults: check whether the battery casing is cracked, whether the battery cover is cracked, whether the seal between the casing and the cover is intact, whether the battery terminals are corroded, whether the battery mounting plate is properly tightened, whether there is any contamination or conductive acidic liquid on the top of the battery, whether the test cable is worn or broken, and whether the connectors are loose or corroded. If any abnormal conditions—such as casing leakage or severe corrosion—are detected, the battery should be immediately marked and replaced without performing an internal resistance test, thereby preventing safety risks during the testing process.

Step 3: Instrument Parameter Settings

Press and hold the power button for 1 second to power on; after a short beeping sound, the device will automatically enter the Battery Test Main Interface. Use the left or right arrow keys to navigate to the System Settings menu to perform the baseline value setup and time calibration. The baseline value is the standard internal resistance value corresponding to the full capacity of the battery being tested (provided by the battery manufacturer or obtained from a previously tested healthy battery); the instrument will automatically calculate the capacity percentage based on this baseline value. The time setting ensures accurate timestamping of the test records, facilitating subsequent traceability. After completing the settings, the device will return to the Test Main Interface.

Step 4: Connect the test fixture

Secure the test fixture's positive and negative terminals to the battery terminals: use the red clamp for the positive terminal and the black clamp for the negative terminal. Ensure that the terminals are firmly clamped; poor contact can cause significant deviations in test data. For batteries with terminals on the side, clamp them onto the circular cable terminal rather than the square terminal; if necessary, replace the side adapter to ensure proper contact pressure. If the voltage display shows zero due to incorrect polarity or absence of connection, correct this immediately. After confirming that all connections are correct, proceed with the test.

Step 5: Launch Testing and Data Interpretation

Press the Enter key to initiate the test; the buzzer will indicate that the test has started. During the test, keep the fixture stable and avoid touching the test cable. When the test is complete, the buzzer will sound again, and the LCD screen will simultaneously display four pieces of data: voltage, internal resistance, estimated capacity, and ambient temperature. Use these baseline values to assess the battery condition: a capacity above 80% indicates a healthy battery; a capacity between 50% and 80% indicates a "warning" status; and a capacity below 50% indicates that the battery is degraded and should be replaced. For abnormal readings, perform a repeat test to confirm the result and avoid misjudgment caused by external contact. The test data is automatically saved to the internal memory, and the record number increments automatically.

Step 6: Batch Testing and Data Export

After completing all battery group tests sequentially, you can view all data in the Historical Records interface by using the up/down keys to navigate through the pages. For further analysis, insert a USB flash drive into the device and navigate to the System Settings> Data Processing menu; then select "Export Data." Once a beeping sound indicates that the export was successful, remove the USB flash drive. The data is saved in the NZY_V20.TXT text file format, containing the serial number, internal resistance value, capacity percentage, and test timestamp. This data can be imported into the companion analysis software to generate trend curves and test reports, create a battery health record, and enable full lifecycle management.

VI. Internal Resistance Interpretation System: Three-Level Battery Health Status Assessment Standard

Health-grade (internal resistance increase ≤ 25%)

When the increase in battery internal resistance relative to the initial baseline value is within 25% and the estimated capacity exceeds 80%, the battery is considered to be in good condition. Such batteries exhibit excellent charge–discharge performance and can be put into service normally; routine periodic inspections are sufficient, and no special maintenance is required. It is recommended to perform inspections quarterly or semi-annually to monitor the trend of internal resistance changes.

Early Warning Level (Internal resistance increase of 25%–100%)

When the internal resistance increases by more than 25% but does not double, and the estimated capacity falls between 50% and 80%, the battery should be classified as being in a "early warning" status. This indicates that the battery has undergone some degree of sulfation or aging, and its capacity is beginning to decline noticeably—though this decline has not yet reached a point where replacement is necessary. For such batteries, the inspection frequency should be reduced to once per month, with close monitoring of the internal resistance's rate of change. If the internal resistance continues to rise rapidly, the battery should be included in the replacement plan for advance procurement. When a significant number of batteries within a battery pack are in this early warning status, consideration should be given to conducting a comprehensive verification discharge test on the entire battery pack.

Degraded grade (internal resistance increase> 100%)

When the internal resistance exceeds twice the initial baseline value and the estimated capacity is less than 50%, the battery is considered to be in a severe degradation state. In such cases, the battery's actual discharge capability has significantly declined, making it unable to provide reliable backup power during critical situations; therefore, it must be included in the replacement list. Particularly in battery packs, a single degraded battery can act as the weakest link, severely limiting the overall discharge duration of the entire pack; such batteries should be replaced with priority. After replacing the batteries with new ones, the baseline value should be re-recorded, and a new battery health record should be established.

Important note: The internal resistance test is a battery health screening method and cannot fully replace the capacity discharge test. For batteries with abnormal internal resistance or whose values are approaching the replacement threshold, it is recommended to perform a verification discharge test to confirm the final capacity and ensure an accurate assessment.

VII. Five Major On-Site Testing Challenges and Solutions

1. Test data exhibits significant variability and poor repeatability.

Problem description: During multiple consecutive tests on the same battery, there is a significant variation in the internal resistance values, resulting in unstable data.

Cause analysis: The most common cause is poor contact between the test fixture and the battery terminals. Factors such as an oxide layer on the terminal surfaces, dust or oil contamination, or insufficient fixture clamping force can all lead to fluctuations in contact resistance; when superimposed on the battery's internal resistance, these fluctuations result in data deviations. Another cause is an inappropriate testing timing—when the battery has just completed charging or discharging and its internal polarization has not yet recovered, the internal resistance reading may be high and unstable.

Solution: Before testing, clean the terminal surface of the electrode to remove any oxide layer using fine sandpaper or a steel wire brush to reveal the original metal color. When clamping the fixture, ensure that the cutting edge fully contacts the side of the electrode and that the clamping force is sufficient. The battery should be left undisturbed in float charge mode for at least 2 hours prior to testing, allowing the internal polarization to fully recover. For each battery cell, it is recommended to perform 2–3 consecutive tests to obtain a stable reading and rule out any potential contact-related interference.

2. Significant differences in the absolute internal resistance values among batteries of the same capacity from different brands.

Problem description: For the same 12V,100Ah battery, the internal resistance values measured by different manufacturers vary significantly, making it difficult to establish a unified standard for determining the battery's quality.

Cause analysis: The internal resistance of a battery is influenced by various factors, including the electrode plate formulation, manufacturing process, and assembly method; therefore, there are inherent differences in the initial internal resistance values among batteries of the same capacity from different brands. It is therefore inappropriate to apply a single absolute value standard to evaluate batteries from all brands.

Solution: Replace the "one-size-fits-all" absolute value interpretation method with the "Base Value Comparison Method." Establish an initial internal resistance baseline for each battery model (either from the first operational measurement or from data provided by the manufacturer); for subsequent measurements, use the rate of change of the internal resistance relative to this baseline as the basis for interpretation. The MY-254 features a built-in base value setting function that allows users to define corresponding baseline values for different battery models; the instrument then automatically calculates the capacity percentage, ensuring that the interpretation criteria for batteries from different brands are unified based on their changing trends – making the approach more scientific and rational.

3. Misjudgment due to generally high internal resistance in low-temperature environments

Problem description: During winter outdoor or low-temperature data center testing in northern regions, the internal resistance of batteries is generally higher than at ambient temperatures, which can easily lead to misinterpretation as battery degradation.

Cause analysis: Temperature is an important environmental factor that influences internal resistance measurement. As temperature decreases, the viscosity of the electrolyte increases, leading to an increase in ion migration resistance and, consequently, an increase in the electrochemical polarization resistance. This effect typically becomes pronounced below the freezing point; at –20°F (approximately –29°C), the internal resistance can reach twice that observed at room temperature.

Solution: Foster an awareness of temperature correction; internal resistance measurements obtained under low-temperature conditions should be comprehensively evaluated in conjunction with the ambient temperature, rather than being directly compared against the reference value at ambient temperature. The MY-254's built-in temperature sensor simultaneously displays the ambient temperature; temperature data are included in the test records, facilitating temperature correction during subsequent analysis. Whenever possible, conduct periodic inspections during periods when the ambient temperature is relatively stable to ensure the comparability of all test data. For critical battery packs, it is recommended to perform baseline tests under a standard ambient temperature range of 20–25°C to establish a baseline reference value at this standard temperature.

4. The internal resistance of all batteries in the entire battery pack is abnormally high; the cause cannot be identified.

Problem description: The internal resistance of the entire battery pack is generally high, although no obvious abnormalities are observed during visual inspection; therefore, a potential issue with the testing equipment is suspected.

Cause analysis: An elevated overall battery pack consistency reading is usually not indicative of an instrument malfunction, but rather suggests that the entire battery pack is operating under a specific abnormal condition. Common causes include: prolonged undercharging leading to sulfation, excessively low float charging voltage, excessively low ambient temperature, or the battery pack being nearing the end of its designed service life due to overall aging. Additionally, loose or corroded connection bars can introduce extra series contact resistance into the test circuit, resulting in an elevated overall pack data reading.

Solution: First, verify whether the float charge voltage is within normal range to rule out any issues with the charging system. Check whether the bolt torque applied to the inter-battery connection bars meets the specified requirements; if not, retighten the bolts and then retest the system. Determine the operational service life of the battery pack – it is normal for the entire pack to undergo aging as it approaches the end of its service life. If a sulfation issue is confirmed, schedule a balancing charge or activation treatment before conducting another test. Establish a trend profile for the entire battery pack and perform a longitudinal comparison of historical data trends; this approach provides greater diagnostic value than analyzing individual absolute values.

5. During testing, the instrument displays a "Overheating – Pause" message.

Problem description: After extensive consecutive testing, the instrument's buzzer emits a continuous "beep-beep" sound, and the testing function is disabled.

Cause analysis: During instantaneous discharge testing, the internal load of the instrument generates heat. When the instrument undergoes high-frequency continuous testing over a short period, inadequate heat dissipation can cause the internal temperature to rise, triggering the overheating protection mechanism. This is an inherent self-protection feature of the instrument, not indicative of a malfunction.

Solution: Upon hearing the continuous beeping alarm, immediately cease all operations and place the instrument in a well-ventilated area to allow it to cool naturally. Once the internal temperature has dropped to a safe range, a short beeping signal will indicate that the instrument has returned to normal operation; at this point, testing may resume. During routine large-scale testing, carefully manage the testing pace by taking appropriate breaks of several minutes after testing 20–30 batteries to prevent heat buildup. During outdoor operations in hot summer conditions, extra attention should be paid to heat dissipation; if necessary, prepare two instruments for rotational use.

VIII. Six Major Typical Application Scenarios

1. Regular inspection of the substation DC power supply battery bank

The substation DC operating power supply serves as the power source for relay protection systems and circuit breaker opening/closing operations; thus, the reliability of the battery bank directly impacts the safe operation of the power grid. According to relevant regulations, periodic internal resistance testing of the DC panel batteries is required to identify any degraded individual cells. The MY-254 handheld device features a portable design, making it ideal for maintenance personnel conducting on-site inspections at substations. With a storage capacity of 1,999 cells, this device enables the comprehensive testing of an entire battery bank—whether comprising 108 or 104 cells—in a single operation. The collected data can then be imported into analytical software to generate a test report and establish a substation battery health record.

2. Data Center UPS Battery Bank Health Assessment

The large number of backup batteries in an IDC data center's UPS system constitutes a critical asset for ensuring uninterrupted power supply. Regular resistance testing is a key component of UPS battery maintenance. The MY-254 supports automatic identification of both 2V and 12V batteries, making it compatible with the commonly used VRLA valve-regulated lead-acid batteries in data centers. When combined with USB flash drive-based data export and analysis software, this solution enables the comparison of multiple data batches, tracks the internal resistance trend of each battery, identifies deteriorating batteries in advance, and schedules their replacement – thereby preventing downtime incidents caused by UPS battery failure during mains power outages.

3. Communication Base Station Backup Battery Operational and Maintenance Monitoring

Mobile communication base stations are widely distributed and numerous, resulting in a significant maintenance workload for their backup batteries. Most base station batteries are 12V valve-regulated lead-acid batteries; prolonged float charging can easily lead to the degradation of individual cells. The MY-254 features a compact design, making it easy to transport to remote rural areas or mountainous regions; its 6-hour continuous operation capability meets the inspection requirements for multiple base stations throughout the day. After conducting station-by-station testing, maintenance personnel export the data and perform centralized analysis of each station's battery health status to develop targeted battery replacement plans, thereby improving operational and maintenance efficiency while reducing maintenance costs.

4. Preventive testing of battery banks in power plant DC systems

The DC power system at a power plant serves as a critical power supply for unit control, protection, and signaling systems; therefore, the preventive testing of the battery bank is an essential part of annual maintenance procedures. The internal resistance test, as a non-destructive testing method, enables comprehensive screening of the entire battery bank without requiring the plant to be taken offline. The accuracy of the MY-254 instrument meets the requirements specified in the pre-test procedure; the test data can serve as a key basis for assessing the technical condition of the batteries and, when combined with discharge tests, can be used to generate a comprehensive battery condition assessment report.

5. Rail Transit Signal System Power Battery Maintenance

Railway signaling systems and metro/light rail systems impose extremely high requirements on power supply reliability; therefore, storage batteries serving as backup power sources must always remain in optimal condition. Signal batteries are typically configured as series-connected strings of 2V individual cells – the failure of any single cell will affect the entire string. The MY-254's dedicated 2V battery measurement range (0–10 mΩ) ensures high measurement accuracy for low internal resistance, making it ideal for precise testing of individual 2V cells. The operations and maintenance department can establish a historical internal resistance curve for each battery cell to implement predictive maintenance, enabling proactive replacement before battery degradation impacts system operation.

6. Battery Distributors and Third-Party Testing Services

Battery distributors, power engineering firms, and third-party testing agencies require professional internal resistance testing tools for battery acceptance, warranty testing, and project handover processes. The MY-254 system allows for the storage and export of test data, ensuring traceability and making it an ideal standardized tool for testing services. By conducting individual-cell testing and archiving the results during delivery acceptance, performing random sampling and comparison during the warranty period, and providing quantifiable data support in the event of disputes, this system ensures that battery quality assessments are based on solid evidence, thereby reducing potential disputes.

IX. Data Management and Analytics Software Ecosystem

Internal data storage and retrieval

The MY-254 features built-in 512 KB of storage capacity, allowing up to 1,999 test records to be saved; each record includes the internal resistance value, estimated capacity, and test timestamp. Data is sorted in descending order based on test time, with the most recent records displayed first. Using the up/down arrow keys on the historical records interface enables seamless paging and browsing of recent test results – no computer connection is required.

USB drive data export format

The test data is exported to a USB flash drive in a standard text file format (NZY_V20.TXT) via a USB interface; the file contains the serial number, internal resistance value (in microohms), capacitance percentage, and test duration. This universal text format is highly compatible – the data can be opened directly in Excel for further statistical analysis or imported into specialized analysis software for processing; there are no data format compatibility issues.

Complementary analytical software features

The companion battery analysis software allows users to import exported test data and generate professional analysis reports. The software supports comparison of data across multiple batches, automatically plots trend curves depicting the internal resistance variation of individual batteries, and intelligently identifies batteries exhibiting abnormally elevated internal resistance (indicating degradation) by displaying them in red for early warning. It enables multi-dimensional classification and management—such as by substation, battery pack, or battery model—to establish a comprehensive battery health management database. For maintenance and operation units managing battery packs across multiple substations, this software significantly enhances data analysis efficiency, transitioning the process from manual interpretation to systematic battery health management.

10. Daily Maintenance and Calibration Service

Instrument Usage and Storage

This instrument is a precision electronic measurement device; during use or transportation, avoid excessive vibration or impact. The test fixture is a critical component that affects measurement accuracy; the elasticity of its clamping jaws and the wear on its cutting edges should be inspected regularly, and these components should be replaced promptly if they show deformation or severe wear. When storing the instrument for an extended period, remove the batteries or recharge it every three months to prevent damage caused by over-discharging of the lithium batteries. Store the instrument in a dry, well-ventilated environment, avoiding high temperatures, high humidity, or exposure to corrosive gases.

Charging and Battery Maintenance

The device features a built-in 12 V, 3000 mAh rechargeable lithium-ion battery, which should be charged using the included charger. When the battery level indicator shows low charge, recharge the device promptly – avoid letting the battery drain completely before recharging – as this helps extend its lifespan. The standby time exceeds 32 hours; under continuous testing conditions, the device can operate for more than 6 hours; for routine inspection tasks, no intermediate charging is required over a single day. For devices not in use for an extended period, it is recommended to recharge them once per quarter to maintain battery performance.

Periodic Calibration Recommendations

As a metrological testing instrument, it is recommended to undergo annual calibration by the metrology authority or the manufacturer to ensure that the measurement accuracy of voltage and internal resistance consistently meets the nominal specifications. For testing facilities with high usage frequency, the calibration interval may be appropriately shortened. If the instrument suffers a drop, is exposed to water, or exhibits significant deviations in test data, it should be immediately taken out of service and submitted for inspection; it may only be reused after confirmation that it meets the required standards.

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