Resistance and continuity
An internal ohmic measurement reads the impedance, conductance or internal resistance of each cell or unit in a stationary battery, usually by injecting a small signal or pulse. Because these values drift as a cell dries out, corrodes or sulfates, the test is a quick way to compare cells and spot a weak one without discharging the whole string.
An internal ohmic measurement characterizes the opposition a battery cell or unit presents to a small current, without taking the battery out of service. Depending on the instrument, the quantity reported is internal resistance, impedance, or its reciprocal, conductance. The instrument is connected across each cell, or across each multi-cell unit in the case of monoblock batteries, and measures how the cell responds to a small alternating signal or to a brief load pulse. Readings are taken cell by cell along the whole string and are stored with their position. The test is used on flooded lead-acid batteries and valve-regulated lead-acid (VRLA) batteries that serve as standby power for substations, switchgear control, generating stations, telecommunications and uninterruptible power supplies. It does not measure capacity. It is an indicator that is correlated with condition, and it earns its place because it takes very little time per cell and can be repeated whenever the string is accessible, which makes it ideal for building a history of each cell.
Batteries rarely fail all at once. Individual cells age at different rates through grid corrosion, loss of water in sealed designs, sulfation, plate shedding, and poor connections. A weak cell in a series string limits the whole string and can reverse during a deep discharge, so the first sign of trouble is often a battery that cannot carry its load when needed. Float voltage hardly distinguishes a good cell from a poor one, and a full capacity discharge test is expensive, time consuming and takes the battery out of service, so it cannot be done often. Ohmic measurements fill the gap. A cell whose internal resistance has risen, or whose conductance has fallen, relative to its own history and to its neighbors is flagged for attention long before the string fails a load test. The test supports decisions on the schedule of capacity tests, on replacing individual cells, and on budget planning for the replacement of the string, and it is called for at both acceptance and maintenance for flooded and VRLA types.
Every cell behaves electrically like a small voltage source in series with an internal resistance, plus capacitive and inductive effects from its plates and connectors. As a cell ages, the active material loses contact area, the electrolyte becomes less effective or drier, and the conductive path through the grids and posts deteriorates. These changes raise the cell's internal opposition to current. The instrument applies a small alternating signal at a specified frequency, or draws a brief pulse, and compares the resulting voltage and current to compute impedance, resistance or conductance. Because the excitation is small, it does not disturb the cell or change its state of charge. The reading depends on the design of the cell, so a value has no universal meaning: only a comparison with the manufacturer's reference, with the same cell at an earlier date, or with cells of identical type in the same string tells anything. Temperature and state of charge also move the numbers, which is why they are recorded. The reading includes the connections around the cell, so a poor link is sometimes mistaken for a poor cell and the two tests are best read together.
The string is normally on float and left in service, and the usual safety practices for stationary batteries apply: eye and face protection, insulated tools, and care with the stored energy and the risk of arcing. The instrument is checked, and the battery type and rated size are entered so that it can apply the right reference. Probes are placed firmly on the terminal posts, or on the dedicated test points where they exist, and the same points are used for every cell. For monoblock units the measurement is made across the whole unit. The reading is taken, allowed to settle, and recorded with the cell number, and the technician moves along the string in order. Probe contact is important, because a loose or dirty contact adds resistance of its own and causes noisy readings. Where a cell is out of line, the connections around it are inspected, cleaned and checked with a low-resistance measurement, and the cell is measured again before a conclusion is drawn. Readings are normally taken at the same time of day and under similar float conditions, and the same instrument model and settings are used on each visit. The aim is a clean dataset that can be compared from one visit to the next.
The record holds the value reported for each cell in its position in the string, the quantity measured, and the instrument with its settings and calibration status. It adds the battery type, its charge condition and the temperature, and notes the as-found state of the cells and any cells retested after the connections were cleaned. These details matter because each reading is useful mainly as a comparison with the same cell on earlier tests and with its neighbors, so the history of a cell can be followed over the life of the string only if the conditions are comparable.
No fixed figure decides whether a cell is acceptable, and this page does not give one. Readings are judged by comparison. The first comparison is with the baseline recorded when the battery was new or recently installed, which describes the healthy state of that particular design. The second is with the other cells in the same string, where an outlier relative to the typical value or the average calls for attention. The third is trending against earlier readings, because a rising resistance or falling conductance over successive tests is more informative than any one result. The fourth is the manufacturer's published reference for the cell model. A flagged cell is usually followed by a connection check and, if the indication persists, by further testing such as a capacity test. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 include the measurement for flooded and VRLA batteries in acceptance and maintenance; for the values and the way each treats the result, consult your own licensed copies and the battery manufacturer's data.
At acceptance the measurement builds the baseline for each cell of a new string, and it also confirms that all cells are consistent with each other after delivery and installation. In maintenance it is a trend and screening tool that guides the interval between more demanding tests. The battery type changes how the numbers move. In VRLA batteries, loss of water and dry-out are major aging mechanisms, and internal ohmic values tend to move earlier and more clearly than float voltage, so the test is especially valued there. In flooded batteries the measurement is helpful but is read together with specific gravity and cell voltage, which are available for these cells. The measurement principle also varies by instrument: some use a small alternating signal, others a brief load pulse, and their readings are not interchangeable, so one method and one reference are kept for the life of a string. Because results can also be influenced by the strap resistance, the same connection resistance survey is often performed on the same visit.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
No. It correlates with condition but it is not a capacity measurement. A cell whose resistance or conductance is drifting is a candidate for a discharge test, and the capacity test remains the reference for deciding whether a battery can still do its job.
Sealed cells cannot be checked for specific gravity or water level, and their aging, such as dry-out, shows up poorly in float voltage. Ohmic values change early and clearly, so they offer one of the few practical in-service indicators of declining cells in these designs.
Not reliably. Instruments differ in signal type, frequency and calculation, so their numbers are not interchangeable. Use one instrument model and method for a given string and compare each reading only with the same method's baseline, the manufacturer's reference and neighboring cells.
It flags a possible weak cell or a poor connection around it. The next step is to clean and check the links with a low-resistance measurement, retest, and if the difference persists, confirm with further testing such as a capacity test before deciding on replacement.
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