A healthy voltage reading can hide poor backup capacity. That uncertainty can undermine maintenance decisions. A controlled discharge reveals what the battery delivers under defined conditions.

An industrial battery discharger applies a specified load and records current, voltage, time, and sometimes temperature and individual cell voltages. It measures delivered charge or energy to an agreed endpoint. Conductance and impedance tests provide condition indicators, but do not directly establish usable discharge capacity.

What quantities does the discharger measure during a test?

The core observations are current, voltage, and elapsed time. From these, a logger calculates delivered charge in ampere-hours and, if voltage is included in the integration, delivered energy in watt-hours. Temperature and cell or block voltages add context. They help explain why the string stopped, whether performance changed during the run, and whether one part of the battery behaved differently from its neighbors.

The calculations are straightforward, but their meaning depends on the measurement boundary. Charge is the sum of current multiplied by each measurement interval. Energy is the sum of voltage multiplied by current and the interval. With time expressed in hours, a constant 40 A discharge lasting three hours delivers 120 Ah. If voltage changes, multiplying 120 Ah by a nominal voltage gives only an approximation to delivered energy. Integrating the recorded voltage produces the relevant result.

Distinguish measured current from commanded current. A display showing a 40 A setpoint does not establish that 40 A left the battery throughout the test. The load may have reached a power limit, overheated, or lost regulation at low voltage. Review the actual current trace and the instrument status alongside the cumulative result. A useful report identifies interruptions rather than silently treating a paused test as continuous service.

A battery discharger may support different load modes and external current sensing, depending on its configuration. The current entering the load is not always identical to total battery current when other equipment remains connected. Specify the relevant measurement boundary and confirm how the proposed configuration records it before interpreting delivered capacity. For a battery capacity discharge tester, confirm which current channel the exported capacity calculation actually uses.

For an installed battery, draw the circuit before attaching sensors. Identify charger current, service current, parallel strings, and the test branch. If the charger supplies part of the load, a three-hour load-bank run may remove much less charge from the battery than expected. If another load remains connected, measuring only the test branch understates battery discharge. Neither result is repaired simply by choosing a more accurate logger.

Why is measured ampere-hour capacity different from rated capacity?

A capacity rating belongs to a discharge condition. The battery manufacturer’s documentation defines a current or power, duration, endpoint, temperature, and preparation basis. Without these conditions, a label such as 200 Ah is incomplete for test interpretation. A battery can deliver a different number of ampere-hours at another rate without either the label or the measurement being wrong.

This is especially relevant when a long-duration standby rating is used to discuss a short, high-power UPS discharge. Lead-acid batteries do not have a rate-independent reservoir of usable ampere-hours. A high-current test changes the relationship between delivered charge and voltage. Comparing its raw Ah total directly with a rating established at a lower current can misrepresent the battery’s condition.

Battery performance tables distinguish discharge duration, load mode, endpoint voltage, and reference temperature. These conditions belong beside the rated value. An endpoint listed for one model and rate is not a universal instruction for another battery. Select the reference that matches the installed product and the purpose of the test.

Consider an illustrative test delivering 180 Ah. If the matching rated condition is 200 Ah, the simple ratio is 90 percent. If the test used a different rate, began partly charged, or ended at a different voltage, that ratio does not establish 90 percent rated capacity. It may instead describe charge delivered during that particular experiment. State the distinction in the report before discussing replacement.

Temperature correction is also a reference comparison, not an improvement to the battery. Preserve the raw measured result and the actual battery temperature, then document any approved correction separately. Use the model’s instructions and the applicable test procedure. Do not apply a remembered correction factor to NiCd or lithium because it appeared in a lead-acid worksheet.

A repeatable record should preserve the battery model, serial or site identifier, cell count, charge preparation, load definition, start conditions, endpoint, and actual stopping reason. These details often make a historical comparison more valuable than an extra decimal place in the final capacity percentage.

Can conductance or impedance replace a capacity test?

Conductance and impedance measurements interrogate electrical behavior with a relatively small test signal. Their value is that they can reveal changes quickly, often while the battery remains in its normal installation. They are useful for screening individual units and establishing trends, provided the method and connections remain consistent.

Conductance and impedance readings do not directly count the charge available through a specified discharge. They can identify changes worth investigating, but cannot independently establish usable capacity. Meaningful trends require consistent measurement methods, configuration, and temperature conditions. Compare readings against an appropriate baseline rather than converting an isolated result into an unsupported capacity percentage.

The practical distinction is between a condition indicator and demonstrated performance. An unusual ohmic reading should trigger investigation; it is not automatically a complete diagnosis. Inspect connections, compare neighboring units, consider charging history, and confirm that the measurement was taken consistently. A favorable reading likewise does not demonstrate that the string can maintain the required voltage for the entire service duration.

The same caution applies to short load checks. A brief high-current test can reveal immediate voltage weakness, but it does not necessarily characterize a several-hour duty. Conversely, a slow capacity discharge does not by itself demonstrate the voltage response to a demanding switching event. Match the test to the question rather than treating every procedure involving current as interchangeable.

A starting-performance test and a capacity test answer different questions. A battery may supply a brief high-current pulse while failing to sustain the longer duty required by backup equipment. Equally, a useful screening reading does not establish delivered ampere-hours. Choose the method from the intended duty and retain the limitations of each result in the maintenance decision.

How should the results be combined?

Which discharge mode answers the operational question?

Constant current is appropriate when the reference performance is specified in amperes. Its interpretation is direct: current is held at the selected level while voltage declines. Constant power suits a reference expressed in watts and can help represent equipment that demands roughly fixed input power. In that mode, current rises as voltage falls, because current equals power divided by voltage.

Constant resistance produces another behavior. Current declines with voltage, and power declines with the square of voltage for a fixed resistance. A resistor selected to draw 100 A at the start will not continue drawing 100 A near the end. This can be appropriate for some experiments, but it is not a constant-current capacity test simply because the initial setting matched the desired current.

Programmable loads can provide different regulation modes, but the modes available depend on the selected equipment. Mode availability does not automatically establish suitability for a high-energy installed string. Verify voltage, current, power, measurement isolation, and connection requirements against the approved test before selecting the configuration.

A service profile adds timing. A substation duty may combine a continuous control load with short operations at defined moments. The cumulative Ah can be modest while the final operation still causes unacceptable voltage depression. The useful output is therefore a trace with the critical events marked, not merely one accumulated total.

Separate two objectives in the work order. A full capacity test compares performance with an appropriate rating. A service-duty test determines whether a specified load sequence can be completed within the installation’s voltage limits. They can inform one another, but passing the service profile does not automatically demonstrate full rated capacity. Failing it also requires checking the load assumptions, conductors, and equipment limits before attributing everything to the cells.

What can the discharge curve reveal about weak cells and system losses?

String voltage is an aggregate. One weak cell or block can reach its limit while stronger units keep the total voltage looking acceptable. Individual monitoring makes this visible. It also allows the operator to distinguish a gradual group decline from one unit diverging sharply near the endpoint.

The voltage curve can suggest where to investigate, but it does not uniquely identify a failure mechanism. A low block could be poorly charged, internally degraded, or affected by a connection problem. Check the physical measurement locations and the relevant connections before treating a plotted difference as an internal cell defect. Keep the original trace so subsequent work can be compared with the initial observation.

Measurement location matters particularly at high current. An illustrative total lead resistance of 5 milliohms produces a 0.5 V drop at 100 A and dissipates 50 W. If cutoff is based on the load terminals, those losses can stop the test before the battery reaches the intended battery-terminal endpoint. If the operational concern is voltage at distant equipment, the same drop is part of the service problem and should be measured there as well.

Lithium packs add a protection boundary. Pack voltage alone may not explain why discharge ended: the battery management system can disconnect when an individual cell reaches its permitted limit. Record the BMS event and relevant cell information rather than assuming a sudden stop means all cells reached the same state. Protection settings must follow the actual pack documentation.

For NiCd, use the selected model’s discharge reference and preparation instructions. Stationary nickel batteries can be designed for different operating duties. Identify the actual product, its rated performance point, and the supported test method rather than assigning one discharge profile to every battery with the same chemistry label.

A partial discharge also needs careful wording. Suppose a planned four-hour test is stopped after one hour because the maintenance window closes. The delivered Ah and the recorded cell behavior are valid observations for that hour. Extrapolating the early voltage slope to claim four-hour capacity is a different assertion and requires a justified method. End-of-discharge behavior can change sharply, and a weak unit may not diverge until later. Record the actual stopping reason and label the result as partial. If the original objective was full capacity, schedule the remaining authorized assessment with appropriate preparation rather than filling the missing interval with a straight line on the graph. This protects the distinction between measured evidence and an estimate.

How should the result support a maintenance decision?

Begin with the question the test was authorized to answer. State whether it was a rating comparison, a service-duty demonstration, or a diagnostic partial discharge. Then report the measured quantity, measurement boundary, conditions, and stopping reason. A clear sentence such as “the string supplied the specified profile until the monitored block endpoint” is more useful than an unexplained pass label.

Interpret comparisons only after confirming that the conditions match or that an approved method addresses the differences. If capacity changed since the previous test, review temperature, preparation, endpoint, and current regulation before declaring deterioration. If those conditions were consistent, the trend becomes stronger evidence and can be considered alongside cell behavior and inspection findings.

Replacement thresholds belong to the applicable maintenance policy, duty margin, manufacturer guidance, and verified governing procedure. There is no single percentage that answers every industrial application. A string may retain useful rated capacity yet have inadequate margin for a revised load, while another may meet today’s duty but require planned replacement because of its trend and operational importance.

Conclusion: A discharger demonstrates delivery under recorded conditions. Preserve the current, voltage, time, endpoint, temperature, and measurement boundary, then compare them with the right reference. Use ohmic trends to guide investigation and service profiles to test operational demands. The strongest maintenance decision explains both the measured result and the conditions that make it meaningful.