Batteries and DC systems
A battery capacity load test discharges a battery bank under controlled conditions and measures how much energy it can really deliver, compared with its rating. It is the only test that proves capacity directly, and it is used at acceptance, at intervals in service and when a battery nears the end of its life.
A battery capacity load test, also called a discharge test, connects a battery bank to a controlled load and removes energy from it at a steady rate, while the terminal voltage of the string and often of each cell is monitored. The test continues until the string reaches its specified end voltage or the planned duration is complete, and the result is expressed as the capacity the battery delivered compared with its rated capacity. Several variants exist. An acceptance test, run on a new installation, proves the battery meets its rating. A performance test, repeated on a schedule or when a battery ages, tracks capacity against the original rating. A service test applies the actual duty cycle that the battery must supply to its connected loads, to prove it can carry that duty. The test applies to flooded lead-acid, valve-regulated lead-acid and vented nickel-cadmium batteries in station, substation, generating-plant and control-power applications, and similar discharge testing is used on battery energy storage systems. Whether the test is performed, and which variant is used, is set by the project specification and the applicable battery standard.
A battery can look perfect on every other measurement and still be unable to deliver its energy when needed. Float voltage, specific gravity, ohmic readings and visual inspection are useful indicators, but only a discharge test shows capacity directly. That matters because the battery exists for the moments when everything else has failed: it must carry breaker control, protective relays, emergency lighting, communications and critical loads through an outage. Plate sulfation, grid corrosion, loss of active material, dry-out, internal shorts and failing cells all reduce capacity, often without obvious warning. A bank that is short of capacity may let a substation lose protection at the worst time. Capacity falls as batteries age, so periodic discharge tests allow replacement to be planned before the capacity drops below the level the loads require, which is far cheaper than an emergency replacement or a failed protection scheme. At acceptance, the test proves that what was delivered and installed is what was purchased, while the cells are still under warranty.
A battery stores energy chemically, and its capacity is the amount of charge it can deliver under a specified discharge rate, temperature and end voltage. Capacity depends strongly on the rate of discharge: a battery discharged quickly delivers less than the same battery discharged slowly, which is why ratings are stated for a particular duration. It also depends on temperature, because cold slows the chemical reactions and reduces delivered capacity, so results are corrected to a reference temperature. During a discharge the voltage of each cell falls gradually and then drops faster as the active material is exhausted. The test stops when the weakest cells reach the end-of-discharge voltage defined for the cell type, because discharging further would damage the battery and risk cell reversal. A healthy battery holds a satisfactory voltage for the full duration, while an aged battery reaches the end point early. The capacity result is the time to the end voltage divided by the rated time, adjusted for temperature, and it is interpreted as a fraction of the nameplate rating.
The battery is fully charged and given the equalizing or settling period that the manufacturer or the standard specifies, so that the test begins from a known state. The load is disconnected from the charger and the battery in the manner the procedure requires; for a station battery the test is often scheduled during an outage window or with temporary power arranged for critical loads. Prior to the test, connections are inspected and intercell connection resistance is considered, because poor connections cause heating and distort the result. A load bank, with a controllable and metered current, is connected across the string. Current is set to the value derived from the battery rating or the duty cycle and held constant, or applied in the profile of the service test. The string voltage, the individual cell or unit voltages and the current are recorded at defined intervals, with more frequent readings near the end of the discharge, along with the electrolyte or ambient temperature. The test is stopped when the end voltage is reached on the string or on a cell, as the procedure requires. Safety includes ventilation and hydrogen precautions for vented cells, protective equipment for electrolyte and arc-flash hazards, and a plan to restore the battery to charge promptly after the discharge.
The record identifies the battery, its type, rating and age, and the test type performed. It documents the discharge current, the time to the end voltage, the voltage readings of the string and of cells or units at the chosen intervals, and the temperature at the start and during the test. The load bank and instruments used are noted, along with the state of charge and charging history before the test and the as-found condition of the connections. The calculated capacity, corrected for temperature and expressed against the rating, is recorded along with any cells that reached the end voltage first, any abnormal behavior and the recharge performed afterward.
Capacity results are judged against the battery's rated capacity and against the criteria in IEEE 450-2020 for vented lead-acid batteries, and against the corresponding recommended practices for valve-regulated and nickel-cadmium batteries. The calculated capacity, corrected to the reference temperature, is compared with the acceptance level for a new battery and with the replacement threshold for one in service, and the trend from earlier tests is as significant as any single figure. A service test is judged pass or fail on whether the battery carries its required duty without the voltage falling below the minimum the loads can tolerate. Individual cells that reach end voltage early, or that fall sharply, point to weak cells. The required levels are set by the named documents and the manufacturer, and ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for the test within their battery sections, so consult them for the governing criteria.
An acceptance capacity test is usually run on a new bank, often after the battery has been installed and charged and before it is relied upon, and it may be performed in the factory as well. A performance test in maintenance is run on a schedule that tightens as the battery ages, and sooner if other indicators decline. A service test is the most direct proof of fitness for the actual load. Flooded lead-acid, valve-regulated and nickel-cadmium batteries all have different end voltages, rates and recommended practices, so the procedure must match the technology. Valve-regulated batteries are particularly sensitive to aging and may need earlier and more frequent testing. Battery energy storage systems are discharge tested through their inverter and battery management system, with the available energy measured against the rating. The test is disruptive, so it is planned with operations and protected from interruption.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Voltage, specific gravity and ohmic readings are useful indicators, but they only estimate condition. A discharge test is the only way to measure how much energy the battery can actually deliver, which is what the load needs in an outage.
A performance test measures capacity against the battery rating using a constant discharge. A service test applies the real duty cycle the battery must supply, to prove it can carry that specific load. Each answers a different question.
A properly run test, stopped at the specified end voltage and followed by a prompt recharge, is considered safe and is a normal part of battery maintenance. Over-discharge or delays before recharging can harm the cells, so the procedure must be followed.
The interval comes from the battery standard, the manufacturer and the owner's program, and it shortens as the battery ages or if other indicators decline. Check ANSI/NETA MTS-2023 and the applicable battery practice for the schedule.
Capacity varies with temperature, because cold reduces the energy a battery can deliver. Recording temperature lets the result be corrected to a reference value so that tests taken in different conditions can be fairly compared.
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