How a Battery Tester Works

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How does a battery tester work? It puts the battery into a known condition and reads one result: the voltage at rest, the voltage while a load draws current, or the conductance of the battery’s internal path. Each method answers a different question, and none of the three reports the full condition of a battery on its own. This explainer covers the principle of each method, what the sources we opened say it measures, and what it cannot show.

Electrical safety

Mains work can kill. Confirm that any meter’s CAT rating suits the circuit, and find that rating in the listing or the manufacturer’s manual. Panels and service wiring are the job of a licensed electrician.

The sources we opened back this up. OSHA’s electrical standards exist to guard against electric shock, electrocution, fires and explosions, and its rule for energized equipment limits work on circuit parts that have not been de-energized to qualified persons. An Illinois workplace safety article explains that a current passes through your body when you touch an energized wire and also a neutral wire, another grounded device, or the path back to ground, and that wet skin or a damp location lets it flow more readily.

This explainer covers low-voltage battery work only. It names no meter, so it gives no CAT rating for any meter.

Hydrogen gas

Vented lead-acid batteries can contain an explosive mixture of hydrogen gas, according to EnerSys’s flooded lead-acid manual, which says to keep smoke, flames and sparks away from the batteries. The Crown Battery safety data sheet, hosted by the University of Nebraska-Lincoln, says charging may form an explosive air and gas mixture, and gives a lower explosive limit of 4.1 percent for hydrogen in air. OSHA calls for ventilation that carries away fumes from gassing batteries, and for precautions against open flames, sparks and electric arcs in charging areas.

Acid

Lead-acid batteries contain sulfuric acid electrolyte. The Crown data sheet says acid causes severe skin burns and eye damage. Trojan Battery’s maintenance page advises protective clothing, gloves and goggles when handling batteries and electrolyte, and EnerSys says to wear rubber gloves and boots. OSHA requires facilities to provide for flushing and neutralizing spilled electrolyte.

Short circuits

A battery can deliver high short circuit current, as the EnerSys manual warns. The same manual advises tools with insulated handles, removing jewelry that could produce a short circuit, and keeping metal parts off the top of batteries. OSHA says tools and other metallic objects must be kept away from the top of uncovered batteries, and Crown says not to let metal touch both terminals at once.

Quick answer

A voltage test reads the battery at rest. For lead-acid batteries that reading tracks state of charge after a rest, but it cannot estimate state of health.
A load test draws a set current and watches how the voltage holds. Battery University prefers it for batteries that power DC loads.
A conductance test sends a small AC signal at one frequency and reads conductance. It cannot read capacity.
The sources we opened give no single pass or fail limit that covers all batteries.

The three methods at a glance

Method	What the tester reads	What the sources say it cannot show
Voltage	Terminal voltage, read at rest	State of health (Battery University, test methods overview)
Load	Voltage while a set current is drawn	State of charge or capacity for the DC load method (BU-902); capacity for the carbon pile method (BU-902a)
Conductance	Conductance or impedance at one frequency	Capacity (BU-902a); the CCA reading also needs a state of charge of 70 percent or higher

Voltage method

A voltage tester reads the voltage across the battery’s terminals, and for lead-acid that reading means something only after the battery has rested. Battery University says voltage-based state-of-charge estimates work reasonably well for a rested lead-acid battery, while a charge or discharge distorts the reading. For a general look at voltage checks, see our battery voltage tester guide.

Battery University reproduces a BCI standard for starter batteries with antimony plates. It lists 12.65 V at 100 percent state of charge, 12.45 V at 75 percent, 12.24 V at 50 percent, 12.06 V at 25 percent and 11.89 V at 0 percent, measured at 26 °C after a 24-hour rest. These are state-of-charge references, not pass or fail limits.

The sources differ on rest time. Battery University gives a minimum of four hours of open circuit rest, and says manufacturers recommend 24 hours for lead-acid. Trojan’s bank check uses about one hour. Check the manufacturer’s instructions for the battery at hand.

Parasitic loads in modern vehicles push the battery into a quasi-closed circuit condition, so the reading is no longer a pure open circuit voltage. Trojan’s FAQ says voltage alone does not confirm a problem. It also says specific gravity, read with a hydrometer, is the most accurate guide to a flooded battery’s state of charge. The hydrometer method sits outside the three methods covered here.

Trojan’s FAQ gives a spread figure for batteries wired together in one system. A spread above .15 volts for 6-volt batteries, or above .22 volts for 12-volt batteries, indicates a problem. That figure compares batteries with one another. It is not a pass or fail limit for a single battery.

Battery University also calls the voltage method impracticable for nickel- and lithium-based batteries, whose discharge curves are flat. The sources we opened give no voltage substitute for them.

Load method

Battery University notes that conventional tests measure stored energy through a full discharge, which takes time and stresses the battery. A load test draws a known current and watches the voltage, so the result depends on how the voltage holds up under that current.

Battery University’s DC load method reads the open circuit voltage, then takes a second reading with the load attached, and Ohm’s law turns the difference into a resistance. It calls the load test the preferred method for batteries that power DC loads. For a multimeter-based approach, see our guide to load testing a battery with a multimeter.

For starter batteries, the carbon pile method loads a battery at a state of charge of 70 to 100 percent with half its rated cold cranking amps (CCA) for 15 seconds, at 10 °C (50 °F) or warmer. The article’s worked example is a 500 CCA battery that discharges at 250 A and passes if its voltage stays above 9.6 V. SAE J537, which the same article cites, requires a 500 CCA battery to deliver 500 A at -18 °C (0 °F) for 30 seconds without dropping below 7.2 volts.

A 2001 Totalflow bulletin for Hawker Cyclone and Genesis batteries puts a 12-ohm resistor across a battery it assumes is fully charged, reads the voltage after 30 seconds, and treats a reading above 12 V as 50 percent or more capacity. That threshold is written for those lines only.

EnerSys’s manual describes a rate-adjusted discharge test. Capacity is the test discharge time to a set voltage divided by the rated time to that voltage, multiplied by 100.

Load results have limits. Battery University says the DC load method gives no state-of-charge or capacity estimate, and it reads a good, partly charged battery much like a marginal, fully charged one. The carbon pile method likewise cannot estimate capacity.

Conductance method

Conductance is an ohmic measurement. Vertiv’s 2003 paper, written by Midtronics staff, calls it an Ohmic technique. It says normal aging can sulfate the plates and shed active material, which lowers conductance over a cell’s life, and it links reduced conductance to a lower expected capacity.

Battery University’s AC conductance method injects one frequency, about 90 hertz for a 70 to 90 Ah lead-acid battery, or 80 to 90 hertz in its CCA article. Most single-frequency testers that read CCA are scalar, which means they compare one reading with a reference value. For one 18650 lithium cell, it gives about 36 milliohms at 1,000 hertz and roughly 110 milliohms with a DC load, and it says both readings are correct.

Battery University says AC conductance cannot read capacity and needs a state of charge of 70 percent or higher. It calls a battery tester an estimation device, and says resistance-based instruments can identify a dying or dead battery. Claiming that such a tester also estimates capacity is misleading.

A 2013 Ehime University paper measured AC impedance and phase detection on lead-acid batteries under constant-resistance discharge and normal charging. Its authors expect impedance to characterize capacity, including state of charge and state of health. It is research, not a review of any tester.

Reading the results

The sources give reference points, and each applies to one method and one kind of battery: the rested state-of-charge table for antimony starter batteries, the 9.6 V and 7.2 V figures in the CCA examples, the 12 V line for Hawker Cyclone and Genesis, and the Trojan bank spread. For conductance, the sources we opened give no absolute threshold. Vertiv compares one cell with its string. In its example, a cell at 400 siemens against an average of 1,500 siemens points to a significant loss of capacity in that cell. Rest, state of charge, temperature and chemistry move every reading, so no single figure travels from one battery to another.

Common mistakes

Reading a rested voltage as a health check. Voltage alone cannot estimate state of health, according to Battery University.
Measuring too soon after use. The rest times in the sources run from about one hour to 24 hours.
Loading a partly charged battery and treating the result as capacity. The CCA example assumes 70 to 100 percent state of charge.
Reading conductance as capacity. BU-902a says AC conductance cannot read capacity.
Applying the Totalflow 12 V line to other battery lines.
Judging a lithium cell by its voltage.
Letting metal bridge the terminals. EnerSys and Crown both warn against it.

How we chose

This explainer covers the three methods that the sources group as the common checks: voltage, load and conductance. We did not rank them. Each source is a manufacturer, battery-test maker, university, government or state agency document, or a company bulletin for a named battery line. Each was opened with curl on 2026-10-22, and each quotation was checked against the fetched text. We left out pages that returned 403, 404 or 405, including a University of Minnesota battery explosion alert and a NIOSH toolbox talk, and a dealer’s support page, because the writing rules allow retail pages only for listing facts. Seventeen sources are listed below.

What we could not verify

The sources we opened give no single pass or fail limit for every battery, chemistry or tester, and they do not say how far the figures above carry to other batteries.

Voltage references for lithium, nickel or alkaline cells. The sources we opened call the voltage method impracticable for lithium and nickel-based cells, and say nothing usable about alkaline cells.
A single rest time. Battery University and Trojan differ, and we could not confirm which applies to a given battery.
How closely conductance tracks capacity across lead-acid batteries. Vertiv links the two, while Battery University says conductance cannot read capacity.
Specific tester models. None is named here, so no claim is made about how a model reads a battery.
The Ehime University paper is research. We rely on its abstract only.

FAQ

Does a battery tester show capacity?

Not in the sources we opened. Battery University says AC conductance cannot read capacity, and it calls a battery tester an estimation device. EnerSys’s capacity test is a timed discharge, which is a separate method.

Why can a battery look fine on a voltmeter and still struggle under load?

Rest voltage shows where a battery sits at rest. A load shows how that voltage holds when current is drawn, which is the question a load test answers.

Is a conductance test the same as a load test?

No. Battery University says AC and DC readings for the same battery can differ, and that both can be correct.

Can I use the voltage method on a lithium battery?

The sources we opened call the voltage method impracticable for lithium and nickel-based cells, because their discharge curves are flat.

How long should a battery rest before a voltage reading?

The sources disagree. Battery University gives at least four hours, with 24 recommended for lead-acid, while Trojan’s bank check uses about one hour. Follow the manufacturer’s instructions for the battery you have.

What does electrical safety mean for a battery tester?

It keeps this explainer on low-voltage battery work. OSHA limits energized circuit work to qualified persons, and a battery tester does not change that rule.

Sources

Battery University, “Battery Test Methods”: https://batteryuniversity.com/article/battery-test-methods
Battery University, “Battery Testing Equipment”: https://batteryuniversity.com/article/battery-testing-equipment
Battery University, “BU-902: How to Measure Internal Resistance”: https://batteryuniversity.com/article/bu-902-how-to-measure-internal-resistance
Battery University, “BU-902a: How to Measure CCA”: https://batteryuniversity.com/article/bu-902a-how-to-measure-cca
Battery University, “BU-903: How to Measure State-of-charge”: https://batteryuniversity.com/article/bu-903-how-to-measure-state-of-charge
Battery University, “BU-905: Testing Lead Acid Batteries”: https://batteryuniversity.com/article/bu-905-testing-lead-acid-batteries
EnerSys, Flooded Lead-Acid Batteries Safety, Storage, Installation, Operation and Maintenance Manual (2016): https://www.enersys.com/globalassets/documents/product-documentation/_enersys/amer/us-fl-iom-aa-0916.pdf
Crown Battery, Lead Acid Batteries Safety Data Sheet, hosted by the University of Nebraska-Lincoln: https://cms.unl.edu/business-and-finance/university-operations/custodial/sites/unl.edu.business-and-finance.university-operations.custodial/files/media/file/crown-battery.pdf
OSHA, 29 CFR 1910.178, Powered industrial trucks: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.178
OSHA, 29 CFR 1910.333, Selection and use of work practices: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.333
OSHA, Electrical overview: https://www.osha.gov/electrical
Illinois workplace safety article, “The Ubiquitous Orange Extension Cord”: https://worksafe.illinois.gov/content/dam/soi/en/web/worksafe/articles/2025/Extension%20Cord%20Article.pdf
Trojan Battery, FAQs: https://www.trojanbattery.com/resources/faqs
Trojan Battery, Battery Maintenance: https://www.trojanbattery.com/resources/battery-maintenance
Vertiv, “Using Conductance Technology to Ensure Battery System Reliability” (2003, Midtronics authors): https://www.vertiv.com/48dfcf/globalassets/documents/battcon-static-assets/2003/using-conductance-technology-to-ensure-battery-system-reliability.pdf
Totalflow Technical Bulletin 59, Lead Acid Battery Inspection and Maintenance Procedures, hosted by ABB: https://library.e.abb.com/public/363ea195a13aae5985256ead00548186/technical%20bulletin%2059.pdf
Ehime University, “Measurements of Impedance of Lead-Acid Batteries by AC Impedance Method and Phase Detection Method” (2013): https://ehime-u.repo.nii.ac.jp/record/2000079/files/AA12158096_2013_12-138.pdf

How a Battery Tester Works