Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
A multimeter across the two terminals of a car battery gives you one number, the open circuit voltage, and that number is the cheapest early warning of a battery going flat on you. It is also the number the internet is most confident and least careful about. Almost every page you will find says 12.6 volts means fully charged. That is East Penn's figure for a flooded battery, and it is not what Odyssey or Optima say about theirs — on Odyssey's own AGM2 table, 12.6 volts is 75 percent charged, not 100. This page shows you how to take the reading without hurting yourself, and then hands you to the manufacturer of the battery you actually own.
The hazards are real and they come from the battery, not the meter. Every lead-acid battery makes explosive hydrogen gas, and every one of them contains sulfuric acid that will burn skin and destroy clothing — East Penn's own document says both in capital letters, and OSHA's battery standard exists for the same reason. Eye protection goes on before you open the hood, not after. This page also stops where honest DIY stops: a voltage reading does not prove a battery can still start an engine. That is a load test, it needs equipment you probably do not have, and East Penn says plainly that it is the best way to find out.
Necessary equipment
| Item | Note |
|---|---|
| Digital multimeter | The multimeter must have a DC volts range. |
| Safety glasses or goggles | Use the eye protection near a battery at all times. |
| Gloves | Gloves give protection from the acid in the battery. |
| Flashlight optional | The light in the engine compartment is often low. |
| Pen and paper optional | Record the value and the date of the measurement. |
| Baking soda material | East Penn tells you to make acid safe with a paste of baking soda and water. |
Prepare the vehicle and yourself
Estimated 2 min
- Stop the vehicle on a level surface away from traffic.<1mReport a problem with step 1, Prepare the vehicle and yourself
- Set the parking brake.<1mReport a problem with step 2, Prepare the vehicle and yourself
- Stop the engine.<1mReport a problem with step 3, Prepare the vehicle and yourself
- Set the ignition to off.<1mReport a problem with step 4, Prepare the vehicle and yourself
- The lamps, the radio and the climate control must be off.
- A load on the battery gives a value below the open circuit voltage.
PUT ON THE EYE PROTECTION BEFORE YOU GO NEAR THE BATTERY.
Hydrogen gas is a product of a lead-acid battery. An explosion at the battery can blind you.
- Put on the eye protection.<1mReport a problem with step 5, Prepare the vehicle and yourself
- Put on the gloves.<1mReport a problem with step 6, Prepare the vehicle and yourself
KEEP FLAMES, SPARKS AND CIGARETTES AWAY FROM THE BATTERY.
Hydrogen gas can explode. Do this work in a space with good ventilation.
- Open the hood.<1mReport a problem with step 7, Prepare the vehicle and yourself
Do a visual inspection of the battery
Estimated 2 min to 4 min
- Find the battery.<1mReport a problem with step 1, Do a visual inspection of the battery
- Some vehicles have the battery below the load floor or below a seat.
- The vehicle documentation gives the position of the battery.
DO NOT TOUCH A BATTERY THAT HAS A CRACK OR A LEAK.
There is sulfuric acid in a lead-acid battery. The acid can burn your skin. The acid can destroy your clothes.
- Look at the case and the cover of the battery for a crack.<1mReport a problem with step 2, Do a visual inspection of the battery
- Look at the terminals for corrosion or damage.<1mReport a problem with step 3, Do a visual inspection of the battery
- East Penn also tells you to look for a leak at the seal between the case and the cover.
- Look at the cable connections for movement.<1mReport a problem with step 4, Do a visual inspection of the battery
- If you find damage or a leak, stop this procedure.<1mReport a problem with step 5, Do a visual inspection of the battery
- Speak to a vehicle repair shop about a damaged battery.
- If the battery gives an odor, stop this procedure.<1mReport a problem with step 6, Do a visual inspection of the battery
- Odyssey tells owners to remove a battery with an odor from service.
Give the battery a rest period
Estimated under a minute
Source: ODYSSEY Battery AGM2 Technical Manual · tier 1 · accessed 2026-07-29
Source: ODYSSEY Battery AGM2 Technical Manual · tier 1 · accessed 2026-07-29
MEASURE AT LEAST FOUR HOURS AFTER THE LAST CHARGE.
Odyssey tells owners that a measurement is not accurate without an adequate rest period.
- Wait at least four hours after the last charge of the battery.<1mReport a problem with step 1, Give the battery a rest period
- Odyssey gives at least four hours after a charge, and at least 30 minutes after a discharge.
Measure the open circuit voltage
Estimated 2 min to 3 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Set the multimeter to the DC volts function.<1mReport a problem with step 1, Measure the open circuit voltage
- The symbol for DC volts is the letter V with a straight line above it.
- If the multimeter has a manual range, select the 20 V range.
- Consult the documentation for your multimeter.
DO NOT LET A TOOL TOUCH BOTH TERMINALS AT THE SAME TIME.
A short circuit across the terminals makes sparks and much heat. Sparks and heat can burn you.
- Touch the red probe to the positive terminal of the battery.<1mReport a problem with step 2, Measure the open circuit voltage
- The positive terminal has the symbol +.
DO NOT PUT YOUR FACE ABOVE THE BATTERY DURING THE MEASUREMENT.
Gas from the battery can injure your eyes.
- Touch the black probe to the negative terminal of the battery.<1mReport a problem with step 3, Measure the open circuit voltage
- The negative terminal has the symbol -.
- Wait until the value on the display is stable.<1mReport a problem with step 4, Measure the open circuit voltage
- Read the value on the display.<1mReport a problem with step 5, Measure the open circuit voltage
- A negative value shows that the probes are on the opposite terminals.
- Remove the probes from the terminals.<1mReport a problem with step 6, Measure the open circuit voltage
Read the value against the correct table
Estimated 4 min to 8 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Source: Basic Rules for Battery Care & Maintenance (form 0004) · tier 1 · accessed 2026-07-29
Source: Basic Rules for Battery Care & Maintenance (form 0004) · tier 1 · accessed 2026-07-29
Source: Basic Rules for Battery Care & Maintenance (form 0004) · tier 1 · accessed 2026-07-29
Source: ODYSSEY Battery AGM2 Technical Manual · tier 1 · accessed 2026-07-29
Source: ODYSSEY Battery AGM2 Technical Manual · tier 1 · accessed 2026-07-29
Source: OPTIMA Battery Charging · tier 1 · accessed 2026-07-29
Source: OPTIMA Battery Charging · tier 1 · accessed 2026-07-29
- Find the type of your battery on the label.1mReport a problem with step 1, Read the value against the correct table
- A flooded battery, an AGM battery and a deep cycle battery have different tables.
- Compare your value with the table from the manufacturer of your battery.1mReport a problem with step 2, Read the value against the correct table
- In the table from East Penn, 12.6 V is 100 percent on a flooded 12 V battery.
- In the table from East Penn, 12.8 V is 100 percent on an AGM battery.
- In the Odyssey table, 12.9 V is 100 percent on an AGM2 battery.
- In the Odyssey table, 12.6 V is only 75 percent on an AGM2 battery.
- Optima gives a range of 12.6 V to 12.8 V for a full starting battery.
- Optima gives a range of 13.0 V to 13.2 V for a full deep-cycle battery.
- This site does not average these values.
- If your value is below the value from your manufacturer, charge the battery.<1mReport a problem with step 3, Read the value against the correct table
- East Penn tells you to charge a battery below 12.4 V.
- Read the instructions from the charger manufacturer before you charge the battery.
- After a charge, wait the rest period again.<1mReport a problem with step 4, Read the value against the correct table
- Do a new measurement.1mReport a problem with step 5, Read the value against the correct table
- If the value stays low, then speak to a vehicle repair shop about a load test.<1mReport a problem with step 6, Read the value against the correct table
Record the result
Estimated under a minute
- Record the date, the value and the type of the battery.<1mReport a problem with step 1, Record the result
- A record of each measurement shows a slow decrease over time.
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- open circuit voltage
- Also called: resting voltage, static voltage, battery voltage, OCV, what my battery reads, no load voltage
- multimeter
- Also called: voltmeter, volt meter, DMM, tester, digital multimeter, electrical tester
- state of charge
- Also called: SOC, charge level, how full the battery is, battery percentage
- lead-acid battery
- Also called: car battery, starter battery, starting battery, SLI battery, 12 volt battery, auto battery
- AGM battery
- Also called: absorbed glass mat battery, sealed battery, maintenance free battery, VRLA battery
- load test
- Also called: battery load test, cranking test, drop test, getting the battery tested at the parts store
- terminal
- Also called: battery post, post, battery terminal, the two lumps on top
When to use a professional
Stop if the case of the battery has a crack. Stop if the battery has a leak. Stop if the battery gives an odor. Speak to a vehicle repair shop about that battery. Do not touch a damaged battery.
This procedure gives the open circuit voltage only. The open circuit voltage does not tell you if the battery can start the engine. East Penn tells you that a load test is the best method to find the electrical performance of a battery. A load test needs a variable load tester and training. Speak to a vehicle repair shop for a load test.
This procedure is for a 12 V vehicle battery only. Do not use this procedure on the electrical installation of a building. Speak to a qualified electrician about the electrical installation of a building.
Common questions
What voltage should a car battery read?
There is no single right answer, and that is the most useful thing on this page. East Penn's chart gives 12.6 volts as 100 percent state of charge for a flooded 12 volt battery and 12.8 volts for an AGM. Odyssey's AGM2 technical manual puts 100 percent at 12.9 volts and calls 12.6 volts a 75 percent charge. Optima says a fully charged starting battery reads about 12.6 to 12.8 volts, but a fully charged YELLOWTOP deep-cycle battery reads 13.0 to 13.2. Take the reading, then look up the table published by whoever made the battery in your car — the name is on the label.
Is 12.4 volts bad on a car battery?
It is the point where East Penn tells you to act: their document says batteries should be boost charged if the open circuit voltage reading is below 12.4 volts, and that you cannot load test a battery below that figure without charging it first. On Odyssey's AGM2 table 12.6 volts is already only 75 percent and 12.2 volts is 50 percent, so 12.4 sits between those two. Either way, 12.4 is a battery worth charging rather than a battery that is fine.
Why do I have to wait four hours before measuring?
Because a battery that has just been charged holds a surface charge that reads higher than the real state of charge, and one that has just been discharged reads lower. Odyssey's manual is specific: the reading must be taken at least four hours after the battery finishes charging, or at least 30 minutes after it finishes a discharge, and measurements will not be accurate if adequate rest time is not allowed. Measuring the moment you take the charger off is the most common way to get a falsely reassuring number.
Do I need to disconnect the battery to test it with a multimeter?
No. This is a measurement taken across the terminals with everything switched off, and nothing needs to come apart. It is also safer that way: East Penn's safety instructions say not to connect or disconnect live circuits, precisely because making or breaking a connection at a battery is what produces the spark. Disconnecting also loses the settings stored in the car's electronics on many vehicles.
Can I test the battery with the engine running?
That is a different test. Measuring across the terminals with the engine started tells you about the charging system rather than the battery, and it is done in a compartment with moving parts and belts a few inches from your hands. This page deliberately stops at the static measurement. If you want the charging system checked, a repair shop will do it in a couple of minutes and without putting your fingers next to a drive belt.
My battery reads 12.7 volts but the car still will not start. What now?
Voltage tells you roughly how charged the battery is. It does not tell you whether the battery can still deliver hundreds of amps for the few seconds a starter needs, and a battery with failing plates can sit at a healthy voltage and collapse the instant it is asked to work. East Penn says it directly: a load test is the best way to determine if the battery is delivering adequate electrical performance. That needs a variable load tester, so it belongs with a shop rather than in a driveway.
Is testing a car battery actually dangerous?
The meter is harmless. The battery is not. East Penn's own document states that all batteries generate explosive hydrogen gas and that sparks, flames and cigarettes must be kept away from batteries at all times, and that batteries contain corrosive sulfuric acid that can destroy clothing and burn the skin. OSHA's battery standard at 29 CFR 1926.441 requires ventilation to prevent the accumulation of an explosive mixture, and face shields, aprons and rubber gloves for anyone handling batteries, with eye-drenching facilities within 25 feet. That standard governs workplaces rather than driveways, but the hazard it describes is the same one sitting under your hood.
Related procedures
- What battery does my car take, and how do I read the group size and CCA rating before I buy a replacement
- Work out why a battery is flat every morning when the battery itself tests good
- How do I test a fuse or check if a wire is broken with a multimeter
- How do I replace the fuse in my multimeter, and how do I know my test leads are still safe to use
- How to test a dryer thermal fuse with a multimeter
- Find the date code on a vehicle battery and work out how old it actually is
- How should I store my cordless mower and trimmer batteries over the winter?
Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 1Basic Rules for Battery Care & Maintenance (form 0004)
- Tier 1ODYSSEY Battery AGM2 Technical Manual
- Tier 1OPTIMA Battery Charging
- Tier 129 CFR 1926.441 - Batteries and battery charging
Revision history
Current version 1 · updated 2026-07-29 · next review due 2027-07-29
- v1 · 2026-07-29First draft. The headline number readers want is exactly the number three tier-1 battery manufacturers disagree on, so it publishes as a conflict under rule 2: East Penn 12.6 V for 100 percent on a flooded battery, Odyssey 12.9 V for 100 percent and 12.6 V for 75 percent on AGM2, Optima 12.6 to 12.8 V for a starting battery and 13.0 to 13.2 V for a deep-cycle battery. No average is taken and the reader is routed to the manufacturer named on the battery. The four-hour and 30 minute rest periods come from the Odyssey AGM2 technical manual. The hazards rest on East Penn's own safety text and on 29 CFR 1926.441. Load testing and charging-system testing are outside the boundary.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.