Free tool · Designed in Canada
Car battery voltage checker
Enter your 12 V battery's resting voltage and its type, and see roughly how charged it is and what to do. The numbers are East Penn Manufacturing's published chart, the same one our car battery voltage guide uses.
An estimate, not a battery test. Resting voltage shows how charged a battery is, not how healthy it is. Only measure after the car has been off for 12 hours or more, with nothing charging or drawing current.
Check a reading
Resting voltage chart
This is the open-circuit voltage: the battery measured after a rest with nothing charging or drawing from it. The figures are from East Penn Manufacturing's battery care guide (Form 0004), which gives separate columns for flooded and AGM 12 V batteries.
| State of charge | Flooded 12 V | AGM 12 V |
|---|---|---|
| 100% | 12.6 V | 12.8 V |
| 75% | 12.4 V | 12.6 V |
| 50% | 12.2 V | 12.3 V |
| 25% | 12.0 V | 12.0 V |
| 0% | 11.8 V | 11.8 V |
A reading between two rows is between those states of charge: 12.3 V on a flooded battery is between 50% and 75%. Other makers publish slightly different numbers (chemistry and temperature shift the curve), so treat the band as a guide, not a precise percentage.
What to do at each reading
| Resting reading | What it means | What to do |
|---|---|---|
| Above 13.0 V | Probably not a resting reading | Let it rest and measure again. No resting figure in the sources goes above 13.0 V (Optima's YELLOWTOP deep-cycle batteries, about 12.8–13.0 V when full); a battery that was just driven or charged reads high for a while. |
| 12.6 V or more (AGM: 12.8 V) | Fully charged | Nothing. |
| About 12.4 V (AGM: 12.6 V) | About 75% | Charge it. East Penn recommends a boost charge below 12.4 V, and Optima says sulfation begins when a battery is left below 12.4 V. |
| About 12.2 V (AGM: 12.3 V) | About 50% | Charge it soon, and find out why it's low. |
| 12.0 V or less | 25% or less | Charge it now. MINI says letting a battery drop below 12.0 V permanently reduces its life expectancy. |
| Below 10.5 V | Deeply discharged | Optima says most basic chargers are built not to charge a battery below 10.5 V; it may need a charger with a recovery function, or a shop. |
Safety when charging: use a charger suited to your battery type. East Penn warns that charging an AGM battery on a charger that exceeds 15.4 V, even once, can greatly shorten its life. Never charge or jump-start a battery that may be frozen.
How to take a resting reading
- Let the car sit for 12 hours or more. Engine off, nothing charging, nothing left on. MINI's service instructions say an accurate reading is only guaranteed after at least 4 hours; we ask for longer to be safe. A battery that was just driven or charged reads high for a while (surface charge).
- Protect your eyes and avoid sparks. Batteries give off explosive hydrogen gas, and the acid inside burns skin.
- Set a digital multimeter to DC volts (the 20 V range if it isn't auto-ranging).
- Red probe on the positive (+) post, black on the negative (−). Read the posts themselves, not the cable clamps, if you can.
With the engine running you are reading the charging system, not the battery. Optima gives about 13.3–15.0 V, typically 14–14.8 V at 10–27 °C (50–80 °F). That reading tells you nothing about state of charge.
What this checker can't tell you
Resting voltage measures charge, not health. A worn battery can recover to 12.6 V overnight and still collapse under the starter's load. East Penn calls a load test on a fully charged battery "the best way to determine if the battery is delivering adequate electrical performance." If a battery keeps reading low after the car sits, the likely causes are a parasitic drain, too many short trips, or simply sitting too long.
Method
Lookup. The checker rounds your reading to 0.01 V and looks it up in East Penn's Form 0004 resting-voltage table for the type you pick. Flooded: 12.6, 12.4, 12.2, 12.0 and 11.8 V for 100, 75, 50, 25 and 0%. AGM: 12.8, 12.6, 12.3, 12.0 and 11.8 V. At or above the 100% row, it says about 100%. Otherwise it takes the highest row at or below your reading: exactly on a row is "about" that percentage (12.4 V flooded is about 75%), and between two rows is "between" them. There is no interpolation. From 10.5 V up to 11.8 V it says flat (about 0%); below 10.5 V, deeply discharged. Above 13.0 V it says the reading is probably not a resting one, because no resting figure in the sources goes higher (Optima: about 12.8–13.0 V for a full YELLOWTOP). Readings below 1 V or above 20 V are treated as typing mistakes.
Assumptions. The reading is an open-circuit voltage taken on the posts after 12 hours or more with nothing charging or drawing current (MINI says at least 4 hours). No temperature correction is applied. The result is a state-of-charge band, not a measure of battery health.
Sources. The table is East Penn Manufacturing's Form 0004; the advice thresholds are East Penn's and Optima's 12.4 V, Varta's 12.5 V, MINI's 12.0 V and Optima's 10.5 V, all listed below and cited in our car battery voltage guide.
Method version 1, .
Sources
- East Penn Manufacturing, Battery Care & Maintenance (Form 0004): the resting-voltage chart, the 12.4 V boost-charge point, the load test, the AGM charger limit and the safety warnings.
- OPTIMA Batteries, Battery Charging FAQs: full-charge readings, the running (charging) range and the 10.5 V charger cut-off.
- OPTIMA Batteries, Why Does a Car Battery Drain Even When Not in Use?: sulfation below 12.4 V.
- MINI USA (via NHTSA), SI M61 01 02: Battery Maintenance Requirements: the 4-hour rest and the 12.0 V life-expectancy warning.
- VARTA Automotive (Clarios), How to Store Vehicle Batteries: charge to 12.7–12.8 V before storage, recharge at 12.5 V or less.
- Battery Council International, BCIS-101 Glossary of Common Battery Terms: open-circuit voltage.
The full explanation, with charging and cranking voltages: What is a normal car battery voltage?