At what voltage is a car battery bad?
At baseline, a car battery is generally considered bad when its resting voltage is consistently below about 12.0 volts after a full charge and rest; a healthy 12V lead-acid battery sits around 12.6–12.8 volts. Voltage readings help diagnose charging status, but should be checked with a load test and system check for a definitive verdict.
Understanding voltage readings: what they imply
The numbers tell a story about charge, health, and charging capability, but temperature, battery age, and battery type (flooded, AGM, or gel) can shift exact values. The following ranges are typical for a standard 12V lead-acid battery when the engine is off and the battery has rested:
- 12.6–12.8 V: Fully charged and healthy
- 12.4–12.6 V: Approximately 75% charged; may need charging or topping off
- 12.2–12.4 V: Around 50% charge; a recharge is advisable
- 12.0–12.2 V: About 25–40% charge; battery is discharged and needs attention
- Below 12.0 V: Significantly discharged; may still be recoverable with proper charging, but capacity is often reduced
- Below about 11.6–11.8 V: Deeply discharged; likely damaged from sulfation or aging; replacement is commonly recommended
Conclusion: Resting voltage is a useful quick check, but it doesn’t tell the whole story about a battery’s health—especially for older batteries or those that are AGM. A full diagnosis should include a load test and an inspection of the charging system.
Voltage during cranking and under load
When you try to start the car, the battery must deliver a surge of current. The voltage during cranking and under load reveals more about capacity than resting voltage alone. Typical expectations:
- During starting, a healthy battery should not fall much below about 9.6–10.0 V. If it dips lower, it’s a strong sign the battery cannot supply the required current and may be failing.
- Under a standard load test (often 15–30 seconds at a defined current), a healthy battery should maintain voltage above roughly 9.5–10.0 V. If it collapses well below this, replacement is likely needed.
Conclusion: A battery that cannot sustain adequate voltage under cranking or load is considered bad, even if resting voltage looked reasonable after charging.
Voltage with the engine running: checking the charging system
Once the engine runs, the alternator should push the battery back toward a full charge. The resting (off) voltage isn’t the guide here—the running voltage is. Normal charging voltage for most cars is:
- 13.8–14.4 V while the engine runs and the alternator is charging
- Below about 13.0 V during operation suggests the alternator may not be charging properly, or there could be a wiring/connection issue
Conclusion: If the alternator isn’t maintaining the expected charging voltage, the problem could lie with the alternator, wiring, or a degraded battery that won’t accept charge efficiently.
Practical steps to diagnose a suspected bad battery
If you suspect the battery is failing, use a structured approach to confirm before replacing components.
- Fully charge the battery with a reputable charger, then let it rest for several hours and measure the resting voltage again.
- Perform a load test or have a professional test the battery’s capacity under load to see if it can sustain current.
- Inspect connections, cables, and terminals for corrosion or looseness; clean and tighten as needed.
- Consider the battery’s age. Most 12V lead-acid batteries last 3–5 years under typical conditions; older batteries are more likely to fail even if readings appear reasonable.
Conclusion: A combination of resting voltage, load testing, and charging-system checks provides the most reliable verdict on battery health.
Summary
Voltage is a practical diagnostic tool for car batteries but must be interpreted in context. A healthy, fully charged battery typically reads about 12.6–12.8 V with the engine off; anything consistently below 12.0 V after charging and resting raises red flags for discharge or damage. Voltage that collapses under cranking or load or charging voltage that remains below the normal 13.8–14.4 V range when the engine runs also signals issues with the battery or charging system. For an accurate determination, combine voltage checks with a proper load test and system inspection, and consider the battery’s age. If in doubt, seek a professional assessment.
