Resting voltage, cranking drop, charging voltage and parasitic drain, with reference values and the pitfalls of smart charging.
When the car is slow to start or the battery light comes on, the cause can be the battery, the alternator or a load draining the battery while parked. Learn how to tell them apart with a multimeter in three simple measurements and which values are normal.
The battery, the alternator and parasitic drain form one system: a good battery goes flat if the alternator does not charge, and a good alternator cannot save a sulphated battery. With an ordinary multimeter you measure resting voltage, the drop while cranking and charging voltage, then, if needed, the current draw with the car locked. On cars with Start-Stop and smart charging, charging values vary normally, so interpretation needs some care.
Original editorial schematic, illustrative only — not a real scan-tool screenshot and not a substitute for engine-specific procedures.
Measure the battery at rest first, then while cranking, then with the engine running. Each measurement rules out a cause, and the order stops you buying a battery when the alternator is to blame.
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2. Main causes| Cause | Clue | Measurement that confirms it |
|---|---|---|
| Discharged or sulphated battery | Slow to start, especially in the cold | Low resting voltage, large drop while cranking |
| Alternator not charging | Battery light while driving, flattens during the journey | Running voltage close to battery voltage |
| Parasitic drain | A good battery flat after a few days | High current with the car locked and modules asleep |
| Corroded terminals or poor ground cable | Intermittent problems, warm terminals | Voltage drop across the cable while cranking |
Check the terminals (white or green corrosion, tightness), the ground cable to body and engine, the alternator belt (cracks, tension) and any swelling of the battery case.
Warning: A battery can give off hydrogen. Do not smoke or create sparks near it, and do not work on a swollen or leaking battery without protection.
After at least an hour standing with everything off, measure at the terminals. About 12.6–12.8 V means a charged battery, around 12.4 V partly discharged and below 12.0 V discharged. Charge it fully before drawing conclusions.
With the multimeter on the terminals, start the engine. Voltage should not fall much below about 10 V on a healthy, charged battery. A bigger drop points to a weak battery or a starter or cable problem.
With the engine running, measure again: on a conventional system usually about 13.8–14.7 V. Switch on headlights, blower and rear screen heater: voltage should stay stable. Close to battery voltage means the alternator is not charging.
On cars with Start-Stop or energy management, the ECU may deliberately lower or raise the voltage. Read the requested voltage and battery state in the scan tool before condemning the alternator.
If the battery goes flat while parked, measure current with the ammeter in series or a DC clamp, with the car locked and after the modules have gone to sleep (this can take tens of minutes). Pull fuses one at a time until you find the circuit drawing current.
Warning: Never start the engine or switch on heavy loads with the multimeter connected in series on the current range: the meter's fuse blows or the meter is destroyed.
Charge or replace the battery, clean the terminals or repair the alternator or the load you found. On Start-Stop cars, register the new battery. Repeat the measurements after the repair.
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Terminals, ground and belt checked Resting voltage measured after standing Cranking voltage drop measured Charging voltage measured with and without loads Smart-charging data read where present Parasitic drain measured if the battery goes flat Measurements repeated after the repairAt rest, after at least an hour, around 12.6–12.8 V. The value alone does not show capacity; the cranking test or a battery tester confirms condition.
Can I test the alternator without a multimeter?The battery light and scan-tool data give clues, but a measurement at the terminals is simple and safe. Never disconnect the battery with the engine running as a "test": you can destroy electronic components.
How much parasitic drain is normal?On modern cars, in the order of tens of milliamps once the modules are asleep. The exact limit depends on the car and is given in the manufacturer's documentation.
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