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Fix a car battery that will not hold charge

We'll check resting voltage, alternator output, parasitic draw, and corrosion—then run a load test to confirm whether to replace the battery or fix the charging system.

Troubleshooting · Home appliances
20–45 min
What you'll need
  • Multimeter (DC volts and amps)
  • Battery charger (if voltage is low)
  • Wire brush and baking-soda solution (for corrosion)
  • Load tester (optional; many parts stores test free)
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Steps

Goal: Confirm the symptom, check voltage and alternator, then isolate the cause—battery, parasitic draw, or charging system.

  • Confirm the battery dies overnight or goes weak after the car sits—not a no-crank or starter click.
  • Good: Starts after charge or jump, then weak or dead after sitting—battery not holding charge. Proceed to Check voltage and alternator.
  • Bad: No crank at all or starter clicks—different problem. See When to get help.

Check voltage and alternator

Goal: Rule out a dead battery and confirm the alternator is charging.

  • Let the car sit at least an hour with nothing on. Set the multimeter to DC volts. Red to positive terminal, black to negative. A healthy battery reads 12.4V or higher. Below 12.2V means low charge or a bad battery.
  • Start the engine. Measure at the battery again. You should see 13.5–14.5V. If the voltage stays near 12V, the alternator is not charging—check the belt and connections.
  • Good: Resting voltage 12.4V or higher and alternator 13.5–14.5V—proceed to Parasitic draw and corrosion.
  • Bad: Low resting voltage—charge fully and retest. Alternator not charging—fix belt or call a mechanic.

Parasitic draw and corrosion

Goal: Check for something draining the battery when off and inspect terminals.

  • Set the multimeter to amps (10A or higher). Disconnect the negative cable. Put the meter in series between the cable and the post. Close all doors, wait 5–10 minutes for modules to sleep. Normal parasitic draw is under 50mA. Over 100mA can drain the battery overnight.
  • Inspect the battery terminals for corrosion. Clean with a wire brush and baking-soda solution. Tighten connections.
  • Good: Draw under 50mA and terminals clean—proceed to Load test.
  • Bad: High draw—pull fuses to find the circuit. Corrosion—clean and retest.

Load test

Goal: Confirm whether the battery can deliver current under load.

  • Charge the battery fully if it was low. Many parts stores run a load test free. Or use a load tester: apply load for 15 seconds. Voltage should stay above 9.6V.
  • Good: Voltage holds above 9.6V—battery is OK. Recheck connections and charging.
  • Bad: Voltage drops below 9.6V—bad cells. Replace the battery.

When to get help

Call a mechanic if:

  • The alternator does not charge and the belt and connections are good.
  • You cannot find the parasitic draw.
  • You are not comfortable with electrical work.

Replace the battery yourself if it fails the load test—most auto parts stores will install it. Dispose of the old battery properly.

Verification

  • Resting voltage is 12.4V or higher after the car sits.
  • Alternator output is 13.5–14.5V at the battery with the engine running.
  • Parasitic draw is under 50mA when the car is off.
  • Terminals are clean and tight.
  • Load test passes (voltage above 9.6V under load).
  • The car starts reliably after sitting overnight.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the battery dies or goes weak after the car sits.
  2. Voltage and alternator Check resting voltage and alternator output with a multimeter.
  3. Parasitic draw and corrosion Test for parasitic draw; clean terminals and connections.
  4. Charge and load test Charge fully, retest voltage, run a load test.
  5. Replace or call a pro Replace battery if load test fails; call a mechanic if alternator or parasitic draw is the cause.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Resting voltage (before and after charge)
  • Alternator output voltage with engine running
  • Parasitic draw reading (mA)
  • Battery age and condition
  • Load test result
  • Steps already tried

Does the battery die overnight or go weak after the car sits?

The car starts after a jump or charge but dies again after sitting. Not a no-crank when hot or starter click.

Confirm the pattern: starts after charge or jump, then weak or dead after sitting. Good: battery not holding charge—proceed to voltage test. Bad: no crank at all or starter clicks—different problem (starter, connections).

Yes No

You can change your answer later.

Is resting voltage 12.4V or higher?

Let the car sit an hour. Measure at the battery with a multimeter on DC volts.

Set the multimeter to DC volts. Red to positive, black to negative. 12.4V or higher: battery may be OK—check alternator. Below 12.4V: charge fully, rest an hour, retest. If still low, battery may be sulfated or bad.

Yes No

You can change your answer later.

Charge and retest

Charge the battery fully with a charger. Let it rest an hour. Measure voltage again. If above 12.4V, check for parasitic draw. If below 12.4V after a full charge, the battery is likely bad—run a load test or replace.

Done

Does the alternator charge 13.5–14.5V with the engine running?

Start the engine. Measure at the battery terminals with the multimeter.

Start the engine. Measure voltage at the battery. 13.5–14.5V: alternator is charging—check parasitic draw and corrosion. Near 12V or lower: alternator not charging—check belt and connections; call a mechanic if needed.

Yes No

You can change your answer later.

Alternator not charging

Check the belt tension and alternator connections. If the belt is loose or broken, fix it. If the alternator still does not charge, it has failed—call a mechanic or replace it. The battery will never hold charge if the alternator does not charge.

Done

Is parasitic draw under 50mA when the car is off?

Set multimeter to amps. Disconnect negative cable, put meter in series. Wait 5–10 min for modules to sleep.

Test parasitic draw with the multimeter in series on the negative cable. Close all doors, wait for modules to sleep. Under 50mA: normal—battery or connections may be the cause. Over 50mA: find the circuit by pulling fuses; fix or call a mechanic.

Yes No

You can change your answer later.

Find and fix parasitic draw

Pull fuses one by one while watching the ammeter. When the draw drops, that circuit has the fault. Common causes: trunk light, glove box light, aftermarket stereo, faulty module. Fix the draw or call a mechanic.

Done

Is there corrosion or physical damage on the battery?

Check terminals, cables, and battery case.

Inspect terminals for corrosion. Check for bulging sides or cracks. Corrosion: clean with wire brush and baking-soda solution; tighten connections; retest. Bulging or cracks: replace the battery immediately. Clean and good: run a load test.

Yes No

You can change your answer later.

Clean terminals and retest

Clean terminals with a wire brush and baking-soda solution. Rinse with water, dry, and apply terminal grease. Tighten connections. Charge and retest voltage. If the battery still does not hold charge, run a load test.

Done

Does the load test pass?

Apply load for 15 seconds. Voltage should stay above 9.6V.

Run a load test—many parts stores do this free. Passes (above 9.6V): battery OK; check connections and charging. Fails (below 9.6V): replace the battery. Bad cells cannot hold charge.

Yes No

You can change your answer later.

Battery OK—check connections

If the load test passes but the battery still dies, recheck alternator output and parasitic draw. Loose or corroded connections can cause intermittent charging. Confirm all connections are clean and tight.

Done

Replace the battery

Replace the battery. Most auto parts stores will install it. Dispose of the old battery properly—stores accept them. After replacement, confirm the alternator charges and there is no parasitic draw.

Done

Call a mechanic

If the car does not crank at all or the starter just clicks, the problem may be the starter or connections—not the battery holding charge. If the alternator does not charge and the belt is good, call a mechanic. If you cannot find the parasitic draw, call a mechanic.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a car battery not hold a charge?
Common causes: sulfation from sitting discharged, age (batteries last 3–5 years), alternator not charging, parasitic draw (something left on or faulty), or bad cells. Check voltage, alternator output, and parasitic draw first.
Can I test a car battery myself?
Yes. Use a multimeter to check resting voltage (12.4V or higher is good). With the engine running, 13.5–14.5V at the battery means the alternator is charging. A load test at a parts store confirms bad cells.
When should I replace a car battery?
Replace if resting voltage stays below 12.4V after a full charge, the load test fails, the battery is over 4–5 years old, or you see bulging or cracks. If the alternator does not charge, fix that first.

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