How to Start a Car with a Bad Alternator

You can often start a car with a bad alternator by fully charging the battery, jump‑starting from another vehicle or a jump pack, or push‑starting a manual transmission, then immediately driving a short distance with all accessories off to reach a repair shop. Expect limited run time on battery alone—sometimes only 15–45 minutes in daylight—and consider a tow if safety is in doubt.

What a “bad alternator” means—and why starting is tricky

The alternator converts engine power into electricity to run the vehicle’s systems and recharge the battery. When it fails or underperforms, the car must run solely on the battery, which drains quickly. Starting may still be possible with outside help, but the engine can stall once voltage drops below what the fuel pump, ignition, and computers require. Your goal is to start it safely, reduce electrical load, and get to a service facility before the battery goes flat.

Quick checklist before you try

Before attempting a start, run through these checks to confirm the problem and avoid damage. This can prevent a wasted jump or a dangerous situation if the belt or wiring has failed.

• Warning lights or symptoms: Battery/charging light on, dimming headlights, whining or grinding from the alternator area, sulfur/burning smell, or repeated stalling.
• Belt check: Ensure the serpentine belt is present, tight, and not shredded. If the belt is off or broken, do not drive—power steering and cooling may fail.
• Battery condition: Look for swelling, leaks, or heavy corrosion. If the case is swollen or leaking, do not jump‑start.
• Voltage reading (if you have a multimeter): Engine off should be about 12.4–12.7 V on a healthy battery. With engine running and a healthy alternator, most cars show roughly 13.5–14.8 V (smart systems may briefly dip lower). If running voltage is near or below resting voltage under light load, charging is likely inadequate.
• Jump posts: On many modern cars with a battery sensor on the negative terminal, use the under‑hood jump posts specified in the owner’s manual.

If these checks suggest a charging issue but no immediate hazards, a controlled start to reach a workshop may be reasonable. If there are safety red flags (belt failure, leaks, or burning smells), arrange a tow.

Ways to get it started

Option 1: Jump‑start from another vehicle or a jump pack

Jump‑starting provides the surge of current needed for cranking. It won’t fix charging, so once the engine runs, minimize electrical load and leave immediately for service.

1. Position vehicles close but not touching. Turn off both ignitions and all accessories.
2. Connect positive (+) clamp to the dead battery’s positive terminal (or designated positive post).
3. Connect the other positive (+) clamp to the good battery/jump pack positive.
4. Connect negative (−) clamp to the good battery/jump pack negative.
5. Connect the final negative (−) clamp to a solid, unpainted ground on the disabled car (or the manufacturer’s specified ground point), not the negative battery terminal if a sensor is present.
6. Start the donor vehicle or power on the jump pack. Wait 30–60 seconds.
7. Attempt to start the disabled vehicle. If it starts, keep it running.
8. Remove cables in reverse order. Do not let clamps touch.

Once running, switch off lights, HVAC, rear defroster, and infotainment. Drive directly to a repair facility—idling in place can drain the battery faster than driving.

Option 2: Charge the battery with a charger

If you have time and access to a wall charger, pre‑charging the battery gives you the best chance of a clean start and a few extra minutes of run time to reach a shop.

1. Turn everything off and disconnect the battery if your charger requires it (follow the charger’s instructions).
2. Connect charger positive to battery positive, charger negative to battery negative or approved chassis ground.
3. Charge until the charger indicates full or near full (often 12.6–12.7 V). A smart charger on a 10–15 A setting may need 1–4 hours depending on battery state.
4. Reconnect (if removed), start the vehicle, and proceed directly to service.

A fully charged battery can temporarily compensate for a failing alternator, but it will drain quickly if charging output is absent.

Option 3: Push‑start (manual transmissions only)

Manual‑transmission cars can sometimes be started by rolling the vehicle to spin the engine without the starter. Do not attempt this on steep or unsafe terrain.

1. Turn the key to ON (not start). Depress the clutch and select 2nd gear.
2. Have helpers push the car to 5–10 mph, or let it roll safely downhill.
3. Quickly release the clutch while gently pressing the accelerator. Once the engine fires, depress the clutch to stabilize idle.
4. Keep the engine running; do not stall. Drive straight to a repair shop.

Push‑starting won’t help if the battery is completely dead (ECU and fuel pump still need power), and it is not suitable for automatics or many hybrids.

How far can you drive on battery alone?

Run time depends on battery size and electrical load. A typical 50–70 Ah 12 V battery might keep a modern gasoline car running for roughly 15–45 minutes in daylight with minimal loads. Fuel pump (7–10 A), engine control (3–6 A), ignition/injectors (6–10 A), and cooling fans (variable) add up quickly. Night driving, rain (wipers), or heavy traffic (fans, brake lights) can cut time dramatically.

To stretch every minute of charge, reduce electrical consumption aggressively using the tips below.

• Turn off HVAC, A/C, heated seats, rear defroster, and infotainment.
• Unplug chargers and accessories; avoid power windows and sunroofs.
• Drive in daylight if possible; if lights are legally required, use only low beams.
• Avoid idling and stop‑and‑go; steady speeds are best for efficiency.
• Keep engine RPM moderate; very low idle with a failing alternator can cause voltage sag.

Even with careful load management, the battery will deplete. Have an exit plan: the nearest safe pull‑off and a tow number saved in your phone.

Keep it running once it starts

With a compromised alternator, voltage stability is fragile. The following practices help you reach service without stalling.

• Do not shut the engine off until you’re at a repair bay; a restart may not be possible.
• Avoid long idles, heavy electrical loads, and tight maneuvers that risk stalling.
• Watch for warning lights, misfires, or surging electronics—signs you’re minutes from stalling.
• If you have a plug‑in voltmeter or OBD app, monitor system voltage; below ~12 V while running is critical.
• Head straight to a shop; don’t attempt multiple stops or errands.

If the engine dies while moving, you may lose power steering and brake assist. Keep both hands on the wheel, press the brakes firmly, and steer to a safe stop.

When you should not attempt to start or drive

Some conditions make a road attempt unsafe or likely to cause further damage. In these cases, call for a tow.

• Broken/missing serpentine belt or seized pulley.
• Burning smell, visible smoke, or glowing/belt debris around the alternator.
• Swollen, hissing, or leaking battery.
• Hybrid/EV systems or start‑stop vehicles showing high‑voltage or charging system faults—these may require specific procedures.
• Deep water intrusion in the engine bay or battery compartment.

Proceeding in these scenarios risks fire, steering/brake loss, or engine damage. Professional recovery is the safest choice.

Troubleshooting: Alternator vs. battery

If you’re unsure which component failed, a quick voltage test can guide next steps and avoid replacing the wrong part.

1. Measure battery at rest (engine off, after 30 minutes): about 12.6 V is healthy; under 12.2 V indicates low charge.
2. Start the engine and measure across the battery: many systems show 13.5–14.8 V. Smart alternators can vary, but running voltage should generally exceed resting voltage.
3. Turn on headlights and rear defroster: if voltage plunges near or below 12.5 V and keeps dropping, charging is likely inadequate.
4. If resting voltage is very low but charges and holds with an external charger, the battery may be aged or drained; if running voltage never rises, suspect the alternator, wiring, or fusible link.

Remember that loose or corroded terminals and bad grounds can mimic alternator failure. Inspect and clean connections before condemning parts.

Costs and next steps

Typical alternator replacement runs roughly $300–$900 parts and labor in many markets; premium models and tightly packaged engines can exceed $1,000. A serpentine belt adds $30–$120 in parts. Some modern vehicles require battery/charging system registration or calibration after replacement; a reputable shop will handle this. If the battery has been deeply discharged repeatedly, budget for a new battery as well.

Summary

To start a car with a bad alternator, charge the battery or jump‑start (or push‑start a manual), then immediately reduce all electrical loads and drive straight to a repair shop. Expect limited run time on battery alone and avoid attempting this if there are safety red flags like a broken belt, burning smells, or a compromised battery. When in doubt, tow the vehicle—it’s often the safest and cheapest option in the long run.

Will a car start with a bad alternator?

Yes, a car can start with a bad alternator because the battery provides the initial power to start the engine. However, the car will only run until the battery’s charge is completely depleted, as a faulty alternator cannot recharge it while the engine is running. A jumpstart is only a temporary solution, and you risk being stranded again quickly once the battery runs out of power. 
How it works:

1. Battery’s Role: Opens in new tabThe battery’s main job is to provide the electrical energy to crank the engine to start it. 
2. Alternator’s Role: Opens in new tabOnce the engine is running, the alternator takes over, providing the power for the car’s electrical systems and recharging the battery. 
3. With a Bad Alternator: Opens in new tabIf the alternator fails, the battery is no longer being recharged. The car will continue to run on the battery’s stored energy until it’s drained, causing the engine to die. 

What to do if your car won’t start with a bad alternator:

• Jumpstart: You can jumpstart a car with a bad alternator to get it running, but it’s a very temporary fix. 
• Get it Repaired: You should get the alternator inspected and repaired as soon as possible to avoid being stranded. 

Signs of a failing alternator: The battery warning light on the dashboard comes on, Dimming headlights or other electrical issues, and Unusual noises.

How to keep a car running when the alternator is bad?

One method to keep a vehicle operational when the alternator is malfunctioning is to utilize a portable battery charger. Attach the battery charger to the vehicle’s battery and start the engine. After that, disconnect the battery charger from the vehicle’s battery and proceed to drive to your destination.

Does banging on an alternator work?

Yes, hitting an alternator with a mallet or hammer can be a temporary, emergency fix, but it’s not a reliable solution, as it only serves to unjam the internal brushes, which may be stuck due to carbon buildup. This is a very temporary measure to restore a connection and should only be used to get your car to a mechanic for a proper repair, which will likely involve replacing the alternator or its internal components, like the voltage regulator. 
Why the “Tap” Might Work

• Stuck Brushes: The main reason a tap can work is that the small, spring-loaded brushes that provide power to the alternator can get stuck. 
• Carbon Buildup: Grease, metal particles, or carbon buildup can cause these brushes to stick, breaking the electrical connection. 
• Temporary Connection: A jolt from a hammer can dislodge this buildup and allow the brushes to make contact again, restoring the circuit for a short time. 

Why It’s Not a Real Fix

• Temporary: The problem will likely return, potentially leaving you stranded again, so it’s a very short-term solution at best. 
• Damage Risk: You risk damaging the alternator’s housing or internal parts if you hit it too hard. 
• Internal Issues: If the alternator’s voltage regulator or other internal components are burnt out, a tap won’t fix the problem. 

What to Do Instead

1. Get to a Mechanic: The best approach is to avoid this temporary fix and have the alternator replaced or rebuilt. 
2. Check Other Issues: Before assuming the alternator is the problem, rule out issues like a bad battery or a loose serpentine belt. 
3. External Regulators: In older vehicles, the voltage regulator might be a separate, external component that can be replaced by itself. 

How do you force start a car with a bad alternator?

Start their car after the jumper cables are correctly. Connected. During this time the car with the dead battery needs to be turned. Off try starting your car with the dead battery.