Car Won't Start: Battery or Alternator? Simple Checks You Can Do

7 min readPublished 15 March 2025

When your car won't start, the problem could be a flat battery or a failing alternator. Both cause similar symptoms, but the fixes are completely different. Understanding how to diagnose which component is at fault saves time and money.

This guide provides simple tests you can perform at home to determine whether your car won't start due to a battery problem or alternator failure, plus when to visit a professional.

Quick Answer

Battery problems cause slow cranking or no cranking with dim lights. Alternator problems cause battery to go flat repeatedly even after charging. Test battery voltage (should be 12.6V+), then test alternator output while running (should be 13.7-14.7V). If alternator output is low, alternator is failing.

Common causes

-Battery age (over 4-5 years) causing weak performance
-Alternator not charging battery (low output voltage)
-Parasitic drain drawing power when vehicle is off
-Loose or corroded battery terminals

Typical fixes

-Test battery voltage - if below 12.4V, battery is flat or failing
-Test alternator output - if below 13.7V, alternator is failing
-Clean and tighten battery terminals
-Replace battery if voltage doesn't recover after charging
-Replace alternator if output voltage is consistently low

Battery vs Alternator Symptoms

Slow cranking, dim lights

Likely cause: Battery problem (flat or weak)

Next step: Test battery voltage, charge if low, replace if voltage doesn't recover

No cranking, everything dead

Likely cause: Battery completely flat or connection problem

Next step: Check battery terminals, test voltage, jump start and test alternator

Starts after jump but dies again

Likely cause: Alternator not charging battery

Next step: Test alternator output voltage - should be 13.7-14.7V when running

Battery goes flat repeatedly

Likely cause: Alternator failure or parasitic drain

Next step: Test alternator output, check for parasitic drain if alternator is OK

Battery Problem Symptoms

Battery problems cause specific symptoms that differ from alternator issues. Recognizing these helps you diagnose the problem correctly.

Slow cranking - engine turns over slowly but eventually starts
Dim lights - headlights and interior lights are noticeably dim
No cranking - engine doesn't turn over at all, may hear clicking
Battery warning light on dashboard
Accessories work but engine won't start
Battery voltage below 12.4V when tested

Alternator Problem Symptoms

Alternator problems have distinct symptoms that indicate the charging system is failing.

Battery goes flat repeatedly even after charging
Battery warning light on dashboard (usually red battery icon)
Dimming lights when idling or at low RPM
Electrical accessories work poorly when engine running
Battery voltage drops while driving (below 12.6V)
Alternator output voltage below 13.7V when tested

Test 1: Check Battery Voltage

Testing battery voltage is the first step in diagnosing starting problems. You need a multimeter to perform this test.

Set multimeter to DC voltage (20V range)
Connect red probe to positive (+) terminal, black probe to negative (-) terminal
Read voltage: 12.6V-12.8V = fully charged, 12.4V = 75% charged, 12.2V = 50% charged, below 12.0V = flat or failing
If voltage is low, battery may be flat (needs charging) or failing (needs replacement)
Test after vehicle has been off for at least 4 hours for accurate reading

Test 2: Check Alternator Output

Testing alternator output determines whether the charging system is working. This test requires the engine to be running.

Start engine (jump start if battery is flat)
Set multimeter to DC voltage (20V range)
Connect red probe to positive (+) terminal, black probe to negative (-) terminal
Read voltage with engine running: Should be 13.7V-14.7V
If voltage is below 13.7V, alternator is not charging properly
If voltage is above 14.7V, alternator may be overcharging (also a problem)
Test at idle and at 2000 RPM - voltage should be consistent

Visual Inspection Checks

Simple visual checks can reveal obvious problems before testing.

Check battery terminals - should be clean and tight, no corrosion
Check battery case - should not be swollen, cracked, or leaking
Check alternator belt - should be tight, not slipping, not cracked
Check battery age - look for date code on battery (over 4-5 years is old)
Check for loose connections at battery terminals and alternator

Test 3: Battery Terminal Connection Test

Poor terminal connections can cause starting problems even with a good battery.

Check terminals are tight - should not move when wiggled
Clean terminals if corroded - use wire brush and baking soda solution
Test voltage at terminals vs battery posts - should be same (0.1V difference max)
If voltage drops significantly at terminals, connections are poor
Tighten terminals properly - over-tightening can damage battery posts

Test 4: Battery Load Test

A load test determines if the battery can deliver power under load, which is what matters for starting.

Battery must be fully charged before load test
Professional load testers apply load and measure voltage drop
Battery should maintain above 9.6V for 15 seconds under load
If voltage drops below 9.6V, battery is failing
Many parts stores offer free load testing
DIY load test: Turn on headlights and try to start - if lights go very dim, battery is weak

When It's a Battery Problem

If tests indicate a battery problem, follow these steps.

Battery voltage below 12.4V: Charge battery and retest
Battery won't hold charge after charging: Replace battery
Battery fails load test: Replace battery
Battery over 4-5 years old and showing symptoms: Likely needs replacement
Battery case swollen or leaking: Replace immediately

When It's an Alternator Problem

If tests indicate an alternator problem, the alternator needs repair or replacement.

Alternator output below 13.7V: Alternator is failing, needs replacement
Alternator output above 14.7V: Alternator overcharging, needs replacement
Battery goes flat repeatedly: Alternator not charging, needs replacement
Alternator belt slipping or broken: Replace belt, retest alternator
Alternator making grinding or whining noise: Bearings failing, needs replacement

Test 5: Check for Parasitic Drain

Parasitic drain occurs when something draws power from the battery when the vehicle is off, causing the battery to go flat.

Normal parasitic drain: 20-50mA (milliamps)
Excessive drain: Over 50mA indicates problem
Test procedure: Disconnect negative terminal, connect multimeter in series, read current draw
Common causes: Interior lights left on, faulty alternator diode, aftermarket accessories, faulty modules
If drain is excessive, identify source by removing fuses one by one

When to Visit a Professional

Some problems require professional diagnosis and repair.

If you're unsure about test results or procedures
If alternator needs replacement (requires mechanical work)
If parasitic drain is excessive and you can't identify source
If battery tests OK but car still won't start (may be starter motor or other issue)
If electrical system is complex (modern vehicles with many modules)

Safety Warnings

Working with batteries and electrical systems requires safety precautions.

Wear eye protection when working with batteries
Don't smoke or create sparks near batteries (batteries produce hydrogen gas)
Disconnect negative terminal first when removing battery
Reconnect positive terminal first when installing battery
Don't touch battery terminals together or to vehicle body
Keep battery away from flames and heat sources

AI-Friendly Summary

Battery problems cause slow cranking and dim lights; alternator problems cause battery to go flat repeatedly.
Test battery voltage (should be 12.6V+), then test alternator output while running (should be 13.7-14.7V).
If battery voltage is low, charge and retest; if it doesn't recover, replace battery.
If alternator output is below 13.7V, alternator is failing and needs replacement.
Visit a professional if unsure about tests or if alternator needs replacement (requires mechanical work).

Frequently Asked Questions

How do I know if it's the battery or alternator?

Test battery voltage first (should be 12.6V+ when fully charged). If battery is OK, test alternator output with engine running (should be 13.7-14.7V). Battery problems cause slow cranking and dim lights. Alternator problems cause battery to go flat repeatedly even after charging. If alternator output is low, the alternator is failing.

Can a bad alternator kill a battery?

Yes, a failing alternator that doesn't charge properly will cause the battery to go flat repeatedly. The battery uses its stored energy to start the engine and power accessories, but if the alternator doesn't recharge it, the battery will eventually go flat. This is why batteries go flat repeatedly when the alternator is failing.

Will a car start with a bad alternator?

Yes, a car can start with a bad alternator if the battery has enough charge. However, once the battery's stored energy is depleted (from starting and powering accessories), the car won't start again until the battery is recharged. The alternator charges the battery while driving, so a bad alternator means the battery won't recharge and will go flat.

How much does it cost to replace an alternator?

Alternator replacement costs vary by vehicle. Parts typically cost £100-£300, and labor adds £50-£150 depending on vehicle complexity. Total cost is usually £150-£450. Some vehicles have easily accessible alternators (lower cost), while others require significant disassembly (higher cost). Get quotes from local garages for your specific vehicle.

Can I drive with a bad alternator?

You can drive short distances with a bad alternator if the battery has charge, but it's not recommended. The battery will drain, and you may be stranded. If the alternator completely fails, the car will stop running once the battery is depleted. Have the alternator replaced as soon as possible to avoid breakdowns.