How to Tell That an Alternator Battery Is Going Bad

When your car's alternator goes bad, you will still be able to drive for quite a while on the charge left in your battery. If your battery is going bad, however, you won't be able to go very far. Learn how to tell that an alternator/battery is going bad before it leaves you stranded. You can do this with a few special tools and decide if your alternator/battery is going bad and needs to be replaced.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Flat-head screwdriver
• Battery gravity tester (hydrometer)
• Voltmeter (optional)

1. Disconnect the negative battery cable and pry up the cell caps from the battery using a screwdriver. The cell caps are the molded black plastic inserts on the top of your battery.

2. 
    
   Some Gravity Testers Have a Tube at one End, Some are Syringe-like

   Insert the battery gravity tester (make sure it matches the amount of cylinders in your car) into a cell. The gravity tester works like an eye dropper: squeeze and release the bulb to draw the battery fluid into the syringe. Count the balls that float in the syringe. There are as many balls as you have cylinders, so if you have a four-cylinder car, a good battery will float all four balls. Test each cell. If two or more cells are bad, take the battery to an auto parts store and get a replacement.

3. Recap your battery cells and reconnect the negative cable. Put your car in"Neutral" with the parking brake engaged, and start the engine. If you cannot start your car because your battery is dead, make sure that all the ground (black wire) connections from the battery and alternator are good and tight. If they are loose, the battery will not charge even with a good alternator.

4. Turn on your headlights. Make sure you are parked somewhere where you can see them reflected (like against a wall) or have someone watch them for you. Look to see if the beam is steady or if it is dimming the longer you leave your lights on--this is one of the first signs your alternator is going bad.

5. Turn on your radio and heater. If your headlights dim, then your alternator is going bad and needs to be replaced.

6. 
    

   Connect the leads of a voltmeter to the battery terminals (positive to positive, negative to negative) and turn the voltmeter to the DC scale. With the engine running, you should read anywhere from 13 to 14 volts from your battery; anything less or more means that your alternator is going bad and needs to be replaced.

Tips & Warnings

• Check that all your electrical connections between the battery and alternator are tight--and that your fan belt is tight as well--before assuming that your alternator is going bad. Typical road use can shake these connections free and cause the battery not to charge.
• When testing a battery cell, wear eye protection and avoid getting any of the internal battery fluid on your skin. The fluid is a form of acid with electrolytes, and can cause severe burns.

References

• Alternator Problems

Resources

• What to Take With You in an Emergency Tool Kit

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