How to Diagnose Undercharging a Car Battery
If you find yourself having to charge or jump your car often, there might be a problem with battery undercharging. If your car starts, but its with a hard crank, this may also be your problem. This can be caused by problems with the alternator or something in the car acting like a parasite.
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Things You'll Need
- Hot water and baking soda mix
- Wire brush
- Ammeter rated above the battery's power level
- Volt meter
Instructions
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Visual Inspection
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Check for cracks in the battery case as well as broken terminals. This can allow battery acid to leak out, known as electrolyte. The battery must be replaced.
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2
Look for cables and connections that are damaged or broken. Replace battery cables as necessary. Tighten any loose connections as well.
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3
See if there's corrosion on the battery terminals. Clean the terminals if there's corrosion or dirt with a mixture of hot water and baking soda. A wire brush may be needed for heavy corrosion.
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4
Inspect the alternator or serpentine belts to be sure they are properly tensioned. They should move less than an inch from their resting place. If they are torn, cracked or frayed, they should be replaced.
Checking the Alternator
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5
Place an ammeter in series between the ground terminal of the alternator and the ground battery lead. Your ammeter must be rated above your battery's power level to avoid damage to your meter.
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Touch the test leads to battery terminals. Negative to negative should be touched, and positive to positive should be touched.
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7
Start your car's engine to get maximum alternator output. As everything is turned on, the battery voltage should drop. At 12.6 volts, the alternator should produce its rated current listed on the label. If the ammeter shows within 10% of this current, your alternator is fine.
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If the voltage rating rises above 15 volts, the battery can overheat and cause acid to spew out. Shut down the car immediately. This indicates a faulty diode pack, which must be replaced.
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Tips & Warnings
If you cannot locate the problem on your own, bring the car to your local auto parts store. Many of them have more sophisticated equipment to test your battery and alternator.
Wear gloves and safety glasses. You might also want to remove your jewelry so you do not get tangled in the car's parts. It's also best to avoid smoking while working on the battery.
Never use spark causing tools or lay tools on the battery.
Do not get battery acid (electrolyte) in your eyes, on your skin, clothing, or the paint of your car. It will eat through everything.
Never reverse battery polarity.
