1. Verify the New Battery:
* Voltage Test: Even new batteries can be defective. Use a multimeter to check the battery's voltage. It should be around 12.6 volts when fully charged. A lower voltage suggests a bad battery, despite its newness.
* Load Test: A more thorough test is a load test, performed at an auto parts store. This simulates the battery under load to determine its capacity.
2. Check the Charging System:
* Alternator Output: This is the most likely culprit. With the engine running, check the voltage at the battery terminals with a multimeter. It should read between 13.5 and 14.5 volts. Lower than that means the alternator isn't charging sufficiently. Higher than that might indicate overcharging (which can damage the battery).
* Alternator Belt Tension: A loose or worn alternator belt can prevent the alternator from spinning fast enough to generate sufficient power. Inspect the belt for wear and ensure it's properly tensioned.
* Battery Cables and Connections: Corroded or loose battery cables and connections can restrict current flow. Clean the terminals with a wire brush and ensure they are securely fastened. Check the cables for damage.
* Wiring Harness: Damage to the wiring harness between the alternator and battery can cause issues. This is more difficult to diagnose and may require a professional.
* Voltage Regulator (part of the alternator on some models): The voltage regulator controls the alternator's output. A faulty regulator can cause inconsistent charging or overcharging. This is often integrated into the alternator itself and requires replacing the alternator as a unit.
3. Consider the Gauge Itself:
While less likely, the gauge itself could be faulty. If all other components check out, it's possible the gauge is giving an inaccurate reading.
Troubleshooting Steps in Order:
1. Battery Voltage Test: Rule out a faulty new battery immediately.
2. Visual Inspection of Cables and Belt: Look for obvious problems like corrosion, looseness, or damage.
3. Alternator Voltage Test (Engine Running): This is the most likely source of the problem.
4. Professional Diagnosis (if needed): If you're not comfortable with electrical testing, take your car to an auto repair shop or parts store for a more in-depth diagnosis. They have the tools to accurately test the alternator and charging system.
Safety Precautions:
* Disconnect the negative battery cable before performing any electrical testing to prevent accidental shorts.
* Be careful when working around the engine and battery. They are hot when the engine is running.
The bouncing needle when using the blinkers strongly suggests a charging problem. The system is struggling to maintain voltage when even a small load (the blinkers) is added. Focus your attention on the alternator and its related components first.
