Battery Not Holding Charge (even with no lights on):
* Internal Battery Failure: The battery itself might be defective, even if it passed a basic load test. Internal shorts, cell degradation, or a failing battery can prevent it from holding a charge. A proper battery load test performed by a qualified mechanic would be more thorough than AutoZone's quick test. Consider getting a more in-depth battery test from a different shop.
* Parasitic Drain: Something in your car's electrical system is drawing power even when the car is off. This could be a faulty component (radio, dome light, alarm system, etc.), a short circuit in the wiring, or a malfunctioning computer module. A mechanic can use a multimeter to measure the parasitic drain to pinpoint the culprit.
* Alternator Issues (despite the test): While AutoZone tests are useful, they aren't foolproof. The alternator might be putting out insufficient voltage at idle or low RPMs, just not enough to be noticeable during a quick test. It may only have an issue at a specific voltage range.
* Poor Connections: Corroded or loose battery terminals, cables, or connections at the alternator or starter can prevent proper charging. Clean and tighten all connections to ensure good electrical contact.
* Voltage Regulator Problem: The voltage regulator within the alternator controls the output voltage. A faulty regulator can cause inconsistent charging.
Lights Dimming at High RPMs:
This is unusual and points to a problem with the charging system, despite the alternators seemingly passing a test.
* Alternator Output Problem (voltage): The alternator might be putting out too *much* voltage at high RPMs, which can actually harm the battery and cause the lights to dim (the voltage regulator is the most likely culprit here). While the alternator may charge at low RPM, it could be overcharging at high RPM. A proper voltage test under load is needed.
* Wiring Issues: High RPMs increase the current demand, potentially exposing a weak point in the wiring harness that leads to voltage drop and dimming lights.
* Grounding Problem: A poor ground connection can cause voltage fluctuations, leading to dimming lights at high RPMs when the current draw is higher.
What to do:
1. Thorough Battery Test: Go to a reputable mechanic or auto parts store that offers more advanced battery testing than just a simple load test.
2. Parasitic Drain Test: Ask a mechanic to perform a parasitic drain test to check for any excessive current draw when the car is off.
3. Voltage Test Under Load: Have a mechanic perform a voltage test while the engine is running at various RPMs, including high RPMs, to assess alternator output and check for overcharging.
4. Inspect Wiring and Connections: Visually inspect all battery terminals, alternator connections, and the wiring harness for corrosion, looseness, or damage. Clean and tighten connections as needed.
Ignoring these issues can lead to premature battery failure and potentially damage to other electrical components. A professional diagnosis is highly recommended to pinpoint the root cause.
