The battery warning lamp is on and the battery goes flat after a long drive.
Charging output is missing or below the regulated voltage.
Suggested test plan
- Measure charging voltage at the battery.
- Compare with the alternator terminal.
- Check the belt, lamp circuit and connections.
- Repair the charging circuit.
Safety: 12 V systems are low voltage but can still cause burns and fire. Disconnect the battery before working on wiring and never short a battery terminal.
The warning lamp is part of the alternator excitation circuit on many vehicles, so a bulb fault can stop charging. Always compare battery and alternator voltages.