Jumper Undervoltage Issue

I had similar symptoms with my Opel Astra. Check the battery voltage with a multimeter when the engine is off and running. This should help identify if the issue is the battery, alternator, or voltage regulator.

When you say check the voltage regulator, what readings should I expect, and what would indicate a problem? I want to understand the diagnostic process better.

For me, the voltage regulator was the problem, so the battery wasn't charging. I'm not a mechanic, so I suggest taking it to a workshop. They can check the power supply and battery health.

Following your suggestion, I had it professionally diagnosed. It was the battery, heavily discharged and not holding a charge. Replacement and diagnostics cost 290€. Thanks for the help!