Renault Spider Undervoltage

Sounds like a classic voltage drop scenario. First, have you checked the battery voltage when the car is off and then again while cranking? What are those readings? A healthy battery should be around 12.6V at rest and not drop below 10V during cranking. Also, have you load tested the battery? That will reveal if it's holding a charge properly.

Thank you for the quick response. I have checked the voltage at rest and while cranking, as you suggested. The resting voltage is around 12.4V, but it drops to about 9V while cranking. I have not performed a load test, could that definitively point to the battery being the problem?

A drop to 9V during cranking is significant and points towards a heavily discharged or defective battery. While other electrical gremlins can cause undervoltage, the load test will confirm the battery's ability to deliver current under load. Given your symptoms, I'd lean towards replacing the battery, but it's always wise to rule out other causes. If the battery is confirmed to be the issue it's time for a workshop!

Thank you, I appreciate the detailed advice. I had the battery load tested, and as you suspected, it was indeed defective. I replaced it, and all the issues have disappeared. The total repair cost was 290€. Your insights were very helpful in diagnosing the problem.