Seat Altea Diesel Battery Sensor Problem

Sounds like a bad battery sensor. I had a similar issue with my old Vauxhall Astra. Weak starts and the lights were all over the place. First thing to check is the voltage across the battery terminals when the engine is running. What does that read?

Voltage reading? How do I check that? I'm not really a car guy...

You'll need a multimeter to check the voltage. If you're not comfortable doing that, I'd suggest taking it to a workshop. If the voltage is erratic, the battery sensor is likely the problem. Replacing it fixed the Astra's issues. Saved me from buying a new car battery.

Took it to the shop and they confirmed it was the battery sensor. 315€ later and the car starts great. Thanks for the advice!