S40 Battery Drain

Hi, I'm having a frustrating issue with my 2001 Volvo S40 diesel. The battery keeps draining. I'm getting an error message in the speedometer, the lights are weak and flickering, and the error memory is storing a fault. The engine is reluctant to start, like the battery is weak. I've checked the usual suspects, alternator seems fine, no obvious parasitic draws. Could it be the battery sensor causing this? Mileage is at 155936 km.

4 comment(s)

Sounds like a pain. Have you load tested the battery itself? What kind of voltage are you seeing across the terminals when the car is off, and then again while it's running?

I haven't done a load test yet, but the resting voltage seems normal, around 12.6V. When the engine is running, it's charging, showing about 14.2V. I'm wondering if a faulty battery sensor could be causing a phantom drain, even with the charging system appearing normal. Can a bad sensor really cause this kind of heavy discharge?

Yes, a defective battery sensor on those models can absolutely cause a significant battery drain. The car's computer gets bad data and doesn't manage the charging properly. It's likely the culprit given your symptoms. Time to take it to a workshop.

Thanks for confirming my suspicions! I took it to my local garage, and they confirmed the battery sensor was defective. Replaced it, and it's solved the problem. Total cost was 315€. Appreciate the help!

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