My VW Golf just won't charge the battery properly. Yesterday the car died on me, even though I had the alternator checked at Bosch only two weeks ago. The battery itself is still relatively new (2024). What could be causing this?
When alternators are tested, they usually only measure voltage, but don't always check the charging curve under load - so your alternator could technically pass, but still not be charging properly. A common culprit is a faulty voltage regulator or worn carbon brushes, especially if your car is older. Have the battery tested at a workshop and check it under load; if it's actually still in good shape, I'd have the alternator checked more thoroughly - look for wear and tear, not just voltage. Also check whether consumers like your radio or lights are drawing power when the engine's off.
The problem could also be with the battery itself, even if it's still relatively new - sometimes batteries come defective from the factory or were stored improperly 🔋. Before you do any further tests, I'd have the battery fully tested at a workshop (capacity, internal resistance, not just the voltage when the engine's off). If the alternator really has been checked but still isn't charging properly, a faulty regulator could be the culprit - that's a separate component that's often overlooked during inspections. Alternatively, it's worth looking at your electrical consumers (seat heater, multimedia, lights) - if something's drawing too much current when the car's at rest, the battery won't actually discharge and charge the way it should.
Don't disconnect the battery yet and don't just drive to the repair shop - if you start the car multiple times in between, you might make a hidden fault worse, one that would otherwise be easier to find. What often gets overlooked: check your power consumers, especially if you're leaving something running after you turn off the engine (radio, lights, chargers). Even a new battery will drain overnight if there's a power consumer in the car that isn't shutting down properly - it could be a faulty control unit connection or even a ground short.
I'd be skeptical about whether that alternator test was really reliable. Most shops only test at idle, and a faulty alternator can look "fine" there even though it's not putting out enough juice under full load (headlights, heating, etc.). You should test it properly: start the car, turn the headlights to full brightness and switch on all the electrical consumers - then check if the voltage at the battery drops instead of rising. If it does, the alternator's probably shot after all, no matter what that first test said.
But before you take the car back to the shop, make sure you check the battery terminals properly. I don't just mean whether they're tight - loosen them and see if there's corrosion (green or white residue) underneath. That's a classic problem and gets overlooked all the time. Just scrub it off with water and a toothbrush, tighten the terminals back down. Also try actually testing the battery (properly testing it, not just "looks okay") - there are cheap battery testers that measure cold cranking amps. If a battery from 2024 is already acting up, the storage conditions at the dealer could've been rubbish.
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