Battery died but tested good!?

[speedydtp247]

Yesterday on my way to work, the volt gauge was reading about 11. Then when I left work it was reading about 10-11 volts. With about 5 more miles away from home the volts was reading 8 volts and as soon I was safely pull off the main road it died. After I got picked up and taken home to grab a spare battery (date code sticker is 2021), some tools and taken back to my avant to switch the batteries around. I started up my avant and the volt gauge now reads 14 volts.

I charged the dead battery (bought new 2 years ago) overnight and took it with me to work. It sat on the passenger floor (still had the spare battery running the avant) while I was work for the day. BTW the volt gauge stayed at 14 when I went to work and left work. I stopped by 2 different auto parts stores that tested the battery (that's not hooked up) and tested the electrical system. The battery and alternator passed the tests at both auto parts store.

What would cause the battery to do dead but the battery and alternator test good? I initially thought I have a failing voltage regulator. This looks like a decent affordable battery tester to buy

[walky_talky20]

One explanation could be that your alternator is intermittently dropping out due to worn brushes. The brushes within the alternator are a wear item and when they run out of material they stop contacting the rotor slip rings. This can cause an intermittent no-charge situation when they get to the end of their life.

For example, my brushes finally wore out around 180k on my Passat 1.8T while I was on a long trip. I was able to hit the alternator with a hammer and get it to charge again for several more miles until I could make it to a parts store (and swap the alternator in the parking lot). Yours may be similarly on the edge. The original brushes usually make it to 100k miles, but usually won't make it 200k miles. I usually suggest people swap in new brushes at 90-100k along with the timing belt job. Obviously I ignored my own advice for my aforementioned Passat.

You can replace the entire regulator and brush-pack assembly or here's a DIY guide I made to replace the brushes only in the Valeo alternators for about $5:

https://www.audizine.com/forum/showt...rushes-(Valeo)