Do I have a battery problem or alternator?
 

Few months ago While I was waiting in my car for my wife, who was on a treatment in hospital, I played music on my cd with car engine off.
Just before she came back, my music stopped and car would not start. I got a boost and everything was normal until 2 days ago and same thing
happened. This time I did not even played any music. Again I got a boost and all seem to be o. k. When I got home I got my voltmeter out and measured
battery , it was 12.5 volts and when measured with engine running it was 13.4 volts. This morning it was 13.8. I would like to know what caused it not to start. Battery is only about 1 year old. Thanks for your help.

You may have a parasitic draw somewhere, check to see if your rear window shadw is up,
If there is an alternator problem, you'll most likely get the battery warning icon.

Alternator is OK. Check the voltage drop while cranking.

Did you charge your battery after the day it would not start. put it on a charger over night?

what is a rear window shadw

no, I drove more then 100km and the battery was at 12.6 volts.

battery is only 8 month old

I would put it on a charger over night to ensure it has a good deep charge. Even better, a trickle charger that repairs batteries.
https://www.optimabatteries.com/expe...dead-batteries

I went to a shop and they told me that everything is o.k. My battery is only 8 months old. Today I looked in my garage and found this gadget that
checks your battery and charging when you plug it into cigarette lighter, When it is plugged in and the car is off, battery shows that is good. When I start the car
the charging is also good, but after 10 minutes it shows that the charging is lower. On voltmeter it went from 13.8 to 13.4 volts.

you must not have a battery charger. perhaps a neighbor has one you can borrow.

I charged the battery, now what?