Two days ago I could not start my car. The battery seemed flat. I seldom use my car and when I use it I only drive short distances. I carried the battery home and charged it with a smart charger. Sadly I forgot to measure the voltage before charging it. Today it started immediately.
I purchased a small voltmeter that plugs into the cigarette lighter. It shows 12.5 V before starting the car. The voltage jumps up to 14 V when idling and 14.4 V when driving. Does this mean that the alternator is working?
I tried to measure the parasitic current with the engine off with my cheap multimeter. I got some strange results: 0 mA. And resistance jumping between 190 Ohm and infinity. I concluded that worst case the car draws 12.5 / 190 = 65 mA when off, which is acceptable?
What further steps can I take to search for problems with either the electrical system of the car or the battery?
At the autoparts store where I purchased the voltmeter they said they could test the battery using a 700 USD battery analyzer. But only if the battery was 100 % full or the result of the test would be misleading. I said that the battery was probably at 95 % but they refused to charge it to a 100% in the store.
I have an Arduino and could with some work log the voltage when starting the engine. According to this video if the voltage does not drop below 9.6 V when starting the car; it is usable. However the guy at the autoparts store said that if it drop to say 9.5 V during starting it will not go up to 12.5 when turning the engine off but rather 10V. So I am a bit uncertain if this test is worth performing? Also I am uncertain about the 9.6 V threshold. Another place I read that it should not be below 8.5V.