Is this new behavior?
Like many modern cars the Tucson reduces battery voltage when it decides the battery is charged “well enough”.
It does this to mainly save a little fuel and also avoid over charging the battery.
I have a voltmeter plugged into a 12V port so I can monitor system voltage while driving.
When the battery is full (or nearly full) system voltage drops to 12.6V, with 4 exceptions.
When steady cruising it raises the voltage to 13.x or 14.x (varies with temperature) volts for a few minutes every ho…
You can get one of these for comparison:
https://www.amazon.com/gp/product/B01M9IKYVH
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