You need:

• DSN-DVM-368 Voltmeter Module, see also here. Buy at many places, e.g. here 

• 12V car connector 

• two 5mm cube magnets 

• 60cm of any 2-wires cable 

Easy PETG print, no supports, layer 0.2mm. (PLA is likely fine as well.)

With key ON but BEFORE starting the motor, you can see the battery voltage: 12V and more is fine, 11.5V should make you worry. With running engine you should see around 14V meaning that charging goes OK.

(You may see nothing while actually starting the engine - that voltage would be interesting, too, but your car may disconnect most other equipment when starting, including the 12V socket.)