Review on 🚗 High-Speed 3-Port USB Car Charger, 12V/24V Input, QC3.0 Output 4.8V, Cigarette Lighter Voltage Meter, Compatible with iPhone, iPad, Samsung Galaxy, and USB Charging Devices - Silver by Kenny Stephens

As this shows the battery voltage (usually 13-14 volts while driving) I put in some petrol a 5 hour drive ago and noticed the battery voltage was as low as 12v , about 11.7. At 11.5V it started flashing. They told me I had an alternator problem, so I put an extra freshly charged 12 volt battery in the car. The next day during my drive my car started flashing the alternator warning and then died after 2-3 miles (not enough warning to do anything!). Since I had a spare battery, I replaced it at the side of the road and got to my destination. That saved me from being towed!