The short answer
A car’s built-in USB port often supplies 5 watts, which is less than navigation with the screen on consumes, so the phone discharges while plugged in. Use a proper adapter in the 12V socket rated for fast charging, with a data-capable cable, and keep the phone out of direct sun.
Charging in a car fails in a specific way: the phone shows a charging icon and the percentage falls anyway. That is arithmetic rather than a fault. Navigation with the screen at full brightness, the radio streaming and the phone warming in the sun can consume more than the charger supplies.
Cause 1: the built-in USB port
The USB ports in most cars, particularly ones more than a few years old, exist to connect a phone for media playback rather than to charge it. Many supply 0.5 to 1 amp at 5 volts, which is 2.5 to 5 watts. A modern phone running navigation can draw three to four times that.
The fix is an adapter in the 12V accessory socket rated for a modern charging standard, ideally 30W or more with USB Power Delivery. That supplies enough to charge while navigating rather than merely slowing the discharge.
Check whether the car’s USB port has a small battery symbol beside it. On many cars that indicates the charging-capable port, while an unmarked or data-marked port supplies far less current. The two frequently sit side by side and look identical.
Cause 2: the cable
- A charge-only cable works for charging and not for CarPlay or Android Auto, which need data.
- A cheap or long cable loses voltage, which reduces charging rate noticeably.
- A worn cable that works at a particular angle, which is common in cars where cables are repeatedly plugged and unplugged.
- A cable not rated for the wattage of a fast charger, which caps it at 60W or lower.
Keep a good short cable in the car specifically, rather than using whichever one ended up there.
Cause 3: heat
Phones reduce or stop charging when hot, and a car in summer is the hottest environment a phone regularly experiences. A phone in a windscreen mount in direct sun, running navigation, will stop charging entirely and may dim the screen or shut down.
Practical steps: mount the phone low and out of direct sun, direct an air vent at it, take the case off on long journeys, and avoid wireless charging in hot weather, since it generates its own heat on top of everything else.
Cause 4: the socket itself
- The 12V socket may be switched with the ignition, so charging stops when the engine does.
- A blown fuse for the accessory socket, which is a common and cheap fix worth checking before buying a new charger.
- A dirty or loose socket, where the adapter wobbles and the connection is intermittent.
- Debris in the socket, which is a genuine hazard as well as a poor connection.
Test the adapter in a different car, or a different adapter in yours, to separate the two.
Cause 5: what the phone is doing
- Navigation with the screen on is one of the heaviest sustained loads a phone experiences.
- Streaming over mobile data with a weak signal makes the radio transmit at high power.
- CarPlay or Android Auto add processing and screen activity.
- A hotspot running for passengers is a substantial additional drain.
- Background updates downloading over mobile data.
Reducing screen brightness and downloading offline maps before a long drive both help considerably.
Very cheap car chargers are a genuine risk. They sit in a socket connected directly to the car’s electrical system, and poor ones have caused fires. Buy from a recognised brand with proper certification, particularly for anything drawing significant current.
Wireless charging in cars
Wireless charging pads and magnetic mounts are convenient and less efficient, producing more heat in the worst possible environment. In cool weather they are fine. In summer, in a sunny car, a wireless charger frequently cannot keep up with navigation and adds heat that makes the phone throttle.
If navigation on long drives is the main use, a cable is the better choice, with wireless reserved for short trips and top-ups.
Why does my phone lose charge while plugged in?
Because the port is supplying less power than the phone is using. Navigation with the screen on can consume more than a basic car USB port provides, so the battery falls despite the charging icon.
Should I use the car's USB port or the 12V socket?
The 12V socket with a proper adapter, unless your car’s USB port is specifically rated for fast charging. Built-in USB ports are often designed for media rather than power.
Why does charging stop on hot days?
Phones stop charging above a temperature threshold to protect the battery. A phone in a windscreen mount in direct sun reaches that quickly, particularly while navigating.
Why does CarPlay not work with my charging cable?
Because it is probably a charge-only cable, which carries power but not data. CarPlay and Android Auto need a proper data cable.
Is wireless charging in the car a bad idea?
In hot weather, largely yes, because it is less efficient and adds heat to an already hot phone. For short trips it is convenient, and for long navigation drives a cable is better.
The short version
Use the 12V socket with a 30W or better adapter from a reputable brand, and a short data-capable cable kept in the car. Mount the phone out of direct sun with a vent pointed at it. Reduce screen brightness and download offline maps for long drives. And check the accessory socket fuse before concluding the charger is faulty.
