Does a 12V socket work with the engine off?

By The FitMyRide deskยทUpdated

Three ways a socket can be wired

  • Switched. Live only when the ignition is in the accessory or on position. Mazda's CX-5 manual says the ignition must be switched to ACC or ON for its accessory sockets to work.
  • Permanent. Live whether the ignition is on or off. Mazda's 2016 CX-9 manual says its USB power outlets can be used regardless of whether the ignition is on or off.
  • Timed or managed. Live after switch-off for a set period or until the car decides to save the battery. Polestar's UK manual for the Polestar 1 says the sockets are deactivated if the engine is switched off and the car is locked, stay active for a further seven minutes if it is left unlocked, and stay active only as long as the starter battery does not get too low.

How to test your own car

  1. Plug a charger with an indicator light into the socket, or plug in a phone and watch the charging symbol.
  2. Turn the ignition on: the light should come on. If it does not, check the socket's fuse in the owner's manual.
  3. Switch off and remove the key (or press the start button to off). If the light stays on, the socket is not purely switched.
  4. Open and close the driver's door, then wait ten minutes. Some sockets switch off when a door opens or after a delay.
  5. Lock the car and look at the light through the window. A socket that is still lit after locking is effectively permanent.
  6. Repeat for every socket and USB port you use, including the one in the boot.

Why it varies from car to car

Manufacturers choose which circuit feeds each socket, and the choice reflects what the socket is for. A front socket meant for a phone charger is often switched, so nothing is left drawing power. A boot socket meant for a cool box may be permanent. Cars with battery management often add a timer or a cut-off, as in the Polestar example. The owner's manual and the fuse box chart are the places to check, but the test above confirms it.

Even where the socket stays live, manufacturers advise against relying on it with the engine off. Mazda's CX-5 manual says not to use the socket for long periods with the engine off or idling, Nissan's Qashqai manual says to use its outlet with the engine running, and Polestar's manual warns that using the socket with the engine off risks discharging the starter battery.

What it means for chargers and dash cams

  • Switched socket: a phone charger or dash cam switches off with the car. No battery worries, but a dash cam plugged in here cannot record while parked.
  • Permanent socket: anything left plugged in keeps drawing power. Unplug chargers when you leave the car for long, and see whether a car charger can drain the battery.
  • Parking mode: rather than relying on a permanent socket, use a hardwire kit with a voltage limiter. See how to hardwire a dash cam.

Common mistakes

  • Assuming every socket in the car behaves like the front one.
  • Testing only with the door open, which can keep some cars awake.
  • Running a high-draw accessory or a double adaptor beyond the socket's rating.
  • Leaving a device charging from a permanent socket for days while the car is parked.

Chargers mentioned on this page

Neither listing publishes a standby current figure, so unplug either one if your socket stays live.

  1. Anker75W car charger with built-in retractable USB-C cable

    Up to 75W total, with a built-in 75 cm retractable USB-C cable (45W) and a separate 30W USB-C port.

    Anker 75W car charger with built-in retractable USB-C cable: see the price on Amazon →
  2. UGREEN63W USB-C car charger

    Two-port USB-C car charger rated 63W with PPS 45W and QC3.0 listed; enough for a phone and one other device.

    UGREEN 63W USB-C car charger: see the price on Amazon →

Affiliate links: as an Amazon Associate we earn from qualifying purchases. Prices are set by Amazon and change often.

Related pages

Frequently asked questions

Why does my phone keep charging after I turn the car off?

Either that socket is wired to a permanent supply, or it is a timed socket that stays on for a while after the ignition goes off. Polestar's manual for the Polestar 1, for example, says the sockets stay active for a further seven minutes if the car is left unlocked, and switch off when it is locked.

Can two sockets in the same car behave differently?

Yes. Front, rear and boot sockets can be on different circuits, and USB ports can differ from the 12V socket. Mazda's 2016 CX-9 manual, for example, says its USB power outlets can be used whether the ignition is on or off. Test each socket you use.

Can I change a switched socket to a permanent one?

Not without rewiring, and it is rarely worth it. If you need power with the engine off for a dash cam's parking mode, a hardwire kit with a built-in voltage limiter does the job more safely than making a socket permanently live.

How much can I plug into a 12V socket?

Check the manual. Mazda's CX-5 manual limits its accessory sockets to accessories requiring no more than 120 W (DC 12 V, 10 A), and Nissan's Qashqai manual gives the same 120 W (10 A) limit and says not to use double adaptors.

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