Can a car charger drain the battery?

When it cannot drain the battery

If the socket is switched, power stops when the ignition goes off. Mazda's CX-5 manual, for example, says the ignition must be in ACC or ON for its accessory sockets to work. With the key out, a charger left in that socket is simply disconnected.

When it can

• Permanent sockets. Some outlets stay live with the ignition off. Mazda's 2016 CX-9 manual says its USB power outlets work whether the ignition is on or off, and warns against using them for long periods with the engine off.
• Timed sockets left unlocked. Polestar's manual for the Polestar 1 says its sockets stay active for a further seven minutes if the car is left unlocked after the engine is switched off, and warns that using them with the engine off risks discharging the starter battery.
• Sockets used with the engine stopped. Nissan's Qashqai manual says to use its power outlet with the engine running and not for extended periods with the engine stopped or idling.

First find out which kind of socket you have. Our guide towhether a 12V socket works with the engine offhas a simple test.

What actually draws the power

1. The charger on its own draws a little for its circuitry and indicator light. Manufacturers rarely publish this figure; neither of the charger listings we checked does.
2. A device that is charging draws much more, for as long as it keeps charging.
3. A dash cam keeps drawing as long as it has power. Nextbase says its dash cams are not designed for continuous 24/7 operation and recommends unplugging the camera for long periods without driving.

How to avoid a flat battery

• Test which of your sockets stay live, and unplug anything in those sockets when you leave the car for more than a day.
• Unplug the device even if you leave the charger in place.
• For a dash cam with parking mode, use a hardwire kit to a permanently live fuse rather than a live 12V socket, because the kit adds a voltage limiter. Nextbase's Series 2 kit listing describes a built-in voltage limiter that protects the battery on a permanent live feed.
• If the car will be laid up for weeks, unplug the dash cam too, as Nextbase advises.

Common mistakes

• Assuming the charger's light going out means the socket is off: some chargers have no light, so test with a phone.
• Plugging a dash cam into a permanently live socket for "parking mode" without any low-voltage protection.
• Forgetting the boot socket, which is more often permanent than the front one.

Related pages

• How to hardwire a dash cam (and when not to)
• Car charger buying guide
• Best car chargers
• Jump starter or jump leads?

Frequently asked questions

Is it safe to leave a charger plugged in overnight?

On a switched socket, yes: it gets no power once the ignition is off. On a socket that stays live, unplug it, or at least unplug the device, if the car will sit for a long time. Polestar's manual, for example, advises disconnecting plugs when not in use to avoid discharging the starter battery.

Does a charger with nothing connected still use power?

A small amount, for its own circuitry and any indicator light. Neither the UGREEN 63W nor the Anker 75W Amazon listing publishes a standby figure, so we cannot give one. A device that is actually charging draws far more than an idle charger.

Will a dash cam drain my battery?

On a switched socket it switches off with the car. On permanent power it can, which is why Nextbase says its dash cams are not designed for continuous 24/7 operation and advises unplugging the camera if the car will not be driven for a long period. For parking mode, Nextbase recommends a hardwire kit to a permanently live fuse.

Does a hardwire kit stop the battery going flat?

It reduces the risk. The Nextbase Series 2 kit listing says its built-in voltage limiter protects the car battery if the kit is connected to a permanent live feed. The listing does not state the cut-off voltage, so check the kit's instructions if you park for long spells.