Is Leaving your car in Accessory (ACC) mode Bad for the Battery? (Explained!)

While you may be able to listen to the car audio or power an appliance from the car’s ACC outlet with the car set to ACC mode, the long-term effect on the health of your car’s battery is often not clear.

In this post, I discuss :

  • the danger of running your car accessories with the car in ACC mode
  • how you can engage/disengage ACC mode, and
  • how you can lower the risk of damage to your battery when you power your car accessories in ACC mode.

Is Leaving your car in Accessory (ACC) mode Bad for the Battery?

Leaving your car’s ignition set to the ACCESSORY (ACC) mode position does not drain your car’s battery as much as when it is in the ON position.

Over time, however, the drain on your battery can be significant with the possibility of your car failing to start more so when accessories like the audio system and others are switched on.

This can lead to battery failure, especially for already weak or aging batteries.

What is Accessory Mode?

Your car’s ignition will likely have 4 modes – LOCK, ACCESSORY (ACC), ON, and START.

When you turn the ignition to the ACC position, it is the electrical accessories connected to the car’s internal ACC wiring circuit that is powered. These are low-power electrical accessories like the radio, and any accessories plugged in the ACC power socket.

Should you need to use the radio or have an electrical appliance plugged into the ACC or cigarette lighter socket you only need the ignition to the ACC position.

The ACC mode supplies power to the low-power electrical accessories and has a lower drain on the car’s battery, unlike the ON mode.

How to switch Accessory mode (ACC) ON/OFF

How to switch ACC mode ON/OFF in a car that used key ignition switch

You can switch off the ACC mode by turning the car’s ignition anticlockwise.

Sometimes the key may not turn easily. In this case, you can rotate the steering wheel first clockwise then anticlockwise.

Then, turn the ignition to the LOCK position to switch off the ACC mode.

Note that in some car models, the car can automatically switch to the LOCK position if the gear lever is in parking and the car’s ignition has been set to ACC mode for at least 30-60 minutes.

Consult your car owner manual to check if this is the case with your car.

To switch ON the car’s ACC mode, turn the car’s ignition clockwise to the ACC position.

How to switch ON/OFF ACC in a car with an ENGINE START/STOP Button

If the car has a Continuously Variable Transmission (CVT) and the Engine START/STOP button is off, then pressing the Engine start/stop button once without pressing the brake pedal, should change the Power mode from OFF to ACCESSORY.

If the Engine START/STOP button is ON then pressing Engine START/STOP button once when the shift lever is not in Parking also changes the power mode from ON to ACCESSORY.

To change from ACCESSORY MODE to Vehicle Off, shift the gear lever to Parking.

Does ACC mode drain the battery ?

Yes, switching the ignition to ACC mode drains some power from the battery though not as much as when it is switched to the ON position. The power drain can increase further when the car’s radio or any other appliance plugged into the ACC outlet is switched on.

How much battery does ACC use?

I conducted a test on a 2015 Toyota Fielder where I measured the current drain with the ignition set to ACC mode. Note: No other accessories were switched on in the car.

The current draw with the ignition set to ACC mode was 370mA.

This means that over a period of 24 hours, this battery loses about 9-10Ah or approximately 14% of its capacity (for a 65Ah battery) to the current drain by simply setting the ignition switch to ACC mode.

Whereas this is likely to vary from one car model to another, it nonetheless gives an idea about how much battery capacity you can lose by simply leaving the ignition set to ACC mode.

How long can a car stay in ACC mode?

While your car may be able to stay in ACC mode longer, it is wise to limit it to about 30-60 minutes to limit the risk of excess battery drain that can cause your car to fail to start successfully.

How long should you leave the Car in ACC mode?

You should limit leaving your car in ACC mode to about 30-60 min maximum if your battery is fully charged and is still in good working condition.

You need to avoid leaving your car’s ignition set to ACC mode if your car’s battery is partially charged or old and you have appliances like the radio or laptop plugged into the ACC outlet.

If the car’s audio system is switched on or there are electrical accessories such as a power inverter or laptop connected to the ACC outlet, you need to keep the car’s ignition to ACC mode for a shorter time to limit battery drain.

Will Leaving the Car in ACC mode kill the battery?

Not necessarily. You can leave your car’s ignition in the ACC position and even run accessories like the audio system for several hours as long as your car battery is not deeply discharged beyond the recommended level.

It is bad for the battery though, and you risk damaging it permanently If you drain it frequently by leaving it in ACC mode or ON especially if it is a partially charged or old battery.

Limit running your car accessories using ACC mode to 30 min or less unless your engine is running to lower the risk of draining and possibly damaging your car battery.

Final thoughts, running your car battery in ACC mode for a short time is not necessarily bad for your battery provided your battery is not deeply discharged.

The risk of damage is higher with aging, partially discharged batteries, or if the accessories are run for several hours unless you run your car engine periodically.

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