Nowadays, with more cars on the road, that commute to work is starting to get much longer. 

This means that you are spending more time in your car. If you are someone who can sit alone in silence with your thoughts I really envy you!

However, if you are like me, you will need some sort of music to fuel your fantasies. 

A radio is a great option, but radio stations tend to play the same songs on repeat and have a lot of annoying advertisements as well. 

So, having the luxury of being your DJ can make your commute more bearable. 

If your car does not have an auxiliary input jack, you can always use a FM Transmitter to play music or audiobooks from a mobile phone or MP3 player. 

Can you leave a FM transmitter plugged in?

Yes, you can leave a FM transmitter plugged into your car as it will not drain the battery when you turn the car engine off. Power is disconnected from auxiliary inputs and outputs like a socket which provides power to devices such as a FM transmitter. 

How a FM transmitter is powered when plugged into your car

If your car does not have its own auxiliary input, or bluetooth capabilities, you will not be able to play your own music.

Fear not, that is not the end of the story!

A FM Transmitter is a device you can use to play your own music if your car does not have the means to do so through an auxiliary input or bluetooth. 

Let’s take a closer look at a FM transmitter and how it is powered which will help you better understand whether you can leave it plugged in.

Without going into too much detail, the FM transmitter is a single transistor circuit which uses radio waves and frequency modulation to transfer information.

It is able to cover a lot of distance using very little power. 

The other key feature that is needed for a FM transmitter is Bluetooth. This way you can connect to it using your mobile phone.

FM transmitter circuit

Below is a common circuit diagram of a working FM transmitter.

Power sources in a car

A car battery is a crucial component in a car. 

One of its main purposes is to provide power when starting the car.

Other jobs it has include powering the lights, wipers, stereo system, GPS, and indicators. 

If you have, or are familiar with older cars, you know that most of them had cigarette lighters. This was a socket that was designed to heat an element which could then be used to light a cigarette. 

Nowadays, they are less commonly used for that but rather for powering portable electronics. 

Newer cars do not tend to have cigarette light sockets, but have their own dedicated power outlets for powering portable electronics. 

The FM transmitters are designed so that they can fit into a cigarette lighter socket, or a power outlet. 

However, both types of sockets provide the right amount of power for the FM transmitter. 

Why it is ok to leave a FM transmitter plugged in

If you have just purchased a FM transmitter, your main concern might be whether leaving a FM transmitter plugged in will drain your car battery. 

Let’s look at the different key ignition positions and when the battery of the car is used to power electronics.

The key ignition of a car has four positions;

• OFF
• ACC
• ON
• START

So, let’s look at each position to see if you could leave a FM transmitter plugged in. 

The first position of the key ignition is ‘OFF’

When the key is in the ‘OFF’ position the engine is off and no power is supplied to any of the accessories like the radio, windows etc. 

In the OFF position the FM transmitter has no chance of draining the battery as no power is being supplied to it anyway. 

But, a note should be made, while most cars do not provide power to auxiliary outputs, some cars might provide power to auxiliary outputs when the key is removed from the ignition.

You will have to research your car brand to see whether it provides power to auxiliary outputs when in the ‘OFF’ position. 

The second position of the key ignition is ‘ACC’ (or accessories)

When the key is in this position, the car supplies power to some of the car dashboard electrical accessories like radio, windows, etc. 

Leaving the FM transmitter plugged in this position is only a problem if you are using the radio as well. This is because the car battery is providing power to operate the radio without being recharged. 

So, you run the risk of draining the battery in this scenario.

However, if you are not using the radio, you can leave the FM transmitter plugged in without any problem. This is because the FM transmitter consumes very little power by itself. 

The third position of the key ignition is ‘ON’

This is the position that the car key sits at when the engine is on. Also, all dashboard electrical accessories have power supplied to them.

An important component of a car is the Alternator, which is a type of electrical generator. 

It has the job of recharging the car battery. It does so by converting mechanical energy (rotation of the wheels) into electrical energy.

If you are at rest (no wheel rotation) while the car is ‘ON’, and are using the radio and FM transmitter for long periods of time, you will drain the battery as it is not being recharged. 

The ‘START’ position is not relevant as it is just used to start the engine when it is off. 

Below is a summary of the different key positions and whether it is ok to leave a FM transmitter plugged in.

Key position	Notes
OFF	It is ok to leave the FM transmitter plugged in as power is disconnected from auxiliary outputs.
ACC	Leaving the FM transmitter plugged in by itself is fine as it consumes little power by itself. However, using the radio along with an FM transmitter will drain the battery.
ON	If the car is in motion you will not have any problems draining the battery. However, using the FM transmitter and radio while the car is stationary for a long period of time will drain the battery.

How to ensure the FM transmitter will not drain the battery

While the FM transmitter does not pose much threat of draining your car battery, there are some things you can do to ensure there is no chance of the FM transmitter interfering with your car.

• If you are unsure whether power is removed from auxiliary outputs, remove the FM transmitter before leaving your car. This way you will eliminate any doubt of whether it will drain the car battery
• Do not use the FM transmitter in conjunction with the radio when the key is in the ACC or ON position of the ignition for long periods of time due to the fact that the car battery is not being recharged.
• Make sure you have a healthy fully charged battery to begin with. A bad battery will drain faster.