Search permanent plus in a car - one of the most common tasks when installing additional equipment: alarms, DVRs, radios or auto start systems. Unlike managed +12V (which appears only when the ignition is turned on), a constant plus is supplied directly from the battery and is always available, even when the car is turned off. But where to look for it, how not to confuse it with other wires, and why is it critical for the safety of the on-board network?

Errors when connecting to a constant plus can lead to battery discharge, short circuit or even fire. For example, if you connect a radio to a controlled +12V Instead of being permanent, it will reset every time the ignition is turned off. And if you reverse the polarity, you risk burning the fuse or the device itself. In this article we will analyze practical search methods, testing and safe connection, which are suitable for both beginners and experienced auto electricians.

How does constant plus differ from controlled + 12V?

There are two types in the car electrical circuit +12V:

  • 🔋 Constant plus - goes directly from the battery through the fuse. Always available, even when the ignition is off. Used to power devices that need to save settings (for example, radio memory) or operate in standby mode (alarm).
  • 🔑 Controlled plus (ignition, ACC) - appears only when the key is turned to position ACC or ON. Suitable for devices that must be turned on with the ignition (for example, interior lighting or a fan).

The main danger is connecting energy-intensive devices (for example, a DVR with a parking function) to a controlled plus will lead to their shutdown when the ignition is turned off. And if you connect to a permanent positive without a fuse, you risk starting a fire if there is a short circuit.

Plus type When available Typical Use Dangers of error
Permanent Always (even with the ignition off) Alarm system, radio memory, DVR in parking mode Low battery, short circuit
Controlled (ACC) Only when the ignition is on Lighting, fans, heated seats The device does not work when the ignition is turned off
Guided (IG) Only in position ON (not in ACC) Main computer, fuel pump Failure of critical systems due to error
⚠️ Attention: Never connect devices with high power consumption (more than 500 mA) directly to the permanent positive without a relay and fuse. For example, LED backlight 10W will drain the battery overnight.

Where to look for permanent plus: 5 proven places

In most cars, the permanent plus can be found in the following areas:

  • 🔌 Fuse box - usually there are separate fuses with constant power (for example, for a cigarette lighter or radio). Look for inscriptions BATT, 30 or ALWAYS ON on the block cover.
  • 🎵 Radio connector - in standard ISO constant plus goes to the yellow wire (B+), and the controlled one - to red (ACC).
  • 🔥 Mounting block under the hood — often there are terminals with constant voltage for connecting additional equipment (for example, for an alarm).
  • 🚗 OBD-II diagnostic connector - in some cars on contact 16 (usually orange or red wire) there is a constant +12V.
  • 🔋 Directly from the battery - the most reliable, but requires laying a wire and installing a fuse.

In Japanese cars (eg Toyota or Mazda) permanent positive can often be found in the fuse box under the steering wheel - look for fuses labeled IG1 or BATT. In European cars (for example, Volkswagen or BMW) the easiest way is to take it from the radio connector or diagnostic port.

📊 Where do you usually look for a permanent plus in your car?
In the fuse box
On the radio connector
From battery
In the mounting block under the hood
Another place

How to check constant plus with a multimeter

Before connecting the device, you need to make sure that the wire you find is indeed a permanent positive. For this you will need multimeter (or a simple 12V test lamp). Procedure:

  1. Set the multimeter to DC voltage measurement mode (DC 20V).
  2. Connect the black probe to ground (any metal part of the body or battery negative).
  3. Touch the red probe to the wire being tested.
  4. Turn on the ignition and check the readings:
    • If the voltage ~12.6V and when you turn off the ignition it does not disappear - this is a constant plus.
    • If voltage appears only when the ignition is turned on, this is a controlled plus.

For reliability, check the wire in three modes: ignition off → ACC → ON. A constant plus should indicate voltage in all cases.

Set the DC 20V mode on the multimeter|Connect the black probe to ground|Check the voltage with the ignition off|Check the voltage in the ACC position|Check the voltage in the ON position|Compare the readings (should be ~12.6V in all cases)-->

⚠️ Attention: Some cars (eg Hyundai or Kia) may have “fool protection” - the permanent plus on the radio connector disappears 10–15 minutes after the ignition is turned off. This is done to save battery power. In such cases, look for the positive in the fuse box.

Typical mistakes when looking for a permanent plus

Even experienced car enthusiasts sometimes make mistakes that lead to equipment not working or electrical damage. Here are the most common:

  • 🔌 Connection to a controlled plus instead of a permanent one — the device will reset or turn off. For example, the DVR will stop recording in parking mode.
  • 🔥 Ignoring the fuse - if you connect directly to the battery without protection, a short circuit can melt the wires.
  • ⚡ Polarity reversal - if you mix it up + and –, you can burn the device or fuse.
  • 📉 Connecting energy-intensive devices without relays - for example, if you connect LED backlight directly to a permanent plus, it will drain the battery overnight.

To avoid these errors, always use a wiring diagram with fuse (at a distance of no more than 30 cm from the power source) and relay (if the device consumes more than 5 A). For example, to connect an alarm, the following scheme is typically used:



[Battery +] → [10A Fuse] → [Relay (pin 30)] → [Device]


[Control signal] → [Relay (pin 85)]


[Ground] → [Relay (pin 86)]
What happens if you connect the radio to a controlled plus?

If you connect the radio memory (yellow wire) to a controlled plus (for example, red), then every time you turn off the ignition the device will lose settings: radio stations, sound equalizers and other user parameters will be reset. In some cases, the radio may even require you to enter an activation code every time you turn it on.

The location of the permanent positive can vary greatly depending on the make and model of the vehicle. Below are typical locations for the most common brands:

Brand Model (example) Where to look for a permanent plus Notes
Toyota Corolla, Camry Fuse box under steering wheel (fuse IG1 or BATT) The yellow wire in the radio connector is a permanent plus.
Volkswagen Polo, Golf Mounting block under the hood (terminal 30) or radio connector B OBD-II connector on contact 16 there too +12V.
Hyundai/Kia Solaris, Rio Interior fuse box (fuse BATT) The permanent plus in the radio connector can turn off after 10 minutes.
VAZ (Lada) Granta, Vesta Mounting block under the hood or cigarette lighter connector B Kalina permanent plus can be taken from the fuse F3 (10A).

For an accurate determination it is best to use wiring diagram of a specific model. It can be found in the repair manual (for example, in Haynes or Autodata) or on thematic forums (for example, Drive2).

💡

If you can't find a diagram for your car, try searching by VIN on sites like AutoZone or Mitchell1. Often there are detailed electrical diagrams even for rare models.

Safe connection: fuses, relays and wire sizes

Finding a permanent plus is only half the battle. The main thing is to connect the device correctly so that there are no electrical problems. Here are the key rules:

  • 🔥 Always use a safety lock - it must be designed for a current slightly higher than the device consumes. For example, for a DVR (0.5A), a fuse of 1A, and for an amplifier (10A) - by 15A.
  • 📏 Wire size - for currents up to 5A sufficient wire cross-section 0.75 mm², for 10–20A needed 1.5–2.5 mm².
  • 🔄 Use a relay for devices with current more than 5A (for example, for additional headlights or heaters).
  • 🛠️ Isolate connections - Use heat shrink tubing or electrical tape to avoid short circuits.

An example of a correct alarm connection diagram using a relay:



[Battery +] → [10A Fuse] → [Relay (30)] → [Alarm]


[Control signal from ignition switch] → [Relay (85)]


[Ground] → [Relay (86)]


[Relay (87)] → [Load (eg interlock)]
⚠️ Attention: Never connect devices directly to the battery without a fuse! Even a short circuit of the wire against the housing can cause a fire. The fuse should be located as close as possible to the power source (maximum 30 cm from the battery).
💡

If the device consumes more than 5A, be sure to use a relay. This will protect the ignition switch contacts from burning and extend the life of the wiring.

Frequently asked questions and problems

Why does the battery drain quickly after connecting to a permanent positive?

Most likely, you connected a device with high power consumption (for example, LED backlight or amplifier) without a relay or through an incorrectly sized fuse. Check the current consumption of the device and install a fuse 20-30% higher than the rating. Also make sure that the device goes into sleep mode (for example, the DVR should turn off 5-10 minutes after turning off the ignition).

Is it possible to take a permanent plus from the cigarette lighter?

Yes, in most cars the cigarette lighter is connected to a constant positive (check with a multimeter!). However, please note that there is already a fuse there (usually 10–15A), and connecting additional devices may overload it. If you plan to connect something powerful (for example, inverter 12V→220V), it is better to pull the wire directly from the battery with a separate fuse.

How to find a constant plus if you don’t have a multimeter?

Can be used 12V test lamp (for example, from an old radio) or LED diode with resistor (220–470 ohms). Connect one end to ground and touch the other end to the wire being tested:

  • If the lamp lights up when the ignition is off, this is a permanent plus.
  • If it lights up only when the ignition is on, it is a controlled plus.
Important: Do not use lamps with a power greater than 5W - they can overload the circuit.
What to do if the constant plus disappears a few minutes after turning off the ignition?

This is a feature of some cars (for example, Hyundai, Kia, Renault), where the constant plus turns off after 10–15 minutes to save battery power. Solutions:

  • Look for the plus in another place (for example, in the mounting block under the hood).
  • Connect directly to the battery with a separate fuse.
  • Use delay relayto make the device work longer.
Is it possible to connect several devices to one permanent plus?

Yes, but with reservations:

  • The total current of all devices must not exceed the fuse rating.
  • Better to use distribution block with separate fuses for each device.
  • Avoid "twists" - use terminal blocks or solder connections.

For example, if you have a fuse on 10A, and two devices consume each 3A, then they can be connected in parallel. But if one of them consumes 8A, and the other 5A, you need a separate fuse.