I have a custom made circuit (device) to be installed in my vehicle.

This device is to work (power up) only when the engine has started. It consumes a reasonable amount of power hence do not want it powered from the battery.

Is there a clean way to access the output from the alternator in the car cabin ?

Any suggestions are welcome.

  • You realize, I hope, that in most vehicles the output of the alternator is directly connected to the (+) terminal on the battery.
    – jwh20
    Jan 19, 2020 at 12:40
  • 1
    The right recommendation toward a solution will depend on whether the 'device' is 12VDC or 110VAC and how many amps it will draw (i.e. total wattage). If it is an electronics device, you might have to add a half or full sine wave inverter. Running a circular saw is different than charging a cell phone.
    – user16128
    Jan 19, 2020 at 13:30
  • you can install a relay that uses power from the ignition switch to turn it off or on,the alternator needs to be connected to the battery at all times or it might burn out the internal wiring of the alternator. Jan 19, 2020 at 19:21
  • @jwh20 Yes i do know that. However i do not want the device drawing current from the battery when the ignition is off and alternator power is down. That will drain the battery.
    – Cerezo
    Jan 19, 2020 at 19:34
  • @Jeeped Yes, it is a 12VDC device. No need for power rectification.
    – Cerezo
    Jan 19, 2020 at 19:35

1 Answer 1


As trond hanson said in the comments, the "proper" way to install accessories is to use a relay, drawing power directly from the battery terminals.

(Technically, when the vehicle is running, your device will be drawing power from the alternator, but the battery terminals offer a good connection point. In theory, connecting to the battery terminals is the same as connecting to the + and - wires coming from the alternator. You should not splice into the alternator wires.)

Below is a wiring diagram of how you could hook up your device.

wiring diagram

Assuming your device will be drawing a lot of amps, you will want to use thick wires, and position your device so that the + and - wires do not have to travel far between the battery and your device. The relay should be in-line with the (red) wire coming from the + battery terminal to your device. The - wire coming from your device could be hooked up to a good ground in the engine bay; you do not have to connect it to the - battery terminal.

It is ok for the wires which connect to the 85 and 86 pins on the relay to be long, and you could use thin 18 gauge wire. It may be convenient to run them into your cabin, so that you can easily turn the device on and off.

(It is often suggested that you connect the 85 pin wire to a wire in the engine bay which is "hot" only when the car is running. This way, when your car is running, your device will receive power. I personally do not like this approach. I prefer to use a simple on/off switch in the cabin instead. You avoid having to find a hot-when-running wire, and then splicing into it. This is how I drew it in the diagram.)

Just make sure you turn off the switch in cabin before you turn off the car!

  • Is there any advantage to piping the (-) back to the battery instead of any ground (most any uninsulated metal) readily available close to the device?
    – user16128
    Jan 19, 2020 at 23:05
  • Thanks for the detailed reply :). First, the device will not draw too much amps (about 300mA). Thank you also for the relay recommendation. Also going by your solution (powering from the battery), i could get away with just sensing if the ignition is in the start position (the one before you crank). Will it be easier to get a wire to be used to sense this start position instead?
    – Cerezo
    Jan 20, 2020 at 0:18
  • @Cerezo - As mentioned in comments above, look at your main fuse box for an unused switched line to tap. 300mA is a minor concern. You might even tap into a used fused connection like the radio. There will be several that are already switched to the ignition (Run or Accessories) so you minimize rewiring and you won't require an inline fuse unless you want double up for device safety. If you go this route, make sure you tap the proper side of the fuse.
    – user16128
    Jan 20, 2020 at 1:15
  • @Jeeped Thanks. I am not used to "auto" terminology. So what I need is to locate a wire that is "switched to the ignition"? I will do some probing with my DMM to see if I can find a "fused", "ignition switched" wire. Thanks again
    – Cerezo
    Jan 20, 2020 at 7:53

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