Starter Solenoid - Good 12v Source?

I need a constant 12v source to power a relay harness and I don't want to run a cable all the way to the trunk to connect it to the battery. Is connecting a wire (with a ring terminal on the end of it) to the 12v post on the starter solenoid acceptable for this?

I've started a thread on another LS forum, and here's what these guys had to say:

http://www.lincolnvscadillac.com/showthread.php?t=38463

Should be acceptable as long as you fuse protect the line leading from the solenoid to your aftermarket relay harness, you perform a good crimp, and you aren't powering a high-current application that will become active as soon as you attempt to start the engine.

To answer this question appropriately, it would be a good idea to post a bit more detail what the relay harness will be used for (car stereo amp, driving lights, underbody lights, remote starter, alarm system, etc.).

-Rod

The harness I built is for an HID system I am retrofitting into my LS. The relays will be activated by the passenger side low and high beam stock harness. So, when the low-beams come on, they trigger the relays to pull 12v and power the HID ballasts which fire up the 35 watt HID bulbs (2 of them). It is designed to power the high beams as well (55 watt) when I switch them on, and keep the low beams on during this time.

The harness consists of 14 gauge wire, except for the 12v input, which is a 10 gauge wire. This is the wire that I want to attach to the +12v terminal on the starter solenoid post. I know someone posted in the other forum about the "surge", but I don't get how running a cable all the way back to the battery would cause the harness to be unaffected by this surge. As an example, if you've got 2 wires coming off the positive on the battery with one going to the harnes, and the other going to the starter solenoid (I guess this isn't really an example per se, because this is the way it is), then I don't understand how that surge would not travel straight down the solenoid wire to the battery, and from there down the wire going to the harness. Just because I run a separate wire from the battery to the harness (instead of from the solenoid terminal to the harness), this does not mean that the circuit is isolated.

Hopefully that all made sense :). And to answer the question about the power being drawn when the car is started, yes I believe with the lights set in "auto" mode they are on when I start the car.

So what do you think? Should I be okay given that I've got a 30 amp inline fuse to protect against and spikes/surges?

For this application I don't really think you'll have an issue. There could be a bit of extra "noise" due to the distance between the battery and the tap, but for an HID system it shouldn't be a problem. The HID system uses ballasts that step the voltage up significantly but shouldn't effect longevity of the bulbs.

The battery of the car essentially acts as a large capacitor to filter noise from the alternator (which produces somewhat noisy rectified AC) as well as current surges, and ideally you want your load tied as close to it as possible to minimize spikes and noise. But I can't really think of a practical application where the surge would be significant enough to cause damage to the end product. I'd expect if a product were damaged by the surge from the starter, it probably was not properly designed for application in an automobile.

-Rod