Making a full size lightbar temporary instead of permanent? Veh. Emerg. Lights

[Sparx MacGyver]

Hi folks. I figured you guys might have some insight into this, and might be able to help me out. I work for a security company, and have been tasked with making a full-size lightbar into a temporary one. That is, it's not hard-wired into the car, and is magnet mounted, for (relative) ease of changing out from car to car.

Basically, the owner is getting some lightbars, kinda like this http://www.uniformswarehouse.com/hal...ars-20635.html (that's the link he gave me in teh e-mail). however, he wants to not have it permanently mounted to the car, which is easy enough with some super strong magnets I suppose.

however, I could use some advice on the wiring. The control box will be wired to it, but other than that it would be somehow not hardwired to the car. I thought perhaps large alligator clips on the battery, so it can be taken on and off, but I would rather ask for some advice than do something foolhardy.

Thanks for any help you can offer.

Note: We have told him we should just mount and wire it in, but he's determined to do it this way, so I figured it's best to ask before he goes and buys something (knowing him, he'll blow $5k - $7k on a light bar) and we somehow jack it up without asking.

I always see tech talk in here, and thought you guys might have some insight.

[Rockphed]

I'm not big on cars, but the only easy to access power in cars tend to be the cigarette lighter and the ODS2 port. And while I am sure that mounting something magnetically to the roof of a car is doable, I am less certain why you would want to. Does he want to be able to re-sell the cars afterwards? Or does he want to be able to have 10 cars but only 5 light-bars? Then there is the problem of running wires from the control box (which probably goes on the dash) to the temporary light-bar (on the roof).

I should probably not get involved in this discussion, but it is a way to procrastinate writing my thesis.

[Algeh]

I have no opinion on the wiring aspect, but rather than magnets might it be possible to permanently install roof rails (as used for mounting various roof racks and commonly found on a variety of cars) and then temporarily mount the lights to those as needed? It seems like that would be a more tested system for attaching things to the roof of a car on a temporary basis.

[Andor13]

Well, his motives escape me, but...

First, mounting the bars themselves, if his goal is that there is no hardware mounted on the car when the bars are off, I don't think it's going to happen. Also I'd avoid magnets personally, I think any magnet strong enough to make that thing safe at highway speeds is also strong enough to present a serious hazard to fingers when mounting/unmounting. I recommend some kind of permanently installed bracket on the car, which the bar then mounts onto. Any number of mechanisms should work, but maybe a clamshell type closure with a hole so you can put a lock on it? I suggest something lockable anyway.

As for the power cord, it depends on the current draw. If it's low enough that the lighter socket can handle it (which seems unlikely since it's halogens and not LEDs) you could just put a lighter plug on it and call it good. Since that's probably not the case you're going to want to hardwire a socket that the bar can plug into. It needs to be weather proof both open and closed. Could be something like this, but there are thousands of options. Note that each vehicle will need to have both brackets and power port installed. Which might not even be cheaper than buying multiple lightbars, but I think you know that.

[Excession]

Attaching the light to a roof rail, as used by roof racks, seems to me like it would work. What type of rail you need will depend on the car. My car has mounting points for one type built in, visible as removable caps in a black strip at both sides of the roof. Other types clamp onto the roof gutter on older vehicles, or to the top of the door frame. You can get rails that remove easily with a key, which sounds like it would be useful to you. This page would be a good starting point if you were in New Zealand. From the look of these instructions, same light bars use similar attachments, not screws through the roof, but they won't be as easy to remove and reattach.

Wiring is something I don't know much about. I do know that LED light bars will drawn far less power, which may make wiring easier and won't drain the battery nearly as fast if left on when the car isn't running. They also add less wind resistance so you will save on fuel.

Edit: it looks like the lights you linked, and others I saw, are more complicated than a simple 12V supply, as they have multiple lights you can control independently. That's going to make wiring even more complex, and simply shutting the cable in the door will be more likely to break one of the wires. One LED bar had separate power and control cables.