wiring a car horn

It MIGHT. The problem is one of power consumption. A horn takes a fair amount of amperage- I'd guess between 3-10 amps depending on a number of factors (coil size, wether or not it's a new horn, if the ground is adequate, etc.). You also need a battery sized for the job- a battery that outputs, say, 750mA will definitely not do the job. A battery charger normally won't do it either- the output from most of them is 'pulsed'- not a steady stream of power. What you could do is use a barely functioning car or motorcycle battery, and hook a charger to it. The charger will keep the battery functional enough, and the battery acts as a filter, smoothing the pulses of the charger. You could also buy yourself one of those rechargable battery boxes (any serious car guy or gal should have one of these anyway, especially since they're so cheap), charge it up, then keep it plugged in and charging during your fright fest. Wiring your horn is far easier. The horn mount or mounting bolt is usually the ground. Make doubly sure it's a clean, solid ground connection to the battery negative, or your blast will wind up sounding like a sick baby elephant. The positive terminal will go to one side of your switch, your horn positive to the other. Use 14ga wire to do your hookups, and use a switch large enough amps-wise to do the job. One of those side-of-the-column ones should do what you want- just make sure you don't ground the terminals to the switch case. I would recommend a fuse (15A), but here I don't think it's a necessity. It will, however make it easier to disconnect quickly should the horn stick on.    

usen a battery charger just get a good one need like 5 amp cont.dont be a pet monkey ..use your dmm