I have a device that plugs into the wall and converts the power to 12v 1.5 amps
I know a car battery is 12 volts but many more amps.
How can I reduce the amps going into the device I want to power with the car battery?

The 12v 1.5amp device that you plug into your wall simply means it provides 12v DC and is capable of sourcing up to 1.5amps. It will only source as much current as it needs to.

You shouldn’t have any problems hooking up a 12v battery straight to your device.

http://www.powerstream.com/dc-dcz1203.htm

I purchased that, the weatherproof one.

What those do is only to reduce from 24v or 13.5v to 12v ..all dc.
If you plugged it into the wall you are doing it very wrong ;p

Now to answer your question Amps do NOT need to be reduced.
They are not a constant input and they are only used if your device needs.
While voltage being a constant input it needs to match your device specification in order to properly function.
But not Amps.

As long as your source if energy produces more amps than the device you will be using, it is good.
You only need to match voltage.

Edit: imagine like this:
A water pump is pumping water through a large pipe..
The speed the water travels through the pipe is the voltage in electricity (abstractly speaking)
The amount of water that travels through the pipe is the amps. If you need more water you open the pump valves, however it travels at the same speed throughout the pipe as with less water.

This post was edited by omghacks2 on Jan 29 2012 03:27pm

I'm plugging it into a car battery. It's output is 1.5 amps.

From other places I've read online if I allow more amps to go into my device (a windpad) it will charge the battery much faster causing it to overheat and possibly fry.

Once again, amps do not go anywhere neither they are used if your device doesn't require them.
You are reading the wrong forums it understanding them wrong, one of the two ^^