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Help! Step down dc without regulating amps?

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First off I'm totally new to the electrical scene and only know what I've learned online and hands on. Which can be a bad thing if your listening to the wrong person. So here it is :

My Problem : I am using an 18 volt dc lithium drill battery to power led lights and a car horn on a project. The battery's charged are reading roughly 20 volts. I have tried to run the setup with a step down board bought on eBay and am having issues. Here is the board: XL4015E. It says it has a peak of 5 amps with extra heatsink. (which mine has) The horn will not work at all. With the lights on as soon as I push the horn button the lights cut out. I'm guessing it's from the high current draw of the horn.(upwards of 10-20 amps of draw according to google)

My Question: Drill battery's can supply upwards of 30-60 amps of dc power needed for the power tools they run. How can I drop the voltage from 20 volts dc to 12 dc without losing the ability to access the higher amperage of the battery? I don't want to use resistors as everyone says they are not efficient. Correct me if I'm wrong but from what I understand linear regulation is similar to the resistor method and I probably want a switching regulator? Is there a chip or something out there to drop the voltage efficiently and supply the current that my components are demanding? I'm looking for a small footprint to fit inside my housing so I can't have bulky capacitors. Putting the horn on a relay direct to the battery would be easy, but I want to be difficult...lol I'm trying to stick with 2 wires coming out of the housing and no more. Positive and negative. Would access to the higher amperage let the led lights draw to much and blow them?
Any help you guys can give me would be great. Thanks in advance, Bob.
 

spec

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Hi,

Everything you say is correct. All you need is a 12V 20A, or greater, buck (step down regulator) in place of the 5A regulator you have.

A horn normally takes massive gulps of current, so you could smooth this current by placing a high value electrolytic capacitor across the horn. Also, you can put a power resistor in series with the horn (and capacitor). Simply increase the value of resistor until the volume from the horn starts to drop. Maybe start with a 4,700uF (25V or greater) capacitor and a 0.47 Ohm resistor.

One of these 12A buck converters may do your job but, if not, two converters is parallel should: http://www.ebay.co.uk/itm/DC-DC-CC-...hash=item3aca5d44ea:m:mR9VGlivIMij3frn-QAQZSg

spec
 
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