Sorta OT: how much heat from buck converter?

Flashlight Modding and DIY Parts
1

As in the title, this one is a bit OT, but I'm hoping you can help.

I have a Dewalt radio -- the DC011 -- that charges batteries and makes noise. Problem is, my phone (source of music) somehow drains its battery through the workday. So I figure I should put in a USB port for charging.

I've found 12.2-13.6 volts available (depending on whether it's on AC or battery, for what that's worth) and don't need much current, so found a couple of options on the 'bay:

http://www.ebay.com/itm/150978204003

http://www.ebay.com/itm/251267679613

These guys don't put out much -- .6 to .8A, which seems about right.

So my question: if it's making 7 volts magically disappear, do I need to worry about heat? I could put the boards on a heatsink, but it would be inside a sealed case. (Though I suppose I could be fancy and push some of the heat sink fins outside the case...) This thing isn't going to be running all the time... probably about 2 hours at a time to charge the phone. Is there something else that would be better suited to my needs? Anything else (like stealing power) that a noob like me should worry about?

Thanks in advance for the advice.

2

My default answer to anything like this is always 'use one of the adjustable LM2596 boards', I've used them and know what to expect though the ones you linked may work perfectly well. The most common version has only a voltage adjustment, but there are more complex ones that also add in an amperage adjustment as well if you think you need it. At the power levels you're talking about the LM2596 boards will run no hotter than ambient temp, they only warm up once you get above 2 amps.

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Thanks, comfychair — good advice, though having too many knobs to twiddle makes me a tiny bit nervous. I think I’ll get a couple of the simple ones and a couple complicated ones and have a play.