I use 3-volt, flat disc, watch batteries to test LEDs. The problem was that holding the leads to the battery was sometimes problematic, at best. This wasn't the case when testing a single 3mm or 5mm LED. Then I could simply fit the battery between the leads and squeeze a little. However, surface mount LEDs and circuits containing LEDs (ie. inside a lighted structure) were a very different story.
So, I made this simple gadget from a clothespin, two brass brads, some bits of wire, and two mini alligator clips. It works quite well. I drilled through the jaws of the clothespin then inserted the brads with their heads meeting inside the clothespin's jaws. I then soldered red(+) and black(-) wires to the brass brads, and cut off the excess length of the brads. I soldered the mini alligator clips to the other end of the red & black wire leads. For good measure, I painted the halves of the clothespin red and black so I could always be sure of polarity when inserting the battery. By the way, don't leave the battery in the clothespin when you're not using it. The leads have a nasty tendency to short to each other and deplete the battery. Also note that I did not include a resistor. With only three volts and limited on time, it's not really necessary to use a current limiting resistor, but you could easily add one if you want to.
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3v battery holder1.jpg
3v battery holder3.jpg