Have a'lot of 3v Coin Cell battery's lying around and want to see which ones work? Use this super easy to make and use battery tester!
Take the cork and cut it in half. There's no need to measure, just take the larger half.
Cut out a slot on one side to hold the battery.
Drill a hole on the other side so the LED's leads can stick through to the battery.
Put the LED leads through the hole so a battery can go in between them and power the LED. If the leads stick out from the slot bend them around the cork.
Use the electrical tape to wrap the cork giving it a cleaner look and holding down the bent over leads.
Put the battery in the slot and look at the brightness of the light to know the power of the battery!