andycivil's video: Test a button cell using a red LED if you have no meter

Use one of the original type of red LEDs to see if a button cell has any power left in it. This is important if you're using up some old batteries in a device that uses more than one. *Different coloured LEDs will not work, so please don't complain about this* They need a higher voltage. The best way to manage your button cell powered toys is to replace all the cells at once, and recycle the old ones. But if you're trying to get every last drop of power from a collection of old cells, it's important to know that each individual cell is good before you use it. Dead cells will leak and corrode if you force them into service by stacking them with good cells. If you have a meter like this, that's perfect because you can check that each cell gives 1.5V. But if you don't have one, I'll show you how to do a quick check using an LED. You have to have one of the original kind of red LEDs. These are the only ones that will turn on with the low voltage that one button cell has. These are rated at 1.85V, but they will JUST about turn on with 1.5V. Test your LED with a good cell, so that you can scale your expectations; remember how bright it was. Don't expect too much, because we're right on the borderline of even being able to turn it on at all. If your LED isn't clear like mine, you might even have to do this in low light. Now you can use that to see if a cell has a little power left like this one, or almost dead like this one. Note that LEDs only work one way round, so make sure you do it the right way. At this low voltage, putting it backwards won't hurt, it just won't come on. Oh, and by the way, don't bother trying this with a 3V lithium cell. Even a flat lithium cell will still have the voltage to light an LED, so seeing this work doesn't mean the cell is good.