You can salvage lithium 18650 cells from all sorts of modern ... Or if you’re a bit more adventurous, you can build this “Battery Health Monitor” designed by [DIY GUY Chris].

So [Travis] decided to upgrade it to make use of the ever-so-common, 18650 lithium-ion battery. Not looking to purchase any components [Travis] performed this hack using simple recycled household ...

"You will, at the very least, burn your hand if it short circuits," he continued ... that consumers can usually identify a hazardous 18650 battery by inspecting its appearance.