A li-ion 18650 battery is a type of lithium-ion rechargeable battery which looks and operates exactly like a standard AA battery and usually has a voltage rating of 3.7. You can use a li-ion 18650 battery to power all kinds of devices such as torches, radios and toys. If you want to find out how much power is left in your li-ion battery you need a simple battery tester that works with AA batteries. These are available for a few pounds from a variety of online outlets including Draper Tools, Amazon.co.uk and 7dayshop.com.
Make sure that your battery's terminals are clean. Wipe them with a dry cloth if they are dusty or greasy.
Insert the battery into the battery tester. Ensure that you put the positive terminal, the end with the nipple, and the negative terminal, the flat end, into the corresponding terminals of the tester.
Read the meter on the battery tester. Most testers have a sweeping hand that moves across a dial marked with red for underpowered and green for powered. If in doubt, consult your tester's documentation.
If you have a battery that won't charge, dispose of it in a dedicated recycling point. Never put batteries in general household waste.