18650 Batteries are rechargeable lithium-ion cells commonly used in flashlights, vapes, and battery packs. They are traditionally 18mm in diameter and 65mm in height (18650). There are a few differences to note when choosing your correct replacement.
Button top or flat top? Usually flat top cells are used in rebuilding battery packs but sometimes, devices are fitted with flat top cells.
Protected or non-protected? Protected cells are protected against over discharging which can damage your battery. The protection can add about 2-3mm to the height of the battery so protected cells are 18mm in diameter and 67-68mm in height. It is important to measure your existing battery to make sure the replacement you choose will fit into your device.