1. Different sizes. Although the 18650 and No. 5 batteries look a bit like the two, the size of the two is actually different. The 18650 is much larger in length or diameter than the No. 5 battery.
2. The voltage is different. The voltage of a lithium battery is generally around 3.5~4.0V, the most common being 3.7V; and the voltage of the fifth battery is 1.5V (non-rechargeable) or 1.2V (rechargeable).
3. Its working principle (internal chemical reaction process) is different.
4.18650 is a rechargeable battery, most of the ordinary 5th battery is a disposable battery, and can be charged very little.