We often hear people talking about protected 18650 and unprotected 18650, but what the difference between the two? To find it out, first we need to figure out what all these numbers of 18650 mean.
18 stand for the diameter of the battery in millimeters, which is 18mm.
650 stand for the length of the battery in millimeters, which is 65.0mm.
Likewise, 14500 means the battery’s length is 14mm and the diameter is 50.0mm.
These measurements are for unprotected or flat top cells. If a cell is protected, an external protection circuit board is welded to the bottom of the bare cell, and flat wire is added to the side of the cell. Protected 18650 can be 2.5mm to 3mm longer, and 0.5mm wider than unprotected 18650 because of added external protection.
What’s the effect of the extra protection? Some flashlights have tight tolerances, so adding the little extra length can leads to the battery not being able to fit the flashlight. Unprotected cells do not have this protection circuit, so they are more flexible to fit sometimes. And they can have more capacity and current capability than protected cells. This is why some E-cigarette cells demand high discharge current to enable the vapers to have big clouds. Besides, the protection circuit makes the protected cells more expensive than the unprotected one.
In spite of this, protected cells have some superiority over unprotected cells. Protected cells are safer to use because it protects against common dangers, such as overcharge, over-discharge, short-circuit overcurrent and overheat. Unlike unprotected cells, they are less likely to ignite and cause damages like "venting with flame." Unprotected batteries may be suitable for experienced users, but for ordinary users, it never a bad idea to have an extra safety feature when it comes to electronics.