18650 battery breakdown: how does it work?

Battery 18650

The e-cig market has exploded in recent years, and the sales of batteries to power these devices have followed suit. The most widely used format is the 18650, which means 18 mm in diametre and 65 mm in length. And to be specific, this is not a battery but a cell, as a battery may be made up of multiple cells.

Tesla automobiles are powered by a series of 18650 cells. The model 70D carries a total of… 5850! You can do the math to figure out how much that would cost.

These batteries are indeed ubiquitous, but do you know what the cylinder is made up of?

BlogVape has done the research so you don’t have to. Warning, do not try this at home! Taking apart cells and batteries in general can be highly dangerous.

This is the outer shell of a disassembled battery:

Battery 18650

The black plastic cap on the left, on the positive terminal, insulates the positive electrode from the negative. This is in fact the only barrier protecting the battery from a short circuit. On the other side, the negative terminal is flush with the battery body, made of a single block. The body is often covered with thermo-plastic sleeve that indicates the brand and size of the battery.

Samsung Patent
Samsung Patent

The metal is nickel-plated steel, while the insulating material is generally made of nylon.

The next component is the PTC or Positive Temperature Coefficient. It is located under the positive terminal and serves to limit the battery’s output current. It is a disk made up of conductive material for which the resistance increases as temperature rises. Thus, if the cell overheats, the PTC will limit the output current thanks to its structural design (rather than an electronic component as most would expect). The threshold is usually in the 120 to 130°C range, but in real conditions, batteries should be kept under 60°C to maintain an optimal life cycle.

Batterie 18650
The components inside of the yellow insulating material are the positive electrode, and on the outside it’s the negative electrode

Under the PTC you will find the CID or Current Interrupt Device. This thick metal disk is vital, as it serves to prevent the battery from exploding in case of degassing. Under pressure, the CID will dilate to let accumulating gas escape the cell (while this is safer, degassing is still very bad news). The CID is triggered at pressures in excess of 2.8MPa, that is to say 28 times Earth’s atmospheric pressure.


Like a chocolate roulé…

BatteryFinally, the interior of the battery body, the electrical storage component, is made up of 8 sheets stacked then rolled into a tube. These layers are made up of an insulating material, sheets of graphite composite, copper, composite layers for the cathode, and finally a sheet of aluminium.

If you wish to know more about the design and technology of these batteries and cells, you can take a look at this NASA (!) research paper on PTC, or this excellent article signed Doctor Maria Wesselmark.