Just Curious (AW 18650 )

I was just curious why there is this wasted space beneath the positive 'teat' on the AW 18650's.
It's a good 3mm deep. Seems like everyone's trying to make things smaller ... with two 18650's that adds up to 1/4" of extra length.

Sleeper said:

I was just curious why there is this wasted space beneath the positive 'teat' on the AW 18650's.
It's a good 3mm deep. Seems like everyone's trying to make things smaller ... with two 18650's that adds up to 1/4" of extra length.

Click to expand...

That part of the cell is commonly referred to as the crimp assembly. In typically 18650 cells it contains the PTC (positive temperature coefficient) device, the CID (current interrupt device) and the vent. It also contains a plastic gasket that seals the cell.

The PTC gets hot when a lot of current passes through it, which increases its resistance by several orders of magnitude. When the PTC cools, the resistance goes back to normal. You can think of it as a resettable fuse, which protects the cell from an external short circuit.

The CID activates when the internal pressure of the cell gets too high. When it activates, the circuit is permanently opened and the cell is dead. Although, if the CID opens because the cell was overcharged, the cell is still potentially dangerous because it is impossible to discharge it.

The vent also activates when the internal pressure of the cell gets to high, and keeps the cell from "rapidly disassembling" due to high pressure.

Although all of these components take up space, they are considered necessary safety features. The crimp assembly is relatively the same size for all 18650 cells, except those that use a welded construction, in which case there is no crimp assembly.

Cheers,
BG