What is an 18650 battery?

An 18650 battery is the sum total of its mechanical and chemical components, neatly wrapped in an occasionally colourful wrap and used in everything from your electric toothbrush to electric vehicles and everything in between. You name it, if it doesn’t require a constant connection to a wall socket, it could have one or more 18650 batteries inside. While we’re writing this from the view of the 18650, the below is true and relevant for almost all lithium cylindrical cells/batteries, including but not limited to 18350, 18500, 20700, 26650 and 21700.

Lets start with the basics;

Lithium cylindrical batteries of all chemistries are “named” by their size. In this case they’re roughly (save for slim manufacturing tolerances and wrap thickness) 18mm wide, and 65mm long. 21700 batteries by comparison are 21mm wide and 71mm long.

They all have a metal outer which comes in several parts;

The main body of the can which is entirely Negative polarity

The Positive terminal

The vent disc (directly below the positive terminal)

The “not so basics”

Internally, there’s a lot going on so we can look to further break this down;

The bottom of the cell can – Actually very little is here, just an insulation disc to isolate the cell windings from the cell can and the Negative/cathode connection from the cell winding to the can itself.

The top of the cell can – This is where the bulk of the internal components sit. Inside the top of the can from lowest to highest, you’ll find most if not all of these components – Anode tab (coming from the centre of the winding and attaching to the bottom of the CID) and mounting disc, PTC (if fitted, in the below image it’s the very thin black disc between vent disc and positive terminal), CID (the slightly curved piece in the below image), vent disc (directly below the positive top) and a gasket (the semi-clear plastic disc separating in the outer cell wall from the rest of the top cap components)  separating the Negative and Positive cell connective crimp (the indentation on either side you can see when you’ve removed the outer wrap)

Everything in between

The cell “winding”, seen below the blue line in the above  cell cross section photo. This part is made up of the components which store energy from charging and release it on demand. The winding is actually a very long (almost a metre in fact), wound roll of multiple layers of components that we’ll get into later on.

Here’s a CT scan of a Sanyo NCR18650B cell, labelling the components you can find inside (we don’t recommend you go looking. Disassembly of lithium cells is highly dangerous!);

Lets look at how all of these components work together

Going back to the top end of the can and the photo above, we can take a closer look at the components and how some of them work both alone and together;

  • Gasket

    Not really visible in the above image (but visible in the cross section further above) due to its material and this being a CT scan, the gasket serves as a water, air and to some extent pressure resistant barrier between the internals and the outside environment. Lithium is highly reactive with Nitrogen (ignites on contact) and since Nitrogen makes up 78% of the air we breathe, you don’t want it getting out or air getting in! Likewise, the chemistry inside (aside from being readily and easily reactive with normal air) is easily damaged and requires protection from normal outside environmental factors like moisture and corrosion causing Oxygen.

  • CID

    The CID or “Current Interrupt Device is a mechanical pressure valve and will (in most cases when acting correctly) permanently disable the cell if the internal pressure reaches over 145psi. This acts as a primary defence to over-charging and external direct shorts. It also works in tandem with the PTC switch.

  • PTC

    Not used in all cells and normally in conjunction with the CID, the “Pressure, Temperature, Current” switch is a disc of composite construction – semi-crystalline polymer and conductive elements. While operating normally, it allows current to pass with little resistance. Under fault conditions (excessive temperature, current flow or extreme ambient temperature) it (in the least scientific terms) melts, causing a sudden high resistance and therefore limiting current flow.

  • Vent or “Burst” Disc

    Found and visible directly under the positive terminal, the vent disc only has one purpose, to stop catastrophic failure of the cell. Rather than simply allowing the cell to explode in any direction and throwing fragments of cell can and dangerous chemicals/components/gasses everywhere, the vent disc has manufactured-in weak points – scored lines either in the form of a circle, lines radiating from the inside outward or both. These are designed to rupture at a specific pressure, allowing a controlled “venting” of the battery’s energy in a single direction. Some newer generation cells from Samsung and Sony/Murata also have them built into the negative end too, visible from the outside as a shiny non-complete circle on the normally matt surface. Rest assured that whilst rather spectacular to watch a vent is not to be taken lightly (normally the primary cause of fire and/or burns ot person(s) or property). If a cell vents, its doing/done its job properly. Something was wrong for this to happen.

    The CID, PTC and Vent/Burst disc are all positive polarity.

  • Can crimp and “Spin Groove”

    The can crimp above and below all of the above components holds and seals them all together, suspended and isolated from the outer cell can (negative polarity) by the Gasket.

    Below is an example of the routing for a normal positive end vent disc;

Here is an image of the new style bottom vent being used on the Sony VC7;

As mentioned above, there is very little going on at the bottom of the cell can. There will be a non-conductive insulation disc which stops and internal shorting of the winding on the outer (Negative/cathode) part of the can. Through this will be one or several (in the case of high drain cells) very small metallic tab(s) connected to the cathode winding and firmly fixed to the bottom of the cell can, normally with a small resistance (spot) weld.

The Winding

Despite (chemical composition aside) this being where all of the magic happens, its actually all very simple, visually. Working through the layers that you can see in the images above and below you’ll find the separator, Cathode current collector, Cathode material, Anode material and Anode current collector in here.

The composition and “blend” of materials of these windings (both chemical and components used) is what determines the overall ability of the cell in terms of capacity, discharge rating and internal impedance (resistance). The anode and cathode materials are applied to both sides of a metallic foil, and these act as the current collectors for current moving in and out of the cell. The anode material is made from Carbon, and the cathode from a Lithium metal oxide. Ordinarily the anode winding will be made from Copper foil and the cathode from Aluminium, much like the tin foil you use at home.

You can probably see by now just how much goes into making an 18650 or other lithium cylindrical batteries. Not only is there a lot going on inside, billions of dollars of revenue are spent on the design and manufacture of these products, from physical components to small tweaks of chemistry to increase capacity or output ability. All of this, creates the mostly widely distributed, energy dense and readily available battery form factor available to date.


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