In the anatomy of a smartphone, maybe the battery isn't the heart of the system. But at the very least it's the stomach churning out the fuel that keeps the processor pounding away.
In a recent post I talked about the limits to thinning down the form factor for smart phones. Note that the battery is not the roadblock. Rather it's the camera thickness that is limited by the physics of basic optics.
If you remember back to an earlier age of cell phones (or open up a lower tier device like a household cordless phone) you'll see that the form factor for rechargeable cells was the cylindrical design similar to a typical ‘AA' or ‘AAA' primary disposable cell.
The conventional cylinder has always made for an easy-to-manufacture and mechanically robust device. Thankfully, rechargeable battery design has evolved to allow greater design freedom for the smartphone product designer. Nowadays, most batteries are very thin bricks. Battery manufacturers can deliver a wide range of dimensions since a given battery energy density will depend only on the volume of the battery. How you get those cubic centimeters is largely up to the designer.
iPhone 4 Battery Image Courtesy iFixit.
Apple iPhone users may not be so intimately aware of the battery in their phones because Apple has always insulated its users from any dealings with the battery. Virtually every other platform requires the battery to be removed in order to insert the SIM card.
Although early portable devices used older technologies like nickel-cadmium (NiCd or often "NiCad"), the most popular secondary battery chemistry today is lithium ion. In particular, your smartphone is almost assuredly a lithium polymer unit which is a specific class of Li ion cell in which the electrolyte is held within a polymer material. The chemistry, more commonly known as LiPo, powers all manner of untethered products including ¬electric vehicles.
Li-Po cells improve the power output for a given weight compared to standard Li-ion cells (around 20% weight reduction without a metal casing). LiPo batteries are attractive for a broad range of applications since they provide a very high discharge to charge efficiency, greater than 99 % compared to less than 90 % for standard Li-ion cells.
Lower weight, form factor design flexibility, and near-perfect discharge efficiency make LiPo cells an obvious choice for smartphones. Luckily for the device you carry in your pants, the demands on cell phone batteries are much less onerous than for electric vehicles.
Watching an hour or two of YouTube on your phone sucks a lot of juice from your battery, but it is nothing like the sudden power spikes (in-flow and out-flow) that an electric vehicle battery will experience. Many of these batteries have started fires thanks to the heat produced in the cells by this type of rapid discharge. More recently and infamously, Boeing Dreamliner battery fire incidents grounded that aircraft.
Not only are your pockets safer, but you won't need tactical pants to carry a high-performance, high energy density device around with you since no special cooling, overload protection (or fire suppression) is required. (Although, it is certainly possible for catastrophic failure in portable electronics considering the litigation and product recall history.)
Since most LiPo battery designs are pouches without the metal casing of older cylindrical designs (external metal casings are usually added in vehicle applications), the cell volume will increase as the device is charged. If charging is not carefully managed, the device can delaminate and reduce the life of the cell. In extreme cases of charging mismanagement, fire or explosion are possible.
Streaming video to your smartphone isn't likely to cause that kind of expansion. On second thought, that might depend on what you're watching.
Recent articles about major components inside today’s smartphones:
Applications Processors – The Heart of the Smartphone
The Guts of Thinner Smartphones