Beyond buttons and coin cells: How small can you go?

Article By : Bill Schweber

It is time to move on beyond buttons and coin cells, which have started becoming too large for the latest in portable products.

Coin and button batteries (cells) are an indispensable item for the many products which need energy in a small form factor and can tolerate very limited energy capacity. They are used both for ongoing operation as in watches and for power-outage/disconnect back-up in devices such as energy monitors and meters. They are available in hundreds of variations, with many differing only in fractions of a millimetre of diameter or thickness, while differ in basic chemistry and voltage.

While these power sources have been a good thing, they can also be a headache: obtaining replacements for some of the less-common ones can be a real challenge. If you need a type 2032 or 357, you can probably get it at a local store, but others are much more obscure. Of course, there are cross-reference tables available from major vendors such as Duracell and less-known manufacturers, as well as from replacement-battery distributors. (Unfortunately, there are many substandard and outright counterfeit batteries out there, too.)

But now it is time to move on beyond buttons and coin cells. Even the tiny button cells are too large for the latest in portable products, such as the wearable health and wellness monitors, smart glasses, and stylus pens. Battery vendors are introducing primary (non-rechargeable) and secondary (rechargeable) cells called "pin batteries," which are cylindrical in shape. Among the latest, but certainly not the last, is the G-320A Pin-Type Li-ion battery from Panasonic (Figure 1).

[Panasonic G-320A Li-ion (cr)]
__Figure 1:__ *An energy source such as this G-320A Pin-Type Li-ion battery from Panasonic is easy to lose on the floor with its thin cylinder body and small dimensions. (Source: Panasonic)*

This rechargeable power source has a nominal capacity 15.0mAh with a diameter of 3.65mm, a length of 20mm, and a weight of 0.6g. The nominal voltage range is 3.0 to 4.35V, with a stop-charging limit of 4.45V. The no-nonsense two-page data sheet is crisp yet informative, and includes the usual dimensional details as well as critical charge/discharge information (Figure 2).

[Panasonic G-320A datasheet (cr)]
__Figure 2:__ *The G-320A data sheet includes the standard maximum/minimum dimension data, but also the critical electrical-protection boundaries for over-charge and over-discharge operation. (Source: Panasonic)*

Of course, a battery is only part of the power-subsystem situation. How it is held mechanically so it makes good contact yet is replaceable (if that is required) is another story. I have seen many otherwise-decent products that had poorly designed or implemented battery holders or contacts with poor fit, insufficient contact pressure or poor materials/plating for the contacts. Vendors of battery holders, such as Keystone Electronics Corp, are likely developing discrete holders for these pin batteries, but some end-product designers prefer to form their own integral holders from the product's enclosure material to save space and cost—these are often of marginal reliability.

It will be interesting to see which pin-battery models become the most popular ones and get stocked at the local stores. I hope end-product designers stick to just a few models and types; we don’t need proliferation of hundreds of pin-button models as we have with button and coin cells.

Is that an unrealistic wish? What's been your experience with "oddball" small batteries?

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