Tiny Power: Coin Batteries

They are all around us. In our watches, laser pointers, hearing aids, toys, calculators, remote control units, and more. Yet we don’t often notice them or give them much thought until they stop working. And then it’s time to poke through the selection at the local drug store or big box retailer, hoping to find one that has the same cryptic number (or at least something that claims to be compatible).

Originally developed for use in hearing aids, so-called coin and button batteries are, as the names suggest, small disc or button-shaped batteries. They come in a variety of formulations and shapes. They can be stacked in series to produce a higher voltage, or wired in parallel for increased current. If purchased in bulk (such as through vendors on Amazon) they are also surprisingly cheap, and they pack a lot of energy into a very small package.

So why aren’t more people using these miniature batteries in their projects? There is no shortage of available battery holders, nor is there any shortage of battery types. I suspect that more than anything it’s largely a matter of inertia: when most people think of batteries they think of AAA, AA, C, and D size cells, not coin or button batteries.

Coin and button batteries offer many advantages in terms of size and power density. Granted, even a large coin battery will not have the same energy density as a AAA alkaline, but if the circuit it is supplying only draws a few milliamps, then it is more than sufficient and will last a while in normal use. So, let’s look more closely at what coin batteries can do. Everything said here about coin batteries also applies to button types; they’re just a different package style, really. The following image shows a selection of the package types available:

coin_batteries

The first two batteries are commonly referred to as button batteries, and the other two are typical coin batteries.

Coin batteries come in four basic formulations: Lithium, alkaline, zinc-air, and silver-oxide. The two most common are lithium and alkaline. The zinc-air and silver-oxide types are typically found in the micro button batteries used in things like hearing aids, and in medical devices and pagers. For a more detailed discussion of battery chemistries see Chapter 5 of my book “Practical Electronics: Components and Techniques“.

Another consideration is power capacity. A coin battery simply does not have the same capacity as something like an AA alkaline battery. For this reason you won’t typically find coin batteries in flashlights, but they are used in low-power devices like calculators, pocket-size digital scales, and digital thermometers.

Coin batteries employ a numbering scheme defined by international standard IEC 60086-3, and once you know the secret code you can look at a full-form battery number and know how large it is, and what type of chemical formulation it uses.

The format for the full-form code is:

 [type] [package] [diameter] [thickness]

The following table shows how to interpret the type code of a coin battery:

coin_batt_chem_codes

The package code is always “R” for round coin and button batteries.

The diameter and thickness are in millimeters, with the thickness being a fractional
value without the decimal. The diameter is specified with either a one- or two-digit
value indicating the diameter of the case in whole millimeters (rounded down). The
diameter value is interpreted as shown in the following table:

coin_batt_size_codes

The thickness always uses just one significant digit (tenths of a millimeter). So, if we have a CR2032 battery, that translates to a round 3V lithium battery 20 mm in diameter, and 3.2 mm thick.

There is also a short-form code. Instead of incorporating the diameter and thickness a numeric code is used for the case dimensions. The format for the short-form code is:

[type] [package] [size code]

In order to interpret this code you need a look-up table for the various case codes, like the one shown below. The capacity ratings (in mAh) are shown for both alkaline (L) and silver-oxide (S) batteries. The ‘x’ in the IEC ID codes can be replaced with either an L or S, as appropriate. For the SR67 and SR68 types no alkaline (L) equivalent is readily available.

coin_batt_codes

LR41 is an example of this type of ID code. This would be a round alkaline battery of size type 41, which is equivalent to an LR736.

Lithium coin-style batteries use the full-form IEC ID numbers. The following table lists some of the more common types you might encounter in the wild. The ANSI designation is also given, where applicable. Capacity is in mAh.

coin_batt_ansi_equiv

So how would one go about using a coin or button battery in a small project? The first is to determine how much voltage you need. You can stack up coin or button batteries in series to create a higher voltage, but it won’t deliver any more current than a single battery. If you have a calculator lying around, pop open the battery compartment and take a look. Many of these employ multiple button batteries laid end-to-end in a tray, while others might have two or more coin types set side-by-side.

The next step is to determine how much capacity you will need. Batteries have a capacity rating (in mAh in the case of coin batteries) but that doesn’t mean you’ll be able to get that much current out over the course of an hour. Alkaline batteries in particular tend to drop off rather quickly, but will continue to deliver current for a time after they drop below their rated output voltage. Lithium batteries are better behaved, but they will still start to fade after a while even with energy left in the cell. In general, however, so long as you aren’t trying to drain the batteries all in one go, then you should be able to determine the expected lifetime as a function of a fraction of battery capacity. In other words, if you are using a lithium battery with a 160 mAh capacity, then draining the battery at 20 mAh should give you at least 6 hours of life. Ideally, however, you would want to avoid doing this and opt for a low-current circuit that only operate on-demand rather than continuously. See Chapter 5 of “Practical Electronics: Components and Techniques” for more on battery capacity.

Once you have a required voltage and a desired capacity, the next step is to figure out how to physically mount the batteries. There are numerous battery holder types available (just do a search for coin battery holder on Google). One type is shown below:

CR2025,CR2032 Battery Holder Through Hole Mount1

This through-hole CR2032 holder is from ITeadStudos, and it sells for about 50 cents. Other sources for CR2032 holders include Adafruit and SparkFun.

If you are handy with tools you can make a button battery holder from an “N” type battery holder (N-type batteries are found in some cameras and wireless doorbells). If you’re looking for 6V output (LR41 batteries are rated at 1.5V per cell), then four LR41 button batteries in series will fit nicely in the holder.

So the next time you need to power a project from batteries, consider the coin battery. They are rugged, have a long shelf life, and they are an excellent compact source of power for low-current applications.

 

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