The lowdown on batteries: Carbon zinc

As you may recall, this series of articles focuses on the various battery technologies available to us. Along the way, in addition to the nitty-gritty technology details, I'm including tips and tricks on selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge. In this article, we consider carbon zinc batteries, but first...

Tip No. 5: Selecting batteries (continued from The lowdown on batteries: Alkaline batteries)

After computing the watt-hours at the load and determining the acceptability of the loss component, we could simply choose a battery with the rated watt-hours (or amp-hours times average voltage). But watch out for the manufacturer's ratings. Not all watt-hours or amp-hours are created equal.

When the manufacturer has discharge curves, you can use your load to find the run time. However, some battery chemistries do not lend themselves to high-rate discharge, so it is more useful to calculate the watt-hours or amp-hours by measuring a smaller load over discharge spans such as five or 20 hours, instead of a single hour. Then the watt-hours or amp-hours are normalized to one hour, which artificially inflates the capacity, making the battery look like it could actually draw the listed watts or amps for one hour. As mentioned earlier, when the manufacturer doesn't specify this time span, the battery usually will be somewhat close to the listed rating when discharged over the course of an hour.

Let's consider an example, courtesy of the LC-R061R3P datasheet from Panasonic. A discharge test is run where the average discharge voltage six volts, the load is a constant current at 65 ma, and the discharge test is stopped at the end-of-life voltage of 5.3 V in 20 hours. The manufacturer lists the rated capacity as: 1.3 Ah (20-hour rate). Some manufacturers list the time as a C rate, such as C/20.

This rate is normalized, 0.065 amps x 20 hours = 1.3 Amp-hours, or 6 V x 1.3 Ah = 7.8 Wh. Note that this does not mean that we can draw 1.3 amps for one hour. In fact, the manufacturer may be nice enough to give us the actual one-hour draw. In this case, the measured one-hour capacity came out to be 0.85 Ah.

The carbon zinc battery
Carbon zinc (or zinc carbon) was originally a wet cell (called the Leclanche cell) and made with a packed block of manganese dioxide. A non-wet version patented by Carl Gassner that used zinc chloride instead of ammonium chloride became available in 1886. In 1898, Conrad Hubert and W. Lawrence formed the Eveready Co. to sell the cells and further develop the technology. Over the years, Eveready and other companies have made improvements, such as sealing technologies and purer materials.