As you may recall, a few weeks ago, Max Maxfield roped me into his ongoing robot project. This led to my writing this series of articles 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, as Max would say. In this article, we consider alkaline batteries, but first...
Tip No. 5: Selecting batteries (continued from this post)
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 (see Tip No. 4). 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.
Carbon zinc batteries are the least expensive of the primary batteries. They are useful for low-current or intermittent devices. They have a moderate shelf life and (these days) are fairly tolerant of abuse.
- Specific energy: 36 Wh/kg
- Energy density: 92 Wh/L
- Specific power: 10-27 W/kg
- Discharge efficiency: 50-60% (medium discharge)
- Energy/consumer price: 3.2 Wh/dollar
- Self-discharge rate: 0.32%/month (newest types, at four-year term)
- Cycle durability: not applicable (primary battery)
- Nominal cell voltage: 1.5 V
- Cutoff voltage: 0.75-0.9 V per cell, loaded (see manufacturer)
- Temperature range: -20 to +55°C
It is interesting to note that the carbon isn't actually part of the important chemical reaction. Graphite and carbon are used only to increase the cell's conductivity.
2MnO2(s) + Zn(s) + 2NH4Cl(aq) → H2O(l) + Zn(NH3)2Cl2 (aq) + Mn2O3(s)
2MnO2(s) + Zn(s) + ZnCl2(aq) + 2H2O(l) → 2Zn(OH)Cl(aq) + 2MnO(OH)(s)
The following two figures are for the AA-size, Zn/MnO2, 1215 battery from Eveready.
Discharge curves using a constant-current load for various cutoff voltages.
Discharge curves for a constant-resistance load, instead of a constant-current load.
Energizer offers a nice application manual on its website. In my next column, we'll look at some more tips and tricks, and we will consider another battery technology. In the meantime, I welcome any questions or comments.