In this article, we focus on zinc-air batteries, including their advantages, disadvantages, and chemistry.
The zinc-air battery
This type of battery has a very high energy density and is very light compared to sealed types. Since oxygen is a reaction consumable, the battery isn't usually sealed, or it would run out of air and quit prematurely. The battery has a high internal resistance, so it is suited to low-rate discharge, except for large-vehicle packs, where forced air improves the overall performance.
The humidity in the atmospheric air has an impact on performance, so this is a factor with regard to the deployment of this type of battery. When sealed from air and kept dry, the industrial version has an indefinite shelf life, whereas miniature cells can be stored for three years or so with little loss. There are two recharging techniques: mechanical (replacing the zinc and/or electrolyte) and electrical (using conventional charging technologies). Also, the battery is well known for its relatively flat discharge curve and low price.
- Specific energy: 442 Wh/kg
- Energy density: 1,673 Wh/L
- Specific power: approximately 100 W/kg
- Discharge efficiency: 50% (rechargeable version) or 60-70% (primary version, low-rate discharge)
- Energy/consumer-price: 2.8 Wh/dollar
- Self-discharge rate: 0.17%/month (sealed, use within three years)
- Cycle durability: high (with good care, rechargeable version)
- Nominal cell voltage: typically 1.45-1.65 V
- Cutoff voltage: 0.9 V per cell, loaded (see manufacturer, some use 1.05 Vpc)
- Temperature range: typically 0 to +50°C (see manufacturer)
O2 + 2Zn → 2ZnO (E0 = 1.59 V)
A very good information guide on designations for battery numbering and tests is IEC 60086-SER, which covers primary cells and batteries in parts 1-5.
The two figures below from Duracell are for the 675-size, Zn/O2 battery at 21°C and 50% relative humidity.
Terminal voltage discharge profile using the standard test protocol.
In the discharge curve shown in Figure 1, using the standard test protocol, the load has a background of 5 mA, with a pulse load of 15 mA that lasts 0.1 seconds (15 mA/0.1S) once every two hours (119 M 59.9 S) for 12 hours per day (12 H/D).
Terminal voltage discharge profile using the high-drain test protocol.
In the discharge curve shown in Figure 2, using the high-drain test protocol, the load has a background of 8 mA, with a pulse-load of 24 mA that lasts 0.1 seconds (24mA/0.1S) once every two hours (119 M 59.9 S) for 12 hours per day (12 H/D).
Figure 3 depicts the effect of temperature on a typical service life.
Effect of temperature on service life (typical).
In my next column, we'll look at some more tips and tricks, and we will consider another battery technology. In the meantime, as always, I welcome any questions or comments.