Figure 2: AA lithium, AA alkaline and lithium-ion internal resistances
As previously mentioned, coin-cell batteries may be able to meet the requirements for size and voltage, but fall short if more than tens of milliamps of current are needed, due to their high internal resistance that leads to large drops in voltage when higher amounts of current are drawn. Although the data in Figures 1 and 2 depict AA size batteries, the characteristics will be the same for the AAA or AAAA sizes. Choosing the right size for a given application is a balance between available capacity and size.
Once a battery has been chosen that provides adequate voltage, current and capacity, the electronics must also be selected appropriately. Generally speaking, the more voltage boost provided through DC-DC conversion, the less current it will be able to deliver. Thus, it is important to consider the application, the battery and the power-conversion design as one complete system where tradeoffs may need to be made.
Potential Solutions and Strategies
The Low-Power, Single-Cell Design
For low-power applications, a single-cell battery with a simple boost converter can be used to replace the rechargeable lithium-ion battery and associated battery-protection circuitry, leading to lower total system cost. This design can provide long runtimes while keeping size and weight to a minimum. Various 1.5V battery sizes can be used, such as AA, AAA or AAAA. If size is most important, AAAA alkaline batteries provide good runtimes in a very small form factor. If long shelf-life, cold-temperature performance, leakage resistance, or energy maximization is most important, primary lithium batteries available in the AA and AAA sizes that have a 20-year shelf life, perform exceptionally well even down to -40°C, are leak-proof, and deliver the highest energy at a high and stable voltage,