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Converting lithium-ion to primary battery

Posted: 17 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:lithium-ion? power source? DC-DC converter? LED? boost converters?

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 trade-offs may need to be made.

Potential solutions and strategies
The following sections describe the strategies that may be employed.

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 maximisation 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 -40C, are leak-proof, and deliver the highest energy at a high and stable voltage, making them well suited for boost-converter applications that require constant power.

A single-cell solution requires a boost converter to step the single-cell voltage range up to a usable, regulated 3.0V or 3.3V system input-voltage range. Readily available boost converters, such as the example from figure 3, integrate the entire switch-mode power solution, including compensation components, and provide the system with a low-current shutdown (figure 4.

Figure 3: AA Single-cell MCP16251 boost converter.

Figure 4: Showing blocking path and current-limit function.

Figure 5: Two series cells-powered MCP1643 application, shown driving high-power LED.

The high-power, two-cell design
For higher-power applications, two cells in series can be used, providing longer run time and higher current. As an example, Microchips MCP1643 high-current boost converter can regulate up to 500 mA from two cells in series for high-power LED applications, while providing true disconnect shutdown, current limit and short-circuit protection (figure 5). A high (1MHz) switching frequency enables small inductors and capacitors, reducing size and cost. The MCP1643 can also be turned off, minimising battery drain to less than 1 A. A regulated current source can be developed with a single integrated device, from two primary lithium batteries (figure 6). A low, 120 mV Vref is used to maximise system efficiency. Optimum performance is enabled by their high and consistent voltage profile across a wide range of discharge rates.

Figure 6: Output current capability of two-cell + MCP1643 solution.

The buck-boost, three-cell design
For some applications, the input-voltage range can be greater than or less than the desired output voltage. This can be true for either primary or rechargeable batteries. For example, lithium-ion battery voltage can range from 3.0V to 4.2V throughout the charge and discharge cycles. Three alkaline batteries connected in series will measure about 4.5V when fresh, but as low as 2.4V when depleted. Similarly, three primary lithium batteries connected in series will measure about 5.1V when fresh, but about 3.6V when depleted. The application, however, may require a regulated 3.3V rail, which suggests that a power supply with buck-boost capability is needed for this configuration.

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