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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?

To overcome the voltage drop, especially if alkaline batteries are used, an energy reservoir needs to provide additional current when needed. A capacitor is the natural element to use, and a wide range of sizes and capacitances is available. With values up to and above 1F available in the lithium-ion voltage range, there remain two additional challenges in implementing a solution. First, the capacitor will have very low internal resistanceparticularly when emptyso a current limit needs to be placed between the primary cells and the capacitor. Second, introducing a current limit assumes an intentional resistance that needs to be controlled and managed to keep the efficiency high. A microcontroller with integrated analogue that controls an external MOSFET solves both of these issues. The remaining design challenge centres on sizing the capacitor and MOSFET.

Figure 9 illustrates the blocks needed to control and protect the capacitor, and which items are integrated into a microcontroller. A suitable microcontroller will contain an internal oscillator, A/D, comparators and op amps, so that the external components needed are limited to just the MOSFET.

Figure 10: Flow chart of software algorithm.

To design the solution, two elements need sizing: Capacitor value and current limit.

For a design example, the assumed current peak will be 1A for a maximum of 500 mS, with a minimum repeat rate of 5 seconds between peaks.

The capacitor needs to keep its voltage value between 3.0V and 4.2V, during the 1A discharge of 500 mS, and recharge in 5 seconds.

Based on the capacitor discharge equation and the voltage assumptions above, the following equation sizes the capacitor:

C = t*Ipeak

A 0.5F capacitor provides the necessary reserve to deliver 1A for 500 mS.

To recharge the capacitor for the next peak current, the circuit must charge the capacitor back up within 5 seconds. The value of R will be determined by the following equation, which is, in turn, based on the charging-capacitor equation and the terminal-charge value:

R = -t/(C*ln(1-V0/Vb))

Solve for t=5s, where C is the capacitor value already determined, V0 is 4.2V, and Vb is the nominal battery voltage. In this example, the R value solves to 3.7 ohms. The current range peaks at 406 mA and drops to 81 mA, during the five-second charging window.

The circuit needs to monitor the capacitor voltage. It turns on the switch when the voltage drops below 4.2V, and turns off the switch once the capacitor voltage matches the battery voltage or arrives at 4.2V. Figure 10 illustrates the algorithm for the firmware in the microcontroller. As shown, the firmware complexity is very simple, leaving room for other functions. For example, a smarter version of this firmware could modulate the switch to achieve a constant current over the five-second window, to lessen the impact on the battery stack.

Conclusion
One of the many design challenges facing engineers today is the choice of which battery technology to use. Many designs use lithium-ion cells when the power requirements and the rate of usage are high. Other designs can attain long life with primary cells and take advantage of some of their less technical benefits, including:
???Lower bill-of-material cost
???No charger solution required (external and internal)
???Lower weight
???Instant use without waiting for recharging

Converting a lithium-ion design to primary cells is sometimes an intermediate step in the design process, and can be accomplished with the example solutions described in this article. These solutions are listed in order of increasing current output and complexity. There are, however, many alternative solutions, since the use cases for batteries are as varied as the designs that use them. Designers need to select a solution that is ultimately based on the needs of their end applications. The methods outlined above are meant to provide just a few approaches from which to begin.

About the authors
This article is contributed by Terry Cleveland, John Haroian and Adam Jakubiak.

To download the PDF version, click here.


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