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.
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