Challenges with Conversion
Any combination of reasons may lead you to consider designing around a primary battery, but you may be hesitant to switch because of the aforementioned highly integrated power supply. It may then be necessary to first prove a primary battery, which fits the device’s form-factor constraints, can even handle its potentially high-power requirements. Although the best prescription for long battery life will almost always be to redesign the power system from the ground up, there may first be a need for an intermediate design that quickly retrofits the existing lithium-ion design, with the various primary-battery solutions under consideration. This design can then be used to quickly and cheaply determine feasibility, while estimating minimum performance.
Designers looking to retrofit an existing lithium-ion-powered device with a primary-battery solution are likely to confront a number of technical issues, and the first one may be voltage. Lithium-ion batteries typically operate between 3.0V and 4.2V; whereas primary alkaline and lithium batteries in the AA form factor typically operate between 0.8V and 1.6V per cell (see Figure 1).
Figure 1: Lithium ion, AA lithium, and AA alkaline voltages during discharge
Lithium coin cells would offer a 3.0V supply; however, they will not be discussed here as they lack the current-sourcing capability that is often needed in lithium-ion applications. Therefore, this intermediate design will likely necessitate the use of multiple cells in series, a DC-DC voltage converter or both, so as to operate within this voltage window. It is also important to understand the range of voltages with which the device will operate, as it is possible that voltages outside of the lithium-ion range could be acceptable.
This leads to the second potential issue that must be addressed—matching the current needs of the device with the sourcing capabilities of the battery and the operating ranges of the DC-DC converter. When it comes to batteries, the amount