Strategies for conversion from lithium-ion to primary battery power

August 19, 2014 // By Terry Cleveland , John Haroian, Microchip Technology, & Adam Jakubiak, Energizer
Why would a designer consider changing the power source? Perhaps to reduce cost, drive increased margins or to reach a new or expanded customer base at a much lower price point. Converting from rechargeable lithium-ion to a single-use primary battery can be an attractive place to reduce costs.

Rechargeable lithium-ion batteries are widely used in today's portable electronic devices; particularly those demanding either significant amounts of power or a heavy usage frequency. When designing with lithium-ion, safety should always be on the forefront of the design engineer’s mind to ensure that proper care is taken—especially during charging. Lithium-ion safety considerations frequently result in a battery that is specially chosen for the given application and is tightly integrated with the system. Making future changes to that system can then be challenging and time-consuming.

If this is the case, why would a designer consider changing the power source? More often than not, the reason for converting comes down to a need to reduce cost. This may be to drive increased margins, to be more competitive with substitutes, or to reach a new or expanded customer base at a much lower price point. Converting from rechargeable lithium-ion to a single-use primary battery can be an attractive place to reduce costs. In addition to the price of the battery itself, lithium-ion designs require sophisticated circuitry to closely monitor charging and discharging, which is unnecessary if primary batteries are used.

Costly redundant systems may have even been employed to intentionally over-design for safety. Aside from bill-of-material costs, additional shipping and labour costs may be required to process returns and repairs. In addition to any cost considerations, the power requirements for modern electronic components have continued to decrease, and the usage of an extreme low power microcontroller in applications that were previously well suited for a rechargeable solution may now enable a primary battery solution instead.

In general, primary batteries that are designed for single use offer some inherent advantages over rechargeable lithium-ion batteries in portable devices. For example, if the device is used often, widely available primary batteries can be quickly swapped out for fresh ones with minimal downtime. There is also no need for packing bulky accessories, such as cables and charging adapters.

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 applications that have been served by lithium-ion. 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 of current capacity they can provide is closely linked to their internal resistance, which can change throughout discharge, as shown in Figure 2.

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.

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