Tips for accurate estimation of battery life(2)

From this information, a simple listing of the different tasks and behaviour of the microcontroller can be documented in order to improve the traditional battery budget "guestimations." A simple example of this can be seen in figure 2. Note that an RMA will also give the developer a comfort level in that all tasks will complete in a deterministic manner, with no deadlines being missed.

Tip no. 3: Chip vendor tools
One of the foggiest areas when it comes to energy consumption is the microcontroller. There are so many variables concerning how these little guys will consume energy. A review of most vendor datasheets will provide wide ranges of energy consumption based on temperature, voltage, peripheral set, altitude, wind speed, birth sign of the developer, and so forth. There is always the question as to where and how these numbers were obtained and if they are accurate or not.

Even with a little bit of "hand waving" though, there are some new tools that chip vendors are starting to supply to developers that will greatly improve how battery budgets are estimated. One that immediately comes to mind is STMCubeMx from STMicroelectronics. This tool is primarily intended to be used to set up and configure an STM32 microcontroller, but the really cool part about it is that it has the ability to simulate current consumption. An example can be seen in figure 3.

Figure 3: STMicroelectronics' STMCubeMx tool. (Click on image to enlarge)

The STMCubeMx software allows a developer to enter in different software assumptions and peripheral configurations, after which it will generate a profile of energy consumption. For example, a developer can enter the run mode, clock speed, and duration, to name just a few parameters. The software can be broken up into steps in the analysis to generate the profile and the average microcontroller energy consumption. There are even common battery types and capacities available for consideration in this analysis.

Tip no. 4: Bench-top experiments
As much as any engineer loves modelling and estimation tools, there is always a tension that exists until those assumptions are tested and proven on the bench. This is why it is imperative that—as early as possible in the design cycle—development kits and prototype parts be tested for real-world behaviour! This doesn't need to be a clean and polished test, but—at a minimum—it should be able to verify basic assumptions about the software, microcontroller energy draw, and other elements of the system.

One of the nice things about bench testing is that it is a quick and inexpensive way to prove that the power portion of the design is on the right path. If there is an error in a datasheet, an oversight in assumptions, etc., then the data taken from the bench can be used to refine the model. The end result is to bring us one step closer to proving that the intended design is, in fact, valid.

Tip no. 5: Battery life-cycle analysis
Creating a model, bench testing, and simulating the energy consumption of the system go a long way with regard to nailing down how much battery life an embedded system can expect. There are a few additional potential pitfalls, however, when it comes to the battery itself.

The first, which concerns rechargeable batteries, is that each charge/discharge cycle decreases the overall capacity of the battery. The result of this decrease in capacity is that a device that has a battery life of nine hours when first manufactured may only last six hours a few months later. This is certainly a factor that needs to be taken into account when performing the battery capacity estimate.