Strategies for extending battery life - Embedded.com

Strategies for extending battery life

Battery life is one of the core challenges in IoT. It relies on almost every specification of the device (hardware and firmware architecture, size, environment…). This is why power consumption must be taken into consideration very early in the design cycle.

To start with, it is crucial for engineers to precisely understand the device’s use case while defining its battery life goal. Otherwise, they might fail to meet market expectations. Then, they can consider several techniques to optimize its power consumption. Engineers need to observe three main criteria to select the best suited solution: performance gain, cost and form factor. Wilfried Dron, CEO and cofounder at Wisebatt illustrates these criteria through some of the most common power management strategies.

To accurately compare these techniques, they were implemented in a basic IOT device composed of a microcontroller, a sensor, a radio, a linear regulator and a battery. This device’s battery life lasts 1h30. It costs around 8 dollars for one thousand units. The schematics in the following illustrations have been designed with the simulation tool Wisebatt.


Figure 1: Wisebatt Schematic

Performance

The first criterion that motivates any design change is the optimization gain: how much additional battery life could one achieve with each strategy? Some simple design adjustments can provide great results. One should start by powering down unused peripherals to minimize power consumption and leakage. In fact, small savings added together can extend the battery life up to 2.5 times.

The use of buck regulators can lead to similar battery life improvements. Yet, buck regulators might not be adapted to all use cases as they might introduce a lot of noise on the power supply due to their switching frequency.


Figure 2: Buck regulators

The addition of another LDO regulator to noise-sensitive components such as sensors or radios can fix this shortcoming. When buck regulators cannot be used, splitting the power domain helps to balance heat dissipation and wear level. One can expect to reach 1,5x the initial battery life when compared to a single power domain.


Figure 3: LDO regulators

Finally, a simple strategy is to use a load switch in order to shut down all the unused components in the system. It can lead to a gain of at least +10% of battery life (depending on the device’s application and the components that are shutdown).


Figure 4: Simulation

With these strategies, one can achieve a significant power consumption decrease. But if it is not enough, a bigger battery should be considered. This choice should be  made only when no other option is possible, as it might imply significant changes in the product characteristics.

Cost

The selection of a specific power management strategy also depends on its impact on the device’s final price. Indeed, changes in the design often imply additional costs. For instance, the highly efficient solution which consists in adding a buck regulator might be overlooked in some cases because of its relatively high price.


Table 1: Cost with buck regulator

On the opposite, the simple fact of powering down each unused peripheral not only provides excellent results, but it also has almost no impact on the final budget.

The use of an additional LDO here proves to be an economically viable choice. While the improvement of power consumption might be less significant, it is a satisfying low-cost strategy.


Table 2: Cost with LDO

At last, aload switch is not only simple to implement but also fairly affordable. Although battery life gains are smaller than with other strategies, it is a must-have for most high-end use cases.


Table 3:  Cost for other components

Obviously, the bigger battery option will end up at the bottom of this list as it would be the most expensive choice.

Form Factor

The last criterion that will influence the choice of a power management technique is its form factor. It will have a more or less significant impact on the decision-making process. It depends, for instance, on available engineering resources, the project’s advancement, the device’s size and weight constraints, etc.

Once again, powering down peripherals shows quite obvious advantages. This simple technique might require rewriting some existing software and drivers but does not need any hardware modification. If you need to add a load switch, the footprint will be very small and the task quite simple as depicted in the following picture.


Figure 5: Footprint & layout of the SPI32431 load switch from Vishay

On the opposite, the selection of a bigger battery will increase the device’s form factor and footprint, with a high probability of having to redesign the mechanical casing. Yet, this modification might have a low impact if made at a very early stage of the design cycle.

Some solutions require adding passives, which slightly increases the complexity of the design, as for the use of a buck regulator (six additional passives) or LDO regulators (four additional passives). Those techniques will impact the PCB footprint (an inductor may sometimes be bigger than the regulator IC itself).


Figure 6: Footprint & layout of the LTC3406A buck regulator from Analog Devices

Here is a sum up of the power management strategies introduced in this article. The results come from my professional experience, as well as battery life estimations conducted on our simulation tool Wisebatt.


Figure 7: Sum Up

With a great understanding of the use case, engineers can find the most adapted technique, with regards to three crucial criteria: performance, price, form factor. A good practice is to test each solution and observe its impact, through simulation. Battery life estimation, done right from the first development steps, will prevent any unnecessary design modification in the future.

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>> This article was originally published on our sister site, Power Electronics News.


Wilfried Dron is Cofounder & Chief Executive Officer, Wisebatt.

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