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Battery Management

PRODUCT HOW-TO: Developing apps on an energy-friendly MCU designed for a 20-year battery life

May 10, 2010 | Rasmus Christian Larsen | 222900911
Rasmus Christian Larsen, Training and Support Manager, Energy Micro, explains that when designing products for battery-powered operation, it is clearly desirable that battery life should be as long as possible. For a rapidly-growing class of devices, extended operation from a single battery becomes more than just a desirable feature in the specification: it is central to the entire product concept.
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In some applications, access to change the battery is difficult or even impossible: in others, the end-user cannot support the cost of replacing the battery.

Such applications employ an MCU that is active on a very low duty cycle, spending perhaps 99% - or even more, 99.9% is not uncommon of its time in a deep sleep state. They wake, either on a periodic cycle or in response to some stimulus, perform an action, and return to their sleep state.

As they spend such a high proportion of their time asleep, it is obvious that a key parameter in obtaining maximum battery life will be the current consumption while in the powered-down state.

However, the difference between a service life of three or four years, and one that exceeds 10 years, extending to 20 years or even more from the same battery lies in paying careful attention to every aspect of how the task uses the resources of the MCU, and how the MCU itself is designed to minimize energy usage in every way.

20 years from a single cell
A widely-used battery in small MCU applications such as remote environmental sensors is the CR2032 coin cell; this is a lithium/manganese dioxide 3V primary cell.

A typical supplier for example, Kodak rates the capacity as 230 mAh to an end-point voltage of 2V into 5.6 kilo-ohms (about 0.5 milliamperes). At that rate its life would be around 400 hours; energy-sensitive applications, by contrast, deal with service lives up to 200,000 hours.

This particular cell has very good shelf life or self-discharge rate; the data sheet gives 10 years to 90% capacity. Very approximately, this is equivalent to a continuous discharge of about 0.25 micro-amperes, which sets a context for the applications average demand if a 10-20 year battery life is to be achieved.

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