Battery

Introduction

Batteries are an important consideration in developing mobile embedded devices. Mobile devices, such as PDA's, often need to be able to run standalone for extended periods of time. A good choice of battery will provide the necessary standalone running time without significantly increasing the size or weight of the device.

The most commonly used type of battery for embedded devices are Lithium-ion (Li-ion for short). Li-ion batteries are rechargeable and have a high energy to weight ratio. They have no memory effect, meaning that recharging a partially charged Li-ion battery does not cause it to hold less charge in the future. Nickel-cadmium batteries sometimes exhibit a memory effect. Li-ion batteries have a very low loss of charge, around 2-3% per month.

A variant of this technology is Li-ion Polymer, which is similar from a user point of view, but can be made in almost any shape, including very thin cells.

For more information, see Wikipedia. There is also a general battery book with lots of information.

Advantages

• Available in a number of shapes and sizes to best suit the form factor of an embedded device.
• Lighter than other battery types.
• Rechargeable.

Disadvantages

• More expensive than other battery types.
• Stricter environmental tolerance than other batteries. Can be dangerous if handled improperly.

Designing Battery Powered Embedded Devices

The choice of battery for a mobile embedded device is based on the target form factor and necessary stand alone power time for the device.

For devices which need a long stand alone power time, while retaining a minimum weight, a removal battery solution may be ideal. A user can then charge one battery externally while powering the device off another.

Li-ion batteries are rated for a given number of milliamp hours. For example a device which draws around 250 milliamps of current, can be powered by a 1000 milliamp battery for approximately four hours. Careful design of an embedded device can help reduce the current drawn and increase the amount of time it can run from battery power. One technique which can help reduce the current draw of an embedded device is power domains. Power domains allow specific parts of a device to be powered off when not in use. For example, a PDA which has a GPS feature does not need to continually supply power to the GPS module. Having power domains would allow the GPS module to be switched on only when the user is directly interacting with it.

Some embedded devices do not need batteries to support stand alone running. For example an in-car navigation system only needs to be enabled while the vehicle is running and therefore can be powered by the standard 12V car battery. A small Li-ion battery or super capacitor (http://en.wikipedia.org/wiki/Supercapacitor) may be necessary to backup a real-time clock in such devices.

Batteries on the Rig 200

The Rig 200 comes with a single 7.2V, 1000 milliamp hour lithium ion battery which is connected to the underside of the baseboard. At a full charge, this is sufficient to power the Rig 200 for around 3-4 hours. The battery is charged when the Rig 200 is powered either externally (power pack or bench supply) or via power over Ethernet (PoE). The battery life is reported using an I2C gas-gauge controller. Bluewater Systems provides tools for configuring the gas-gauge and support in both Linux and Windows CE for configuring the battery. A second battery may also be connected to provide a longer stand alone power time.