Start • Design Requirements • System Architecture • External Charging Unit • Internal Charging Unit • Charging Coils • Charging Data • Power Up • Sources and PDF

DESIGN REQUIREMENTS

Mobile and portable battery-operated wireless sensors are all the rage these days. For example, it costs less than $20 in parts to stick a ZigBee radio on a smart sensor and have the sensor data continuously report back to a main control center in real time, even in an environment where connecting the sensor to the power mains is impossible. But ultimately, every battery pack deployed in a sensor package must be replaced or recharged. Recharging typically involves connecting the battery pack through a charge controller to the electric power mains via a wired connection.

You probably plug your cell phone into a wall-wart charger each night. But for a variety of reasons, it may not be possible to plug every embedded application into a power source (even for a few hours to recharge the battery pack). An example is when a doctor inserts an embedded sensor in a patient’s body. When the implant incision heals, there are no connections to the outside world. Another example is when sensors are placed in industrial settings where wired connections may be undesirable for safety reasons. In both situations, a mechanism for charging the battery wirelessly is necessary. 

As part of our research at the University of Iowa, we were asked to develop a telemetry application that required a portable, rechargeable battery-based power supply. We were told the power supply had to provide continuous power to the embedded telemetry controller while maintaining the standards of long life and high reliability. The application would not be accessible for connecting a power supply, so the power supply would have to be inductively charged as well. 

There were additional constraints on the quality of the power required by our application. The amount of transient switching behavior that typically occurs when a single battery pack supply switches between powering the application from the battery pack and powering the application from the charging circuit during battery recharging was more than our application permitted.

This was because of the nature of the charging circuit. An inductive charging circuit involves a much more tenuous connection than a wired charging circuit. Even with the hysteresis permitted by the internal charging unit, an inductive charger tends to switch into and out of Charge mode. As a result, it presents more transient switching behavior than a wired charger. Also, a two-pack supply can provide enhanced reliability for mission-critical applications because of the redundancy provided by the second pack.

We had several requirements for our application: high quality, continuous power (100 mA with a 24-h charge cycle), long life, and high reliability. To satisfy these requirements, the system we designed has two separate battery packs. One battery pack provides power to the application. A second battery pack charges when necessary. To ensure that the batteries don’t get damaged during the charging process, we use a smart battery controller device to monitor the temperature of the battery packs. The controller can now stop the charging process if the temperature happens to rise too high.

Because battery technologies are continuously improving, we included a charge controller with adjustable parameters. This has permitted us to optimize system performance based on the particular kind of batteries in use.

We determined that inductive charging would be the most suitable method for providing recharging energy to the battery packs. Inductive charging essentially uses two coils to couple energy wirelessly into the battery-charging system. The coil connected to the electric power mains is like a transformer’s primary coil. The coil connected to the battery charging circuit is like the same transformer’s secondary coil. In an ordinary transformer, the coils are typically wound together, often around a layered iron core to enhance coupling.

Our coils are wound separately and positioned close together in order to charge batteries in the internal charging unit (ICU). When the electromagnetic flux lines from one coil intersects the second, energy can be transferred from the electric power mains to the battery-charging circuit.