Design Constraint: 24 Hour Performance on a Charge

Basic concept:

Our system will need batteries. No getting around it. Therefore, the question becomes how long must the battery life be. We believe there is a big difference between 6h of battery life and 24, but not much difference between 24 and 36. Why? Once you reach 24 hours (or close), the sensors can be recharged once per night. A person can get into a routine of charging by placing the devices on the bedstand or by the phone (and charging the phone). But no interaction is required during the day.

A device that lasts 36 hours does not encourage this type of habit forming. The extra battery life, in some respects, makes failure more likely.

We therefore want to permit someone to get up in the morning, take off one set of sensors, replace them with a new set that have been charging the previous day, and then forget about them until the next day. One action, once a day, taken at a predictable time.


  • This is a well-defined constraint that helps to constrain the design space somewhat.


  • Achieving 24h performance with real-time transmission of data in a form factor small enough to meet other design constraints is a major challenge.


  • Many people get into a routine of plugging in mobile phones once a night and they never worry about battery life (unless they have cutting-edge phones and are power callers or use wi-fi or GPRS or bright displays heavily).
  • Many people get into morning or evening routines where they take actions such as weighing themselves, replacing watches, etc.