PROMETHEUS Intelligent Multi-Stage Energy Transfer System for Near Perpetual Sensor Networks Xiaofan JiangJoseph PolastreDavid Culler Electrical Engineering Department University of California, Berkeley Computer Science Department University of California, Berkeley

2 IPSN 2005 A Typical Solar Powered Sensor Node Charging Circuit Voltage Regulation DAYNIGHT Stress on Battery ENERGY DAY NIGHT DAY

3 IPSN 2005 Energy Storage Element Designed for Pulsing Applications Is it enough? Supercapacitor Medium Capacity High Leakage Infinite recharge cycles Rechargeable Battery High Capacity Low Leakage recharge cycles vs

4 IPSN 2005 Multi-Stage DAYNIGHT ENERGY DAY NIGHT Stress on Battery ENERGY DAY NIGHT DAY

5 IPSN 2005 Inefficient and complex hardware? NO! Simple and efficient hardware Complete and powerful control using software Intelligent

6 IPSN 2005 Architecture

7 IPSN 2005 Environmental Energy Solar  Characteristics  Sizing considerations  Typically 18mW/cm2 under direct sunlight Vibration / kinetics Sound / wave Heat

8 IPSN 2005 Wireless Sensor Node Duty cycle Intelligence – self-aware of power levels and able to exert control Ultra-low power – Telos  Low operating voltage

9 IPSN 2005 The Larger the Better? NO! Without LoadWith Load MINIMUM OPERATING VOLTAGE

10 IPSN 2005 Secondary Buffer Li+ vs NiMH Dedicated charging chip vs software+simple hardware

11 IPSN 2005 Case Study: Prometheus

12 IPSN 2005 Control Loop and Charging by Software Sensing ADC Piggy-back Voltage divider trade-off Actuation uC I/O Digital switch Charging Dedicated charging chip? MOS switch vs Digital Switch DC/DC  current limiting

13 IPSN 2005 Temperature Compensation

14 IPSN 2005 Driver piggy-backed on application active duty cycle 1. if TempV > BattV = BattV (TempV − 2.2) 3. if CapV < SwitchCap = FALSE 5. if CapV > SwitchCap = TRUE 7. if CapV > 4.4 and BattV < ChargeBatt = TRUE 9. if CapV < ChargeBatt = FALSE 11. call Radio.send(STATS)

15 IPSN 2005 Duty-Cycle Adaptation

16 IPSN 2005 Test 1 / Data Reported by Telos Scenario: Supercapacitor is less than half full Battery is half full 1% duty cycle No light t 0 : Source = Cap; Vref = 2.5 Expect: t 1 : Vref = 1.5 t 2 : Source = Batt; Vref = 2.5

17 IPSN 2005 Result 1 SupercapacitorBattery t0: Source = Cap t1: Vref = 1.5 t2: Source = Batt; Vref = 2.5

18 IPSN 2005 Test 2 / Actual Measured Data Scenario: Supercapacitor very low Battery is half full 1% duty cycle From dark to sunrise t 0 : Source = Batt Expect: t 1 : Source = Cap t 2 : Charge = True t 2 +dt: Charge = False

19 IPSN 2005 Result 2 SupercapacitorBattery t0: Source = Batt t1: Source = Cap t2: Charge T2+dt: !Charge

20 IPSN 2005 Conclusion Intelligence + Multi-stage + Simple Hardware == Perpetual operation * * Direct sunlight