1. “Power for Wearables” Wearables Studio Spring 2009 Zach Eveland, 2009

2. Power for Wearables Special power needs of wearables: Long operation High power Comfort Durability Integration with soft circuits

3. Batteries

4. Coin Cells Small size Low power Rechargeable and non-rechargeable types

5. Cylindrical Cells Medium size Medium power Rechargeable and non-rechargeable types

6. Rechargeable Packs Chemistry varies Size varies Typically high power

7. Cell Chemistries Non-rechargeable:  Alkaline  Lithium Rechargeable:  Lithium Ion (LiIon) and Lithium Polymer (LiPoly)  Nickel Metal Hydride (NiMH)  Nickel Cadmium (NiCad)  Sealed Lead Acid (SLA)

8. Safety LiIon and LiPoly cells like to explode Protect against short- circuits Never charge while wearing

9. Technical Terms

10. Power Terms Current: Amps, mA, or A Voltage: Volts or V Resistance: Ohms or Ω Getting Started in Electronics – Forrest Mims III

11. Calculating Power Ohm's Law says: V = IR or – voltage equals current times resistance when voltage is measured in Volts, current in Amps, and resistance in Ohms Also, I = V/R or – current equals voltage divided by resistance

12. Calculating Power With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread:

13. Calculating Power With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread: I = V/R

14. Calculating Power With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread: I = V/R I = 3 V / 100 Ω

15. Calculating Power With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread: I = V/R I = 3 V / 100 Ω I = 0.03 A ( or 30 mA )

16. Calculating Power With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread: I = V/R I = 3 V / 100 Ω I = 0.03 A ( or 30 mA ) Enough to light an LED, probably not enough to run a motor

17. Battery Terms Capacity: mAh Internal resistance: Ω Duty cycle: % Battery Life = Capacity / Current Getting Started in Electronics – Forrest Mims III

18. Reading a Datasheet

23. Battery Calculations Add up current consumption for all parts in your design – use values given on datasheets Add 10% extra for wiggle room This gives total current consumption – can be used to calculate battery needs and runtime

24. Battery Calculations With a 280 mAh coin cell battery, an Arduino and two LEDs:

25. Battery Calculations With a 280 mAh coin cell battery, an Arduino and two LEDs: Current required for Arduino and 2 LEDs is 70 mA – add 10% overage for 77 mA or 0.077 A

26. Battery Calculations With a 280 mAh coin cell battery, an Arduino and two LEDs: Current required for Arduino and 2 LEDs is 70 mA – add 10% overage for 77 mA or 0.077 A Battery Life = Capacity / Current

27. Battery Calculations With a 280 mAh coin cell battery, an Arduino and two LEDs: Current required for Arduino and 2 LEDs is 70 mA – add 10% overage for 77 mA or 0.077 A Battery Life = Capacity / Current Battery Life = 0.28 Ah / 0.077 A

28. Battery Calculations With a 280 mAh coin cell battery, an Arduino and two LEDs (requiring 70 mA): Current required for Arduino and 2 LEDs is 70 mA – add 10% overage for 77 mA or 0.077 A Battery Life = Capacity / Current Battery Life = 0.28 Ah / 0.077 A Battery Life = 3.64 hours (or about 218 minutes)

29. Considerations Evaluating your needs:  How much time do you need?  How much current do you need at once?  Should the battery be rechargeable?  How big can the battery be? Consider how you will charge or replace batteries and how often

30. Mounting Batteries

31. Mechanically tricky Electrically tricky

32. Mounting Batteries Other options: 9V battery Stashed battery pack Coin cells Magnets

33. Not Batteries

34. Solar Great for very bright sunlight or very low power Usually must be supplemented with another power source

35. Voltage Regulators Many fixed voltages available Variable voltage also possible

36. Wall Warts Cheap, easy, inefficient Difficult to wear Great for charging, testing, or fixed locations

37. Power Supplies Very difficult to wear Best for high-power applications

38. Super Capacitors Cheap, simple way to collect and store energy Useful for solar- powered applications

39. Resources Class site: http://itp.nyu.edu/wearables/Links/Technology Battery FAQ: http://www.powerstream.com/BatteryFAQ.html