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“Power for Wearables”
Wearables StudioSpring 2009
Zach Eveland, 2009
Power for Wearables
Special power needs of wearables: Long operation High power Comfort Durability Integration with soft circuits
Batteries
Coin Cells
Small size Low power Rechargeable and
non-rechargeable types
Cylindrical Cells
Medium size Medium power Rechargeable and
non-rechargeable types
Rechargeable Packs
Chemistry varies Size varies Typically high power
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)
Safety
LiIon and LiPoly cells like to explode
Protect against short-circuits
Never charge while wearing
Technical Terms
Power Terms
Current: Amps, mA, or A Voltage: Volts or V Resistance: Ohms or Ω
Getting Started in Electronics – Forrest Mims III
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
Calculating Power
With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread:
Calculating Power
With a 3 Volt coin cell battery and a 100 Ohm piece of conductive thread:
I = V/R
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
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 )
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
Battery Terms
Capacity: mAh Internal resistance: Ω Duty cycle: % Battery Life =
Capacity / Current Getting Started in Electronics – Forrest Mims III
Reading a Datasheet
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
Battery Calculations
With a 280 mAh coin cell battery, an Arduino and two LEDs:
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 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 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
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)
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
Mounting Batteries
Mounting Batteries
Mechanically tricky Electrically tricky
Mounting Batteries
Other options: 9V battery Stashed battery pack Coin cells Magnets
Not Batteries
Solar
Great for very bright sunlight or very low power
Usually must be supplemented with another power source
Voltage Regulators
Many fixed voltages available
Variable voltage also possible
Wall Warts
Cheap, easy, inefficient
Difficult to wear Great for charging,
testing, or fixed locations
Power Supplies
Very difficult to wear Best for high-power
applications
Super Capacitors
Cheap, simple way to collect and store energy
Useful for solar-powered applications
Resources
Class site:http://itp.nyu.edu/wearables/Links/Technology
Battery FAQ:http://www.powerstream.com/BatteryFAQ.html
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