“Power for Wearables” Wearables Studio Spring 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 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 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 A Battery Life = Capacity / Current Battery Life = 0.28 Ah / 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 A Battery Life = Capacity / Current Battery Life = 0.28 Ah / 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

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: Battery FAQ: