“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