Battery Models

EMF and voltage What is EMF? – Electro Motive Force What is the difference between EMF and battery voltage? – The battery has internal resistance and capacitance – This reduces the voltage drop across the load

Definitions Cycle Life - One cycle of a battery is a discharge from full charge to full discharge and a return to full charge again Fast charging time - A control circuitry is used to rapidly charge the batteries without damaging the cells' elements. Most such chargers have a cooling fan to help keep the temperature of the cells under control. – Other charging types – pulse, USB, induction Overcharge Tolerance – Poles can get reversed if overcharged. Amount of tolerable overcharge. Self Discharge Rate – The rate at which the battery discharges mA/hr Gravimetric Energy Density - is a term used for the amount of energy stored in a given system or region of space per unit volume

Discharge Characteristics Non linear discharge

Temperature dependence The battery performance decreases with cold temperature and increases with heat. Heat increases battery performance but shortens life by a factor of two for every 10°C increase above 25–30°C (18°F above 77–86°F). Although better performing when warm, batteries live longer when kept cool.

Memory Effect Effect - Recharging the batteries after partial discharge Loss of battery capacity Peak voltage is not maintained for longer duration Reason – Shallow Discharge Battery negative plate is clogged with crystals resulting in increase in internal resistance. Solution – Let your phone shutdown on its own – Battery reaches 90 % discharge state

Safety issues in battery Toxic chemical leakage Internal short circuit – Due to poor quality anode and cathode – Shock due to fall Figure 1: Lithium-ion battery damages a laptop.

Circuit model for battery discharge R1 is the internal resistance -Reduces the efficiency of the battery -The lower the value the better is the battery R2 is the leakage resistance -Causes leakage of charge from the battery -The higher the value the better What happens when you connect a load to this circuit?

Sustainability in Mobile apps Energy Perspective Eco-friendly Equipment 1.Lower battery usage –Batteries are fail safe options 2.Energy Harvesting –Extract energy from the environment 3.Energy efficiency –Low energy operation of the devices 1.Recycle computing equipments 2.Battery less equipment design 3.Reduce carbon footprints of computers 4.Reducing waste energy Energy Neutrality Variable Power Demand Intermittent Scavenging Source Limited Energy Storage Limited Energy Storage Maximize Battery Lifetime Maximize Scavenging Source Usage Never deplete battery

Optimal Wearables Design [2] Challenges – Scarce and intermittent energy resources – Form Factor – Non-linear and location dependent interactions with human physiology – Trade of in safety, sustainability and security 10

Results Case 1: Single Sensor Interactions

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