Low Volatge And Battery

ELECTROTECHNOLOGY I By Sulaiman Olanrewaju, Oladokun

Objectives • Differentiate between primary & secondary cell • Operation (with aid of sketches): – Lead-acid battery – Alkaline battery

• Battery charging system

Sources of Power: Batteries

WHAT IS A BATTERY? •A battery is a device consisting of one or more galvanic cells, which store chemical energy and make it available in an electrical form. •A battery has a voltage, measured in volts, an internal resistance measured in ohms, and a capacity, measured in ampere-hours, which may vary due to many factors including internal chemistry, current drain, and temperature. •There are two types of batteries, primary and secondary, both of which convert chemical energy to electrical energy. •A primary batteries can only be used once, as they use up their chemicals in an irreversible reaction. Secondary batteries can be recharged because the chemical reactions they use are reversible; they are recharged by running a charging current through the battery, but in the opposite direction of the discharge current.

BATTERY HISTORY •The story of the modern battery begins in the 1780s with the discovery of "animal electricity" by Luigi Galvani, which he published in 1791. He created an electric circuit consisting of two different metals, with one touching a frog's leg and the other touching both the leg and the first metal, thus closing the circuit. He noticed that even though the frog was dead, its legs would twitch when he touched them with the metals. •By 1791, Alessandro Volta realized that the frog could be replaced by cardboard soaked in salt water, employing another form of detection. Volta was able to quantitatively measure the electromotive force (emf) associated with each electrodeelectrolyte interface (voltage) in volts, which were named after him. In 1799, Volta invented the modern battery by placing many galvanic cells in series, literally piling them one above the

•In 1836, Daniell cell provided more reliable currents and were adopted by industry for use in stationary devices, particularly in telegraph networks where they were the only practical source of electricity. These wet cells used liquid electrolytes, which were prone to leaks and spillage if not handled correctly. Many used glass jars to hold their components, which made them fragile. •Near the end of the 19th century, the invention of dry cell batteries, which replaced liquid electrolyte with a paste made portable electrical devices practical. •The battery has since become a common power source for many household and industrial applications. According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales annually.

HOW A BATTERY WORKS •A battery is a device that converts chemical energy directly to electrical energy It consists of one or more voltaic cells. •Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. Each cell has a positive electrode (cathode), and a negative electrode (anode). These do not touch each other but are immersed in a solid or liquid electrolyte. In a practical cell the materials are enclosed in a container, and a separator between the electrodes prevents the electrodes from coming into contact.

•The electrical potential difference across the terminals of a battery is known as its terminal voltage, measured in volts. The terminal voltage of a battery that is neither charging nor discharging is called the open-circuit voltage, and gives the emf of the battery. •The voltage developed across a cell's terminals depends on the chemicals used in it and their concentrations. For example, alkaline and carbon-zinc cells both measure about 1.5 volts, due to the energy release of the associated chemical reactions.

TYPES OF BATTERIES. •There are various types of batteries depends on its sizes and chemical properties. Generally there are two main types of batteries: 1. non-rechargeable (disposable) 2. rechargeable •Non-rechargeable (disposable) •Disposable batteries, also called primary cells, are intended to be used once and discarded. They are not designed to be rechargeable. These are most commonly used in portable devices with either low current drain, only used intermittently, or used well away from an alternative power source.

•Rechargeable Batteries •Rechargeable batteries are also known as secondary batteries or accumulators .They can be re-charged by applying electrical current, which reverses the chemical reactions that occur in use. Devices to supply the appropriate current are called chargers or rechargers. •The oldest form of rechargeable battery still in modern usage is the "wet cell" lead-acid battery. This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging. A common form of lead-acid battery is the modern wet-cell car battery.

Battery Capacity and Discharging •The more electrolyte and electrode material there is in the cell, the greater the capacity of the cell. Thus a small cell has less capacity than a larger cell, given the same chemistry though they develop the same opencircuit voltage. •The capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature, and other factors.

•The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 A·h will deliver 5 A over a 20 hour period, but if it is instead discharged at 50 A, it will run out of charge before the theoretically expected 2 hours. •The relationship between current, discharge time, and capacity for a lead acid battery is expressed by Peukert's law. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates.

Environmental Considerations •Since their development over 250 years ago, batteries have remained among the most expensive energy sources, and their manufacturing consumes many valuable resources and often involves hazardous chemicals. For this reason many areas now have battery recycling services available to recover some of the more toxic and sometimes valuable materials from used batteries. Batteries may be harmful or fatal if swallowed. It is also important to prevent dangerous elements found in some batteries, such as lead, mercury, cadmium, from entering the environment.

The Electric Battery •

A BATTERY is a source of electric energy. • A simple battery contains two dissimilar metals, called ELECTRODES, and a solution called the ELECTROLYTE, in which the electrodes are partially immersed.

The Electric Battery • An example of a simple battery would be one in which zinc and carbon are used as the electrodes, while a dilute acid, such as sulfuric acid (dilute), acts as the electrolyte. • The acid dissolves the zinc and causes zinc ions to leave the electrode. • Each zinc ion which enters the electrolyte leaves two electrons on the zinc plate. • The carbon electrode also dissolves but at a slower rate. • The result is a difference in potential between the two electrodes.

The Dry Cell •The Dry cell is relatively inexpensive and quite portable. •It has many uses such as in flashlights and radios. •The anode consists of a Zinc can in contact with a moist paste of ZnCl2 and NH4Cl. •A carbon rod surrounded by MnO2 and filler is the cathode. •The cell reaction appears to vary with the rate of discharge, but at low power the probable reactions are as follows:

Lead Storage Cell The basic features of the lead storage cell are electrodes of lead and lead dioxide, dipping into concentrated sulfuric acid

Both electrode reactions produce lead sulfate, which adheres to the electrode. When the cell discharges, sulfuric acid is used up and water is produced. The state of the cell can be determined by measuring the density of the electrolyte solution (the density of water is about 70% that of the sulfuric acid solution).

Primary cell • Chemical action eats away one of the electrodes (usually -ve side) • When happened, electrode must replaced or cell discarded • In galvanic-type cell, zinc electrode & electrolyte must replaced • Dry cell - cheaper to buy a new one

Secondary cell • Electrodes & electrolyte altered by chemical action when cell delivers current • Cells may restored to original condition by feeding current in opposite direction • Metal plates & acid mixture change as battery supplies voltage • Metal plate become similar & acid strength weakens – discharging • Recharging - applying current to battery in reverse direction, restored battery materials • Example - automotive lead acid batteries

Battery capacity • Capacity of battery to store charge - ampere hours (1Ah = 3600 coulombs) • 1 Ah - battery can provide 1A) of current (flow) for one hour • Factors affecting battery performance: – Chemical reactions within cells – Discharge conditions – current magnitude, duration, battery terminal voltage, temperature etc

• Battery is discharged at constant current rate over fixed period of time such as 10 or 20 hours, down to set terminal voltage per cell • So, 100Ah battery is rated to provide 5A for 20hours at room temperature • Battery efficiency - different at different discharge rates • When discharging at low rate, battery's energy is delivered more efficiently than at higher discharge rates - Peukert's Law

General description • Rated at 24V DC - some cases use 110V or 220V DC – large emergency lighting, vital & battery is the only single source • 2 main types of rechargeable battery: – Lead-acid – Alkaline

Lead acid battery • • • • • •

Nominal cell voltages - 2V Thus, 12 lead-acid cells must connected in series - 24 V More cells connected in parallel - increase battery capacity Battery capacity – rated at 10 hrs discharge 350 Ah – will provide 35 A for 10 hours Will have lower capacity at shorter discharge rate – checked manufacturer's discharge curves • After 10 hour discharge, cell voltage will fallen to approx 1.73 V • State of charge indicated by its electrolyte SG using hydrometer

Lead acid battery

Hygrometer tester

Lead acid battery (cont/…) • Fully charged lead-acid cell – SG about 1.27-1.285 (1270-1285) • Falls to about 1.1 (1100) when fully discharged • Cell voltage also falls during discharge – can also state of charge indication • Safely discharged until cell voltage drops to approx 1.73V • Open-circuit (no-load) voltage readings – can’t interpret that cells are in healthy charged state (due to high voltage) • SG values quoted at 15°C ambient temperature • SG corrections at any other ambient temperature: – Add 0.007 to reading for each 10°C above 15°C – Subtract 0.007 from reading for each 10°C below 15°C

• e.g. hydrometer reading at an ambient temperature 25°C is 1.27 • Thus, equivalent SG value at 15°C is 1.27 + 0.007 = 1.277

Alkaline battery

Alkaline battery (cont/…) • Nominal cell voltages - 1.2V • Thus, 20 alkaline cells must connected in series to produce 24 V • After 10 hour discharge, voltage fallen to approx 1.14 V • SG value – cannot determine state i.e. electrolyte density doesn’t change during charge/discharge cycles but gradually falls during battery lifetime • New cells have SG around 1190, reduces down to 1145 take up to 5~10 years depending on duty cycle) • Electrolyte must completely renewed or battery replaced thereafter • Discharge of cells should discontinued when voltage fallen to 1.1 V

Battery Characteristics • Important characteristics: • • • • • •

energy density (Wh/liter) and specific energy (Wh/kg) power density (W/liter) and specific power (W/kg) open-circuit voltage, operating voltage cut-off voltage (at which considered discharged) shelf life (leakage) cycle life

• The above are decided by “system chemistry” • advances in materials and packaging have resulted in significant changes in older systems » carbon-zinc, alkaline manganese, NiCd, lead-acid

• new systems » primary and secondary (rechargeable) Li

Modeling the Battery Behavior • Theoretical capacity of battery is decided by the amount of the active material in the cell • batteries often modeled as buckets of constant energy » e.g. halving the power by halving the clock frequency is assumed to double the computation time while maintaining constant computation per battery life

• In reality, delivered or nominal capacity depends on how the battery is discharged • discharge rate (load current) • discharge profile and duty cycle • operating voltage and power level drained

Battery Capacity from [Powers95]

• Current in “C” rating: load current nomralized to battery’s capacity • e.g. a discharge current of 1C for a capacity of 500 mAhrs is 500 mA

Battery Capacity vs. Discharge Current • Amount of energy delivered is decreased as the current (rate at which power is drawn) is increased • rated as ampere hours or watt hours when discharged at a specific rate to a specific cut-off voltage » primary cells rated at a current which is 1/100th of the capacity in ampere hours (C/100) » secondary cells are rated at C/20 or C/10

• At high currents, the diffusion process that moves new active material from electrolytes to the electrode cannot keep up

Battery Capacity vs. Discharge Current: Peukert’s Formula • Energy capacity: C = k/Iα • k = constant dependent on chemistry & design ∀ α = 0 for ideal battery (constant capacity), up to 0.7 for most loads in real batteries » also depends on chemistry and design

• Good first order approximation • does not capture effects of discharge profile

• Battery life at constant voltage and current L = C/P = C/(V.I) = (k/V).I-(1+α)

Ragone Plots (log-log plot)

Specific Power W/kg

Specific Energy Wh/kg

Amount of Computation during Battery Lifetime • Consider a system modification that changes performance by factor n and power by factor x • total work (= speed x lifetime) will change by n.x -(1+α)

• e.g. reducing the clock frequency by xN reduces power by xN (N>1) & reduces performance by xN, • work done changes by (1/N)x(1/N) -(1+α) = Nα » > 1 for α>0

• however, can’t just go on reducing frequency » static power dissipated even at zero frequency » P = V.I = V.(S+Df)

∀ ∃ optimum frequency to maximize computation

Alternate Equivalent View of the Battery • Manufacturer’s often give battery efficiency (%) vs. discharge rate (or discharge current ratio) – discharge rate = Iave/Irated

• Battery cannot respond to instantaneous changes in current – so, a time constant τ used to calculate Iave

• Given actual energy drawn by the circuit, one can use the battery efficiency to calculate the actual depletion in the stored energy in the battery • Example: battery efficiency is 60% and its rated capacity

Modeling Battery Efficiency

RI = N bat =

I ave

1 = N bat

I ave I rated τ Tcycle

N bat

∑I

cycle = 0

system

(cycle)

Ebat = (1 − ηbat ) I aveVbatTcycle from [Simunic01]

Digression:Metrics to Relate Power and Performance • MIPS/Watt: millions of instructions per Joule – problem: running faster gives better MIPS/Watt – increasing frequency by N • MIPS go up by xN • power goes up < xN due to static power • MIPS/Watt will increase!

• W/Spec2 has similar problem • Total computation during battery lifetime is better – shows diminishing returns of increasing frequency

Capacity & Variable Discharge Current: Constant vs. Pulsed

• Capacity can be extended by draining power in short discharge periods separated by rest periods • also works with constant background current

• Battery relaxes and partially recovers the active material lost during the current impulse • longer the rest period, the better is the recovery • longer rest period needed as the discharge depth becomes greater • battery voltage also goes back up

Benefits of Pulsed Discharge • Higher specific power for a given specific energy • impulses of several times the limiting current value can be obtained by choosing short pulses and long rest periods

• Higher specific energy for a given specific power • ideally, want specific energy = theoretical capacity • depends on pulse and rest periods

Exploiting Pulse Discharge • Gain in battery life if system shutdown is done taking into account the pulse discharge • Examples: • protocols in case of radios where power during transmission is a lot higher than during receive and idle periods • shutdown of CPUs and variable speed CPUs • shutdown of disks

Alternatives to Batteries? • Small batteries are the only choice for consumer products upto 20W • But – – – –

heavy expensive expire without warning require replacement (disposal problem) or recharging (time problem)

• Are there alternatives?

No Batteries Needed! Energy Harvesting/Scavenging • Power requirements for ICs continually getting lower • The requisite power may be supplied by sources in the environment, instead of the battery – lots of energy sources around us: light, wind, vibration etc.

• E.g. computers worn on one’s person are jostled when one walks, and electric power may be generated – Media Lab’s “Parasitic Power Harvesting” project for devices built into a shoe • http://www.media.mit.edu/resenv/power.html • piezoelectric shoe inserts, shoe­mounted rotary magnetic generator – 20­80 mW of peak power during brisk walk, 1­2 mW average

• a system had been built around the piezoelectric shoes that periodically  broadcasts a 12­bit digital RFID as the bearer walks

Self-powered Chips • Power generated using motion or solar cells, and stored in a backup source (e.g. large capacitor) • no batteries needed • applicable to sensors on vehicles, body etc. • e.g. Embedded power supply processor from MIT [Amirtharajan97] Back-up Source (large capacitor)

Generator

Processor

Fuel Cells • Invented in the 1990s: liberate energy from H atom • Theoretically, quiet and clean like batteries • Plus, amazing energetic potential • up to 20x more than NiCd of comparable size

• No length recharging: rapidly refueled • Costs coming down considerably

• sophisticated engineering, and reduced amount of expensive platinum required for catalysts » while, $/J have gone up with energy-dense batteries

• example:

» NiCd weighs 0.5 kg, lasts 1 hr, and costs $20 » comparable Li-Ion lasts 3 hrs, but costs > 4x more

Electrochemistry of Fuel Cells ELECTRONS

+ + +

+

OXYGEN +

HYDROGEN

ELECTROLYTE (specialized polymer or other material that allows ions to pass but blocks electrons)

ANODE

CATALYST (e.g. platinum)

CATHODE WATER

Theoretical Energetic Potential of Fuel Cells Stored Chemical Energy Wh/Kg Wh/liter FUEL CELLS Decalin (C10H18) Liquid hydrogen Lithium borohydride (LiBH4 and 4H20) Solid metal hydride (LaNi5H6) Methanol Hydrogen in graphite nanofibers RECHARGEABLE BATTERIES Lead acid NiCd Ni-metal hydride Lithium-ion

2400 33000 2800

2100 2500 2500

370 6200 16,000

3300 4900 32,000

30 40 60 130

80 130 200 300

Also Important: Modeling the DC-DC Converter Efficiency • The dependency of efficiency on the output current IC =

EC VC Tcycle

I bat =

I out η DC

E DCbat = I batVbatTcycle E DC = E DCbat − Eout from [Simunic01]

Battery charger • Due to internal leakage between terminals, fully charged battery will get discharged even if unused – took place over period of weeks, leads to fully discharged of battery • Charged by constant voltage method – quickest • Fully discharged battery – damaged beyond repair – plates heavily sulphated • Float / trickle charge – charge battery when battery fully charged state • Compensates loss of battery capacity due to internal leakage i.e. small make up current for topping up, ensure battery fully charged at all times • Float charging voltage > rated battery voltage (27V) – allow sufficient charging current to compensate internal current leakages

Battery charger components • MCCB – for switching supply to charger & provide SC protection • SD transformer – step down 3 phase supply from 440 to 35V • Potentiometer – varies charging voltage as necessary • Silicon diode rectifier bridge – convert AC supply to DC for charging • Electronic filter – smoothing DC output from rectifier • Batteries & transformer protected against SC by fuses or CB • Keep battery on float condition & supplies power to all 24V DC loads, as automatic switching system • Indication provided on main swbd, if battery are discharged

Battery charger operation • When black out occur, charger cannot supply the DC 24V load due to no power input • So batteries automatically supply all the 24 V loads • When power restored, charger gets normal AC power input • Charger automatically supplies quick charge to charge the discharged battery • At same time, supply to all 24V DC loads • At end of quick charge, charger automatically adjusts the voltage to float charge the battery

Quick charge • When battery discharged, needs to charge ASAP & shortest time possible without damaging the battery • 30V (2.5V/cell) applied to lead acid battery during quick charging • Charging current is initially high, but reduces as battery voltage rises • After quick charge completed, resume to float charge • For nickel cadmium battery, float charge is 1.4V/cell & quick charge is 1.7V/cell

Methods of control • Charge discharge • Float charge

Charge discharge • Battery initially charged from mains • When fully charged, allowed to discharge to load • If load is continuous type, two sets of batteries are provided – one on charge whilst the other on discharge • Rectifiers – besides supplying DC to battery, also ensure battery on charge does not feed back into main supply network, if supply failure occur • Essential to have individual c/o switch operated independently i.e. each has an ‘off’ position

Charge discharge (cont/…) • This enables both batteries working in parallel to load during c/o period – ensuring supply continuity at all times • ‘Off’ positions essential to avoid excessive overcharging • Each battery should off charge once adequate, left on open circuit until required for another discharge • Excessive charging - electric power wasteful, shortened battery life & more frequent cell topping up

Battery charging system • • • • •

•

•

•

Use transformer/rectifier arrangement to supply required DC voltage to cells Voltage size depends on battery type & mode of charging, e.g. charge/discharge cycle, boost charge, trickle or float charge Do not allow electrolyte temperatures to exceed about 45°C during charging. A lead acid cell will gas freely when fully charged but an alkaline ceil gases throughout the charging period. The only indication of a fully charged alkaline cell is when its voltage remains at a steady maximum value of about 1.6 to 1.8V. Generally, alkaline cells are more robust, mechanically and electrically, than lead acid cells. Nickel cadmium cells will hold their charge for long periods without recharging so are ideal for standby duties. Also they operate well with a float charge to provide a reliable emergency supply when the main power fails. For all rechargeable batteries (other than the sealed type) it is essential to replace lost water (caused during gassing and by normal evaporation) with the addition of distilled water to the correct level above the plates. Exposure of the cell plates to air will rapidly reduce the life of the battery. On all ships and offshore platforms there are particular essential services which are vital during a complete loss of main power. Such services include switchgear operation, navigation lights, foghorns, fire and gas detection, internal communications, some radio communications, alarm systems. To avoid the loss of essential services they are supported by an uninterruptible power supply or UPS. These can be for battery supported DC supplies or AC supplies both of which can be configure as continuous UPS or standby UPS.

UPS DC battery charger

System description • Shows typical continuous UPS DC supported supply system • Essential DC services supplied from 440V through charger 1 continuously in trickle charges • During power loss, battery 1 maintains transitional supply while emergency generator restores power to emergency board & charger 2 • Either battery is available for few hours if both generators are unavailable • Some critical emergency lights - have internal battery supported UPS i.e. battery charge continuously during non emergency conditions

Care & handling • Main hazards – hydrogen explosion in battery compartment & short circuits • Release hydrogen & oxygen when in charged • Hydrogen easily ignited in concentrations 4~75% in air • Short circuit – cause burns due to arcing, heavy current flows & flash may cause explosion • To avoid explosions & other hazards, proper care, handling & maintaining batteries should strictly adhered

Care & handling (cont/…) 1. Kept compartments adequately ventilated – remove dangerous gases 2. Smoking & any type of open flame prohibited in compartment – no smoking & naked light sign displayed at entrance 3. Battery circuits should dead when leads connected or disconnected – avoid sparks 4. If battery in section, advise to disconnect jumper leads between sections before commence works 5. Vent plugs should screwed tight while making or breaking connections 6. Light bulbs in battery compartments - protected by gas tight glasses

Care & handling (cont/…) 1. Never lay metal tools (spanners, wrenches etc) on top of batteries – sparking & short circuiting may occur + explosions 2. Battery connections – clean & tight, dirty & loose connections lead to local sparking 3. Compartment should never used as storage place for inflammable material or gas 4. Rings should removed from fingers or heavily taped – short circuit through ring will heat it rapidly & cause severe burns 5. Always transported in horizontal position with sufficient manpower – heavy concentrated load & cause painful strains or injury to individual handler

Care & handling (cont/…) • All cables / wires should adequately insulated & guarded – any open high current transmission equipment is potential danger • When preparing electrolyte, concentrated acid should added slowly to water • If water added to acid – heat generated cause steam explosions, acid spattering over handler • To neutralize acid on skin / clothes, thoroughly & frequently clean with fresh water • Only fresh water should be used for eyes • Eyewash bottles & container of FW should kept in compartment for immediate use – clearly label to avoid used by acid

Care & handling (cont/…) 1. Goggles & rubber gloves should worn when handling acid 2. Corrosive products may formed round the terminals – injurious to skin & eyes, use brush to remove them 3. Protect the terminals with petroleum jelly 4. Excessive charging rate causes acid mist to be carried out of the vents into adjacent surfaces, contact with which may burn the skin. If this happens, the affected areas should be cleaned off with diluted ammonia water or soda solution. 5. The general safety precautions with this type of battery are the same as those for the lead acid battery with the following exceptions: • The electrolyte in these batteries is alkaline and corrosive. It should be allowed to come into contact with the skin or clothing. In the case of burns to the skin, the affected part should be covered with boracic powder or saturated solution of boracic powder if available. • Eyes should be washed out thoroughly with plenty of clean fresh water followed immediately with a solution of boracic powder. This solution should always be readily available when the electrolyte is handled.

Care & handling (cont/…) • 19. Unlike lead acid batteries, the metal cases of alkaline batteries remain live at all times and care must be taken not to touch them or allow metal tools to come into contact with them. • 20. Alkaline and lead acid batteries should never be kept in the same compartment. (this is because rapid electrolyte corrosion to metal work and damage to both batteries is certain). • 21. Instrument and utensils (hydrometer, topping up jars and bottles) used for lead acid batteries should not be used on an alkaline installation and vice versa or else thoroughly washed before using.

Why worry about power? Intel vs. Duracell PS

)

16x MI

ity )

r(

ac

so

(c

oc

ap

es

12x

Ha rd

Di s

10x

k

Pr

Improvement (compared to year 0)

14x

8x

) ity

c pa

6x

ry

(ca

o

m Me

4x

Battery (energy stored)

2x 1x 0

1

2

3 4 Time (years)

5

6

• No Moore’s Law in batteries: 2-3%/year

System Design for Low Power • Need to explicitly design the system with power consumption or energy efficiency in mind • Fortunately, IC technology still continue to help indirectly by increasing level of integration – more and faster transistors can enable low-power system architectures and design techniques – e.g. system integration on a chip can reduce the significant circuit I/O power consumption

• Energy efficient design of higher layers of the

System Design for Low Power (contd.) • Energy efficiency cuts across all system layers – entire network, not just the node – everything: circuit, logic, software, protocols, algorithms, user interface, power supply... • complex global optimization problem

• Need to choose the right metric – e.g. individual node vs. network lifetime

• Trade-off between energy consumption & QoS – optimize energy metric while meeting QoS constraint

• Power-awareness, and not just low power – right energy at the right time and place

Where does the Power Go? Processing

Peripherals

Disk

Display

Programmable µPs & DSPs

ASICs

(apps, protocols etc.)

Battery

DC-DC Converter Radio Modem Power Supply

Memory

RF Transceiver Communication

Power Consumption for a Computer with Wireless NIC Other 7% CPU/Memory 21%

Hard Drive 18%

Display 36%

Wireless LAN 18%

Energy Consumption of Wireless NICs (Wavelan) Specs

Measured

2 Mbps (Bronze)

Sleep Mode Idle Mode Receive Mode Transmit Mode

9 mA -------280 mA 330 mA

14 mA 178 mA 200 mA 280 mA

11 Mbps (Silver)

Sleep Mode Idle Mode Receive Mode Transmit Mode

10 mA -------180 mA 280 mA

10 mA 156 mA 190 mA 284 mA

Power Consumption in Post-PC Devices

• Pocket computers, PDAs, wireless pads, wireless sensors, pagers, cell phones • Energy and power usage of these devices is markedly different from laptop and notebook computers – much wider dynamic range of power demand – share of memory, communication and signal processing subsystems become more important • disk storage and displays disappear or become simpler

• Design of power-aware higher layer applications and protocols need to be re-evaluated

Example: Power Consumption for Berkeley’s InfoPad Terminal DC/DC 25%

Wireless 18%

Video Display 40%

DC/DC 42%

µProc. 6%

LCD 6% Misc I/O 7% 1%

With Optional Video Display Total = 9.6W (with processor at 7% duty cycle)

I/O 2% Misc 11% LCD 10%

µProc. 6%

Wireless 29%

Without Optional Video Display Total = 6.8W (with processor at 7% duty cycle)

Example: Power Consumption for Compaq WRL’s Itsy Computer •

System power < 1W – doing nothing (processor 95% idle) • 107 mW @ 206 MHz • 77 mW @ 59 MHz • 62 mW @ 59 MHz, low voltage

– MPEG-1 with audio • 850 mW @ 206 MHz (16% idle)

Itsy v1 StrongARM 1100 59–206 MHz (300 us to switch) 2 core voltages (1.5V, 1.23V) 64M DRAM / 32M FLASH Touchscreen & 320x200 LCD codec, microphone & speaker serial, IrDA

– Dictation • 775 mW @ 206 MHz (< 0.5% idle)

– text-to-speech • 420 mW @ 206 MHz (53% idle) • 365 mW @ 74 MHz, low voltage ( < 0.5% idle)

•

Processor: 200 mW – 42-50% of typical total

•

LCD: 30-38 mW – 15% of typical total • 30-40% in notebooks

Example: Power Consumption for Compaq’s iPAQ

206MHz StrongArm SA-1110 processor 320x240 resolution color TFT LCD Touch screen 32MB SDRAM / 16MB Flash memory USB/RS-232/IrDA connection Speaker/Microphone Lithium Polymer battery

* Note CPU is idle state of most of its time Audio, IrDA, RS232 power is measured when each part is idling Etc includes CPU, flash memory, touch screen and all other devices Frontlight brightness was 16

Metrics for Power • Power • sets battery life in hours • problem: power ∝ frequency (slow the system!)

• Energy per operation • fixes obvious problem with the power metric • but can cheat by doing stuff that will slow the chip » Energy/op = Power * Delay/op

• Metric should capture both energy and performance: e.g. Energy/Op * Delay/Op • Energy*Delay = Power*(Delay/Op)2