Battery TechnologyNovember, 2010

1. range: function of energy density of the battery.Compare gasoline @ 12,000 (theo.) / 2600 Wh/kgwith the lead-acid battery @ 175 (theo.) / 35 Wh/kg

2. time to refuel: charge 40 kWh in 5 minutes?220 V × 2200 A!!!When you pump gasoline @ 20 /min,your energy transfer rate is about 10 MW!(Hint: energy density of gasoline is 10 kWhth/.)

3. cost:(a) light but safe means higher materials costs,e.g., less steel, more aluminum; and higher processing costs,e.g., fewer castings, more forgings...(b) to reduce load on the battery requires high efficiency appliances = costly(c) low cycle life — batteries priced @ $4,000 to $8,000lasting about 2 years

Problems with EV propulsion

Specific Energies of Battery Chemistries

(Wh/kg) (MJ/kg)lead acid 35 0.13NiCd 45 0.16NaS 80 0.28NiMH 90 0.32Li ion 150 0.54gasoline 12,000 43

NOTE: 1 Wh/kg storage capacity equals about 1 mile driving range

Battery Basics

what is a battery?a device for exploiting chemical energy to perform electrical worki.e., an electrochemical power source

the design paradigm?choose a chemical reaction with a large driving force (ΔG) and fast kinetics to cause the reaction to occur by steps involving electron transfer

Types of lead-acid batteries

1. Car battery“SLI” - starter lighting ignitionDesigned to provide short burst of high currentMaybe 500 A to crank engineCannot handle “deep discharge” applicationsTypical lifetime of 500 cycles at 20% depth of discharge

2. Deep discharge batteryMore rugged constructionBigger, thicker electrodesCalcium (and others) alloy: stronger plates while maintaining low leakage currentMore space below electrodes for accumulation of debris before plates are shorted

3. “Golf cart” or “forklift” batteries Similar to #2Bigger, very rugged, Low cost — established industryAntimony alloy, Strong big electrodesBut more leakage current than #2Can last 10-20 years

AGM Battery

Lithium Ion Battery

Tesla BatteryThe pack weighs 990 pounds, stores 56 kWh of electric energy, and delivers up to 215 kW of electric power, 375 volts. Battery cost is about $30,000

Each cell is 18mm in diameter by 65mm length, The small cell size enables efficient heat transfer, allows for precise charge management, improves reliability, and extends battery pack life. Each cell is enclosed in a steel case which effectively transfers heat away from the cell. The small size makes the cell essentially isothermal, and its large surface area allows it to shed heat to the ambient environment.

Sixty-nine cells are wired in parallel to create bricks. Ninety-nine bricks are connected in series to create sheets, and 11 sheets are inserted into the pack casing. In total, this creates a pack made up of 6,831 cells.

Charge management (lead acid)

Over-discharge leads to “sulfation” and the battery is ruined. The reaction becomesirreversible when the size of the lead-sulfate formations become too large

Overcharging causes other undesirable reactions to occurElectrolysis of water and generation of hydrogen gas (explosive)Electrolysis of other compounds in electrodes and electrolyte, which can

generate poisonous gassesBulging and deformation of cases of sealed batteries

Battery charge management to extend life of battery:Limit depth of dischargeWhen charged but not used, employ “float” mode to prevent leakage

currents from discharging batteryPulsing to break up chunks of lead sulfateTrickle charging to equalize charges of series-connected cells

Charge profile

A typical good charge profile:Bulk charging at maximum power

Terminate when battery is 80% charged(when a voltage set point is reached)

Charging at constant voltageThe current will decreaseThis reduces gassing and improves

charge efficiency“Absorption” or “taper charging”

Trickle charging / float modeEqualizes the charge on series-connected

cells without significant gassingPrevents discharging of battery by

leakage currentsOccasional pulsing helps reverse sulfation

of electrodes

A 12v automotive battery is actually made up of cells…

Power Configurations

In series In Parallel

My EV bug is a 72V system, 6 x 12v batteries in series…

Specifications & Performance:

http://evw.tech.purdue.eduDrive system: 72 volts (lead acid)Top Speed: 40 mphDistance: 25 milesCost: $5,000 (total)

Vehicle: $1,500Batteries: $700Electronics: $2,000Batt Chargers: $500Mechanical: $300

Explain the TOTAL impact of adding an additional 6 (12V) batteries…

1)In Series (144V system)2)In Parallel (to the existing battery set, remains a 72V system)3)As a secondary drive system

Consider (a) vehicle performance, (b) needed electrical component upgrades, (c) mechanical considerations

Assignment (email answer to mdkane@purdue.edu)