batteries lecture

Battery Technology
November, 2010
Problems with EV propulsion
1. range: function of energy density of the battery.
Compare gasoline @ 12,000 (theo.) / 2600 Wh/kg
with 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 $8,000
lasting about 2 years
Specific Energies of Battery Chemistries
lead acid
Li ion
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 work
i.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 ignition
Designed to provide short burst of high current
Maybe 500 A to crank engine
Cannot handle “deep discharge” applications
Typical lifetime of 500 cycles at 20% depth of discharge
2. Deep discharge battery
More rugged construction
Bigger, thicker electrodes
Calcium (and others) alloy: stronger plates while maintaining low leakage current
More space below electrodes for accumulation of debris before plates are shorted
3. “Golf cart” or “forklift” batteries Similar to #2
Bigger, very rugged, Low cost — established industry
Antimony alloy, Strong big electrodes
But more leakage current than #2
Can last 10-20 years
AGM Battery
Lithium Ion Battery
Tesla Battery
The 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 becomes
irreversible when the size of the lead-sulfate formations become too large
Overcharging causes other undesirable reactions to occur
Electrolysis of water and generation of hydrogen gas (explosive)
Electrolysis of other compounds in electrodes and electrolyte, which can
generate poisonous gasses
Bulging and deformation of cases of sealed batteries
Battery charge management to extend life of battery:
Limit depth of discharge
When charged but not used, employ “float” mode to prevent leakage
currents from discharging battery
Pulsing to break up chunks of lead sulfate
Trickle 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 voltage
The current will decrease
This reduces gassing and improves
charge efficiency
“Absorption” or “taper charging”
Trickle charging / float mode
Equalizes the charge on series-connected
cells without significant gassing
Prevents discharging of battery by
leakage currents
Occasional 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:
Drive system: 72 volts (lead acid)
Top Speed: 40 mph
Distance: 25 miles
Cost: $5,000 (total)
Vehicle: $1,500
Batteries: $700
Electronics: $2,000
Batt Chargers: $500
Mechanical: $300
Assignment (email answer to
Explain the TOTAL impact of adding an additional 6 (12V) batteries…
In Series (144V system)
In Parallel (to the existing battery set, remains a 72V system)
As a secondary drive system
Consider (a) vehicle performance, (b) needed electrical component upgrades, (c) mechanical considerations