Batteries and Fuel Cells

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Batteries and Fuel Cells

• Portable Electric Energy

2
The Battery

• A cell consists of two electrodes of different
  metals immersed in a weak acid
• Multiple cells can be stacked in series to make a
  battery
• The positive terminal is called the anode and the
  negative terminal the cathode

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Connecting Batteries in Series

• Batteries connected end to end will have a
  voltage equal to the total voltage of the
  individual batteries
• Disposable dry cell batteries have a typical
  voltage of 1.5 V

1.5 V
3 V



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Amp-hours

• The total energy contained within a battery can
  be described using Amp-hours
• Example A battery that can provide 4 A-hrs can
  generate 4 A for 1 hour, 2 A for 2 hrs., etc.
• Example A 12 volt car battery can provide 60
  A-hrs. of energy. How many joules is this?
• Solution If it drew 60 A of current at 12 V,
  that would be (60 A) x (12 V) 720 W. And 1 hr
  3600 s, so (720 W) x (3600 s) 2.59 x 106 J
• In other words Energy (J) (Amp-hours) x
  (Volts) x (3600)

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How a Battery (Cell) Works

• Both electrodes slowly dissolve in the acid
• At the anode, electrons are used in chemical
  reactions as the metal dissolves
• At the cathode, electrons are absorbed into the
  electrode as the metal dissolves
• The net result is a buildup of electrons at the
  cathode

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Disposable and Rechargeable Batteries

• A rechargeable battery can be connected to an
  electric current so that dissolved metals reform
  on the electrodes
• Examples lead acid, nickel cadmium, lithium,
  etc.
• The chemical reactions that power a disposable
  battery cannot be reversed
• Examples alkaline dry cells, etc.

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The Lead Acid Battery

• Two electrodes, one of lead, the other of lead
  dioxide (PbO2) immersed in sulfuric acid
• Lead ions (Pb) dissolve, leaving two electrons
  behind
• Two electrons flow through the circuit and are
  used to help lead dioxide dissolve

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Disposable Batteries

• A typical disposable battery contains a carbon
  (graphite) and a zinc electrode
• The electrolyte is a paste of ammonium chloride
• Disposable batteries may leak if too much of the
  zinc can is dissolved

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How Disposable Batteries Work

• Both electrodes generate electrons when they
  dissolve
• The cathode generates more than the anode
• To remove electrons from the anode, positive ions
  plate or stick to the anode
• A membrane separates the A and B ions
• Eventually, positive ions accumulate near the
  cathode and are depleted near the anode

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Electric Vehicles

• Electric vehicles use electric motors powered by
  rechargeable batteries
• Both Honda and GM (among others) manufacture
  electric vehicles
• Electric vehicles have been around as long as gas
  powered cars!

The GM EV-1
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How and Electric Vehicle Works

• An electric motor replaces the gas engine
• Electric motors are extremely reliable
• During braking, electric motors can act as
  generators and recharge the batteries
• A rechargeable battery pack in the trunk provides
  electric power
• Battery packs are heavy and costly
• They must be replaced eventually

Electric Motor
Battery Pack
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Why Nobody Buys Electric Cars

• The battery challenge is vast. Even with our
  most advanced experimental power pack, operating
  costs in 1998 would be unacceptable to the vast
  majority of drivers. Essentially, its like
  asking the customer to buy a car with a 15,000
  gas tank--a 15,000 gas tank that holds the range
  equivalent to 3 gallons of gasoline a 3 gallon
  tank that takes 8 hours to refill, compared to a
  few minutes at a self-service gas station.
• D. Wilkie, 1994

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Comparing Gasoline and Batteries
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Pros and Cons of Electric Cars

• Pros
• Pollution occurs at the power plant where it can
  be more easily contained
• Lower operating expenses (repairs, refueling)
  than gas engines

• Cons
• Limited range
• Batteries must be replaced frequently
• Very expensive
• Same total amount of pollution (when including
  the power plant that generates electricity)

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Fuel Cells

• In a fuel cell hydrogen is burned by mixing
  with oxygen in such a way that it creates a
  voltage across two electrodes
• Only water is produced as a by-product
• Hydrogen can be fed in directly or as part of
  larger molecules such as methane (natural gas)

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How They Work

• Hydrogen molecules give up their electrons to the
  first electrode
• Electrons pass through the circuit to the second
  electrode
• Electrons are returned to the molecules when
  hydrogen and oxygen combine to make water

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Comments on Fuel Cells

• Fuel cells have been around for 100 years
• Fuel cells can be made to burn other molecules,
  such as methane, propane, etc.
• Hydrogen can be extracted from gasoline before
  being fed into a fuel cell
• Fuel cells cannot store energy, so they must be
  used in conjunction with a storage battery

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Fuel Cells in Electric Cars

• Prototype cars have been developed that generate
  electricity using fuel cells
• Pros
• Very little pollution
• Cons
• Expensive
• Hydrogen gas is explosive

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Fuel Cells to Replace Batteries

• Fuel cells can be used to power a laptop
• Micro fuel cells have been developed that are
  small enough to fit into a cell phone
• Fuel cells weigh less and last much longer than
  rechargeable batteries

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Why are Fuel Cells so Uncommon?

• Methods are being developed to store hydrogen in
  a porous material rather than as compressed gas
• Fuel cells require expensive catalysts
• How do you pump compressed hydrogen at a
  self-service gas station?
• What happens to the hydrogen tank in an accident?

21
Flywheels?

• A flywheel (i.e. a heavy disk) spinning in a
  vacuum can store a large amount of energy
• Electrical energy can be extracted (and stored)
  using magnetic fields
• What happens to a disk spinning at 100,000 rmp
  when you hit a bump?