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How dense batteries can we make?

  1. Jul 4, 2010 #1
    I took a two-hour trip with my electric mini crosser (mobility scooter, electric wheelchair, whatever), and I started thinking about how much energy we can actually pack into batteries.

    So how "dense" (watts per volume unit or whatever) are the practically applicable, and the most advanced batteries we can make? What limits us?
  2. jcsd
  3. Jul 4, 2010 #2
    Current batteries are 100 - 200 times less dense than petrol.. Wh/kg. There are new battery technologies such as aluminium batteries and lithium-air which have much high Wh/kg.
  4. Jul 4, 2010 #3
    The numbers I found are:

    Lead Acid Battery: 83 kJoules per kilogram
    Lithium Polymer Battery: 500 kJoules per kilogram
    Gasoline: 10 million Joules per kilogram
  5. Jul 5, 2010 #4
    Interesting. Anyone know more about this?
  6. Jul 5, 2010 #5
    A lot of people think that Li-Air batteries will lead to the next big break through for electrifying cars. With Li-Air you basically only have to carry about half the cell as you are using ambient oxygen for the other half. The problem is that the electrolyte/separator must only allow oxygen to migrate and not water. Currently silica or glass separators are used to do this but the have very poor conductivity and aren't very good at keeping water out.
  7. Jul 5, 2010 #6
    Aluminium-air too? Pretty much an instantaneous reaction there...
  8. Jul 5, 2010 #7


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    Metal air batteries do have the current record for highest energy density. Al-air for instance provides ~4.5 MJ/kg; lithium air will likely go higher, maybe 10 MJ/kg. However, as Topher stated above they have poor internal conductivity, which translates into low power density, meaning they must be discharged very slowly. And there are yet to be solved problems with recharging them without making the metal electrode unusable. So for the moment Li Ion has the highest energy density of practical rechargeable batteries, at about 0.9 MJ/kg. Gasoline is ~45 MJ/kg, or about 50X better than the best Li Ion battery.
  9. Jul 6, 2010 #8
    Oh, right, how do these numbers compare to fuel cells?
  10. Jul 6, 2010 #9


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    Fuel cell 'energy density' would be the density of the chemical fuel supply - hydrogen, methane, etc, keeping in mind that a fuel cell will waste 30-60% of the chemical fuel's energy as heat. Power (electrical energy produced per unit time) and power density are performance figures appropriately attached to the fuel cell itself, especially since cost is directly dependent on the FC power level ($8 per Watt and up).
  11. Jul 6, 2010 #10
    So, let's see if I get it right, fuel cells waste a larger portion of the energy if you try to increase the power?

    When people talk about "Hydrogen fuel cells powering cars", or the like, what HFCs do they actually mean, or rather, which are the most promising HFCs? Solid oxide fuel cells?

    And if I haven't screwed up my math... Hydrogen has an energy density of ~242,000 kJ/kg (excluding energy transmission efficiency), but is simply less dense than hydrocarbons even at very high pressures?
  12. Jul 6, 2010 #11


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    Google, but I don't think anyone has a Solid oxide fuel cell suitable for a vehicle (yet), perhaps because of the high temperatures required for SOFCs.

    http://en.wikipedia.org/wiki/Energy_density" [Broken]
    Last edited by a moderator: May 4, 2017
  13. Jul 6, 2010 #12
    Fuel cells tend to be very efficient engines when compared to thermal engines (internal combustion engines). Fuel cells are not energy storage devices like batteries although they work on very similar principles.

    When people talk about hydrogen fuel cell powered cars, they are referring to "polymer electrolyte membrane" or "proton exchange membrane" fuel cells. Also known as PEMFCs. PEMFCs are most suitable for cars because they operate at low temperatures, have a high power density, and can handle transient operation.

    Solid Oxide fuel cells or SOFCs are mostly used for stationary power generation due to their high operating temperature and their inability to perform with transient operation.

    Hydrogen has the greatest gravometric energy density (143 MJ/kg) of any other chemical based fuel in existence. However, it has a very poor volumetric energy density so storing it in small places (like a car) is a problem. Currently very high pressures and metal hydrides are used.
  14. Dec 20, 2010 #13
    Well, which is it? Does gasoline have 10 MJ/kg, or 45 MJ/kg? http://en.wikipedia.org/wiki/Gasoline#Energy_content_.28High_and_low_heating_value.29"puts it closer to 45 MJ/kg, but I suspect the much lower 10 MJ/kg for gasoline and 500 kJ/kg for the Li ion battery represents the energy delivered to the tires, after conversion losses, not the raw energy content of the sources themselves. Electric motors have significantly less conversion losses than do IC engines.
    Last edited by a moderator: Apr 25, 2017
  15. Dec 20, 2010 #14


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    45 MJ/kg sounds like heat of combustion of gasoline, that means (between other things) water as a liquid product. That in turn means these values can be used to compare energy content of different fuels, but they are not applicable (directly) to the real life applications.
  16. Dec 20, 2010 #15


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    Gasoline energy content is indeed 45MJ/kg, that is, this is the amount of heat energy released if its combusted, just as the wiki link states. You're on the right track with the rest: most of the heat energy must be inevitably lost out the tailpipe, at least half and typically two-thirds or more, and is thus not useful for propelling the vehicle.

    On the other hand most all of the energy stored in a Li Ion battery can be converted into tractive power via an electric motor (~85% or so). The bottom line is not even how far can the vehicle travel per unit mass of fuel or battery, as the vehicle system's overall makeup is very different - fuel/oil pumps, radiators, exhaust systems in the one, not so much in the other. The useful metric is how far can can two comparably performing vehicles travel (passengers, acceleration, cost, etc), one fuel-combustion and the other battery-motor. With the current technology the answer seems to be three or four to one in favor of the combustion vehicle, e.g. 300-400 miles combustion vs 100 miles battery-motor. Twenty years ago the ratio was maybe 10:1, a hundred years ago maybe 20:1.
    Last edited by a moderator: Apr 25, 2017
  17. Dec 20, 2010 #16
    The table in this website (linked above)


    gives 44.4 MJ per kilogram or 115,400 net Btu/gallon for gasoline. There are actually two energy contents for gasoline; LHV or lower heating value, and HHV or higher heating value. The difference is whether the residual heat content in the gas combustion products is used.

    US gallon
    LHV = 115,000 Btu/gallon = 121 MJ/gallon = 32 MJ/liter .
    HHV = 125,000 Btu/gallon = 132 MJ/gallon = 35 MJ/liter


    http://bioenergy.ornl.gov/papers/misc/energy_conv.html [Broken]

    These URLs also list the energy content of several other liqiud fuels.

    [added] The conventional internal combustion automoble engine is about 35% efficiency maximum but only at a specific torque and RPM. See the thread


    Bob S
    Last edited by a moderator: May 5, 2017
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