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Confused about battery's specific power and energy

  1. May 17, 2010 #1
    Hello there,

    I am so confused about the term battery specific energy (in Wh/kg) and specific power (in W/kg)

    Actually, I know what it means by power and energy, of course P = E/t

    But looking at the text, which quotes like "Ni-Cd batteries have high specific energy (than Ld-acid) but the disadtantage is they have low specific power"

    and the table says,

    Lead-acid 40 Wh/kg 180 W/kg
    Nickel-cadmium 50 Wh/kg 120 W/kg
    Nickel-metal hydride 70 Wh/kg 200 W/kg
    Lithium-ion 130 Wh/kg 430 W/kg

    What causes this to happen? How come, for the Ni-Cd which has higher Wh/kg has a lower W/kg compared to Ld-acid?

    Isn't power derived from energy divided by time?

    Thank you
  2. jcsd
  3. May 17, 2010 #2
    Maybe an analogy will help:

    The energy density of a battery is comparable to how many people a room can hold (i.e. the size of the room), while the power density is comparable to how quickly the people in that room are able to escape/leave the room (i.e. the number/size of exit doors). A high energy-density battery with a low power-density would then be equivalent to a large room full of people, but with only a single small exit door.

    Typically, the electrical equivalent of the small door would be a high internal (current-limiting) resistance.
  4. May 17, 2010 #3
    Hi gnurf,

    This is so clear. I not get it.

    Thank you so much
  5. May 18, 2010 #4


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    There are two important parameters which describe a battery. One is the amount of Energy it can store and the other is the Power it can produce. Because a battery is based on chemical reactions, the two parameters wouldn't be expected to 'track' one another.

    I think that the Energy per kg is more fundamental than the Power per kg (the mass of the plate material must be very relevant here). If you increase the area of the plates then there is no inherent limit to the power you could get out of it; you can make the plates thinner and thinner and have a bigger and bigger area, allowing more and more current to flow. There will, of course, be practical limits to this, which must account for the figures which are quoted. A new construction technique could give an improved specific power for any given type.
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