Comparison of energy in AA battery vs Ultracapacitor

Discussion in 'General Electronics Chat' started by jut, Mar 20, 2008.

  1. jut

    Thread Starter Senior Member

    Aug 25, 2007
    224
    2
    Just for fun, I wanted to compare the energy stored in a rechargeable AA battery sitting here on my desk vs and ultracap that I found on the internet.

    My battery: 1.2 V Ni-MH, 1700 mAH
    The ultracap: 2.7 V, 310 F

    My battery energy:
    = (voltage)(amperes)(time)
    = (1.2 V)(1700 mA)(1 Hr)
    = (1.2 V)(1.7 A)(3600 sec)
    = 7344 Joules

    The ultra cap energy:
    = (1/2)(C)(V^2)
    = (1/2)(310 F)(2.7 V)^2
    = 1129 Joules


    Hmm, seems fishy. I don't believe my battery can really supply 1.7A for one hour. Maybe half that if I'm lucky-- bringing it to 3672 Joules. That's still 3x more energy than this particular ultracap can store.
     
  2. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    that's interesting. i've never done that before :)
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why not look at the datasheet of an AA Ni-MH cell on a battery manufacturer's website like Energizer?

    Their AA Ni-MH cell is rated at 2500mAh when its voltage has dropped to 1.0V.

    The voltage stays nearly the same for most of the discharge.
    The voltage of a discharging capacitor drops quickly like a brick.
     
  4. royocir

    New Member

    Jul 9, 2008
    1
    0
    Big advantage of ultracapacitor is very quick charge. Nicads and NiMH steadily lose charge, and usually when you go for your cordless tool it turns out to be dead, and needs an overnight charge. If it were UC powered, it would charge in a few seconds. Does anybody know of a UC in AA battery form - I'm ready to buy a bunch.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A Ni-MH battery loses its charge if it is sitting for months doing nothing. But a very low trickle charge current keeps it at full charge forever. Look at the datasheet for your battery to see.

    Your AA cells are old. Modern Ni-MH AA cells have a capacity of 2500mAh.
     
  6. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    The amp/hour rating really means that the battery would last 10 hours if discharged by 1/10th of the amp/hour rating. That means 170mA for your 1700mA/h batteries. If discharged at the battery rating, they would last way less than the expected hour.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    True only for old carbon-zinc and alkaline non-rechargeable cells.

    The datasheet for Energizer's AA NI-MH cell shows that its capacity (discharged to 1.0V) is the same for its full 1.0C rating.
     
  8. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    Wow! that's very good, indeed. There's even a curve for 2C.
     
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