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Rechargeable Battery ideas

  1. Sep 10, 2014 #1
    Trying to get my head around some ideas about "rechargeable" batteries and getting in a muddle!

    So I get the idea that rechargeable is a rubbish word for them but I have 2 questions:

    1) Does heat loss (i.e. from bulbs) in a DC circuit result in eventual lower PD across a cell? Thinking in terms of energy if PD provides the energy to make current flow, then surely if some of that energy is lost during transmission, by the time it returns to the cell some of that energy is lost and the PD will be reduced...

    2) Is it energy that is replaced in a battery when it is "recharged"? i.e. the electrical energy is converted into chemical energy and stored within the battery until it "discharges" that energy into a circuit?
     
  2. jcsd
  3. Sep 10, 2014 #2

    rcgldr

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    During operation a rechargeable battery will be discharged, and the voltage will decrease over time during usage, following a curved path with a somewhat linear mid section. The voltage is not supposed to drop to zero, but down to some minimal point, where discharging below that voltage point end up harming the battery.

    During a recharge operations, the chemical potential energy is restored, but after many discharge / recharge cycles, the maximum chemical potential will degrade.
     
  4. Sep 10, 2014 #3

    Drakkith

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    1. The potential difference (voltage) across the battery does not change (In an ideal battery at least. In a real battery the voltage does drop over time and changes depending on the resistance of the circuit, but let's just look at an ideal case for now). A 9v battery always has 9-volts of potential difference across its terminals. The key here is that potential difference and energy are not the same thing. The potential difference is what causes current to flow but it is not "lost" or "used up" as the battery discharges like energy is. Apply 9-volts to a circuit of 10-ohms results in more current flow than applying 9-volts to a circuit of 100-ohms, but in both cases the voltage is 9-volts. However in the 10-ohm circuit the energy lost per time (power) is 10 times more than the 100-ohm circuit. The math:

    9v/10 ohms = 0.9 amps
    9v/100 ohms = 0.09 amps

    Power is volts x current:

    9v x 0.9 = 8.1 watts
    9v x 0.09 = .81 watts

    So as you can see, the energy lost per time is greater in the 1st case, but the voltage is the same.

    2. Yes, energy is stored in the battery when it is recharged.
     
  5. Sep 10, 2014 #4

    SteamKing

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    It's not clear why you think 'rechargeable' is a rubbish word.

    When you 'recharge' these batteries, you are not adding additional electrical charge to the cell, but you are reversing the chemical reaction which produces the electricity in the first place.

    You should study what happens in these types of rechargeable cells:

    http://en.wikipedia.org/wiki/Lithium-ion_battery

    http://en.wikipedia.org/wiki/Nickel–cadmium_battery

    http://en.wikipedia.org/wiki/Nickel–metal_hydride_battery

    and the granddaddy of them all:

    http://en.wikipedia.org/wiki/Lead–acid_battery
     
  6. Sep 10, 2014 #5
    1) it doesnt matter if the energy of the battery goes into heat or mechanical work (such as in the case of a DC motor turning a wheel). As long as there is current I between the points of P.D=V(between the poles of the battery) then there is power P=V*I offered from the battery to the circuit. This power(Power=Energy per unit of time, energy offered per sec) can go to heat or mechanical work or whatever, but once it is offered it cannot returned to the battery (dont get confused because the current is returning to the other pole of the battery, this doesnt mean that the same happens to power/energy).


    2) This is absolutely correct. When we charge a battery , we offer energy to the battery and this energy is converted to chemical energy with the help of some chemical reactions that happen inside the battery.

    As to why exactly the voltage between the poles of the battery is *slowly* but steadily dropping as the battery is offering energy to the circuit (discharging), this is not so easy to explain, it has to do with the chemical reactions that happen inside the battery, as these reactions convert chemical energy to electrical energy some of the electric charge in the poles of the battery is stored in chemical compounds inside the battery and is being removed from the poles. The opposite happens when we charge the battery, this electrical charge that is stored inside the battery is realeased back to the poles.
     
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