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From: none on 18 Feb 2010 23:03
Is the "power" of a battery a constant? I assumed it should be, but
then I was thinking that if V=IR, say you have a battery with voltage 9
and circuits with resistance 3 or 4.5. The current should be 3 or 2,
respectively. Then, the power would be 27, or 18, if power = voltage *
current, right? So does the power of a batter change depending on the
resistance of the circuit? Seems odd...

From: Peter Webb on 18 Feb 2010 23:15

"none" <none(a)none.com> wrote in message
news:2010021822033416807-none(a)nonecom...
> Is the "power" of a battery a constant? I assumed it should be, but then I
> was thinking that if V=IR, say you have a battery with voltage 9 and
> circuits with resistance 3 or 4.5. The current should be 3 or 2,
> respectively. Then, the power would be 27, or 18, if power = voltage *
> current, right? So does the power of a batter change depending on the
> resistance of the circuit? Seems odd...
>

Shouldn't seem odd. If the resistance is infinite (ie it is an open circuit)
then no power is consumed. The current flow obviously depends on the
resistance.

One thing that should remain more-or-less constant is the output voltage.
However, real batteries have internal resistance (shorting them does not
produce an infinite current). This increases as the batteries discharge. The
internal resistance places an upper limit on current flow and hence power
output; this is the main reason that batteries stop working in high drain
appliances.


From: Androcles on 18 Feb 2010 23:23

"none" <none(a)none.com> wrote in message
news:2010021822033416807-none(a)nonecom...
> Is the "power" of a battery a constant?

No.


I assumed it should be, but
> then I was thinking that if V=IR, say you have a battery with voltage 9
> and circuits with resistance 3 or 4.5. The current should be 3 or 2,
> respectively. Then, the power would be 27, or 18, if power = voltage *
> current, right?

Wrong. Power = voltage * current * time.

>So does the power of a batter change depending on the resistance of the
>circuit? Seems odd...
>
Power is measured in kilowatt hours or watt hours. That's what
you pay for.

A one watt-hour battery may deliver a milliamp for a 1000 hours at 1 volt
into a 1000 ohm resistor, or it could deliver 1 amp for 1 hour into a 1 ohm
resistor.
A car battery is 12V nominal and delivers 100 amps (1200 watts) into the
starter motor, but it won't do it for long. Hopefully it doesn't need to,
the engine fires up.
It can deliver 3 amps into a headlight for much longer (36 watts)
but it would take 6 amps for two headlamps and only last half
the time.





From: Darwin123 on 18 Feb 2010 23:29
On Feb 18, 11:03 pm, none <n...(a)none.com> wrote:
> Is the "power" of a battery a constant? I assumed it should be, but
> then I was thinking that if V=IR, say you have a battery with voltage 9
> and circuits with resistance 3 or 4.5. The current should be 3 or 2,
> respectively. Then, the power would be 27, or 18, if power = voltage *
> current, right? So does the power of a batter change depending on the
> resistance of the circuit? Seems odd...

Ideally, the voltage of the battery is constant. The current
changes with the resistance of the circuit. The power can vary
greatly.
The voltage of the battery is determined by the chemistry of
its components. The voltage is physically determined by how tightly
the ions hold on to their electrons. You can read up on how the
voltage of the battery is determined in any introductory chemistry
textbook.
The voltage of the battery can't be 100% constant, of course.
There are variations in voltage due to the current, the age of the
battery, the temperature of the battery, etc. As a first order
approximation, one can model the battery as being a constant voltage
source. The power changes drastically.
There are a few approximations that enable one to describe these
variations in voltage in simple model. For example, there is a
phenomenological parameter called the internal resistance. The battery
acts like it is a constant voltage source with an internal resistance
in series with it.
The battery does have a maximum current. This puts an upper
limit to power. Usually, the changes in voltage of the battery are
modeled by assuming the battery has an internal resistance. Then the
maximum current of the battery is the voltage of the battery divided
by the internal resistance. The internal resistance of the battery may
increase as the battery ages.
The concept that I sense you are grouping for is total capacity.
The total capacity of a battery is a measurement of potential energy.
When the battery is fresh, it has potential energy equal to its total
capacity. Energy is conserved.
Each battery has a maximum amount of energy it can put out,
which increases with the size of the battery. The potential energy of
the battery decreases as the battery is used. The bigger the power
that the battery puts out, the faster the energy is used up. The
internal resistance of the battery increases as the potential energy
is used up. Eventually, the battery runs out of potential energy. The
internal resistance reaches infinity (which to an engineer simply
means a very big value). Then, the battery is dead..
From: Sam Wormley on 18 Feb 2010 23:36
On 2/18/10 10:03 PM, none wrote:
> Is the "power" of a battery a constant? I assumed it should be, but then
> I was thinking that if V=IR, say you have a battery with voltage 9 and
> circuits with resistance 3 or 4.5. The current should be 3 or 2,
> respectively. Then, the power would be 27, or 18, if power = voltage *
> current, right? So does the power of a batter change depending on the
> resistance of the circuit? Seems odd...
>


http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html

Forget not that there is also a resistance in the battery itself.


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