Re: TI-H: Extending the battery life
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Steve Wrobleski wrote:
>
> ti-hardware@lists.ticalc.org wrote:
> >
> > With all this talk about putting circuits in internally, it only
> > seems natural to me to point this out, your batteries will drain a lot
> > sooner. So I was wondering, wouldn't 2 9volt batteries outlast 4 aaa
> > bateries? Common sense tells me it would, and it also looks like it's
> > possible to add 2 9 volt batteries in there, or at leats one. and with a
> > couple of resistors you could drop the voltage down to 4.5( I figured this
> > out by plugging my calc into an AC adapter at 4.5 volts).. Would this
> > work??
> > -Alex
> >
>
> Actually, resistors dont drop voltage. Youd want two resistors and make
> a voltage divider to make it around 4.5 but this would drain even when
> no power was being used. Best bet would be just to open a 9-volt and
> rewire it so that instead of 9 volts, it makes 4.5 volts. It would be a
> bitch to put it back together probably and every time you needed a new
> battery
Here's a little insight on batteries and resistors. The voltage rating
of the battery has *no* effect on the power the battery is capable of
delivering. The power capacity depends solely on the physical size of
the battery. This is because batteries are made with plates of different
materials inside them. A chemical reaction between the two plates
produces the voltage; the size of the plates determines how much current
will be available. All this means: 4 AAA batteries will run your calc
longer than a single 9V battery (assuming they're the same type: eg,
alkaline, rechargable, lithium, etc).
Now for resistors. They use Ohm's law, which states V=IR with Voltage,
Resistance, and current (I). I don't know how 'I' was picked for
current.. Now, if you look at the formula, and put nearly 0 current in
and the resistance in, then you see that the voltage drop over the
resistor is next to nothing. However, if you increase the current,
leaving the resistance the same, then the voltage *will* drop. The
amount that the voltage drops depends on the current going through the
resistor and its resistance. So if the TI-85 (or whatever else) had a
constant current draw (it doesn't..) then you could use just a resistor
to lower the voltage for the TI-85. But, as I said, the TI draws more
current while it is graphing, for example, than it draws while it is
turned off. Using a resistor voltage drop would cause a wide voltage
swing on the TI.. definitely something that computers don't deal with
well.
<pre>
--
Greg Hill
http://braves.bhs.davis.k12.ut.us/
ghill@lab.tech.bhs.davis.k12.ut.us
greg_hill@email.state.ut.us
</pre>
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