Re: TI-H: Extending the battery life

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Re: TI-H: Extending the battery life

On Sun, 10 Nov 1996 09:02:44 -0800, you wrote:
>> Actually, resistors dont drop voltage.=20


Actually, they do.  That's why a voltage divider works.


>> 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.=20


Good point.


>> 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


It's impossible to "rewire" a 9V 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.=20


True.


>The power capacity depends solely on the physical size of
>the battery.=20


Not really...  This only holds if you're comparing batteries of the
same type, i.e. alkaline to alkaline or lithium to lithium.


>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).
>


Neither the voltage nor the current sourcing capability determines the
power a battery has.  The most important statistic is the maH rating
(milliamps per hour).  Roughly speeking, a 250 maH battery can produce
250 ma for 1 hour, 1 amp for 15 minutes, etc.  In actuallity the maH
rating is never exact, but it gives you an idea of the power
capability of a battery.  The reason 4 AAA batteries will last longer
is because their collective maH rating is much higher than a single 9V
battery.  Actually (if memory serves), even two AAA batteries can beat
the power capability of a single 9V battery.  The only problem is that
you'll need a charge pump DC-DC converter if you want to get the
required 6V out of two AAA batteries.


>Now for resistors. They use Ohm's law, which states V=3DIR 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.




You're on the right track, but here's some clarifications: =20


First of all, the TI-85 (as well as most other microprocessor devices)
uses a voltage regular to automatically bring the voltage down to
exactly 5.0 volts.  It doesn't matter if the input was 6V or 10V, it
will still bring it down.  In fact, a voltage regulator *requires*
that the input voltage be 20 to 50% higher than the rating.
Therefore, you should be able to directly connect a 9V battery and get
it to work.


That 5.0 volts will (theoretically) not change under any load.
Therefore, the voltage drop is ALWAYS the same, 5.0 volts, under any
resistance anywhere.  When the processor isn't doing anything special,
the resistance is relatively high.  Using ohms law with 5.0 volts and
a high resistance, this forces the current draw to be very low.
However, when you do something else (like graphing), the resistance of
the TI-85 drops, forcing the current draw to be higher.  The voltage
never "swings," only the resistance does.


-Mel


<pre>
--
The TI Memory Expansion Homepage
http://pilot.msu.edu/user/tsaimelv/expander.htm
</pre>
References: