Re: TI-H: Expander, etc.


[Prev][Next][Index][Thread]

Re: TI-H: Expander, etc.



> > What I meant was why not use lithium batteries for the expander-type
> > devices and use normal memory chips.  The TI uses ONE lithium battery
> > that powers the RAM probably 90% of the time (more or less depending
> > on your calc usage).
> Actually, the lithium battery is used only when the AAAs are not
> installed, so not really that 90% of the time...


The lithium battery backup in the calculator is only used when there is no main
power available, kind of like a "failsafe".  This job can also be done by a
high-valued capacitor, but generally lithium backups are cheaper.

Lithium "button" batteries are generally only rated for about 0.1 to 0.5
milliamps continuous.  They can supply this current for quite a long time.
However, most static RAM chips draw in excess of 15 to 20 milliamps when they
are being accessed at full speed.  This would far overload a lithium battery.
Even low-power static rams access maybe 3 or 4 milliamps (as in the TI-85,
etc.).  So in the TI-85 and other TI calculators, the lithium battery backup is
only used as a "failsafe" and is generally not used at all (I'd say it's used
maybe 5% of the time, just when the AAA batteries aren't installed).

Lithium battery backups are great for when static RAM chips are in the
"suspend" or "sleep mode" (or more techinically, when the chip enable line is
inactive and its data and address lines go high-impedance).  In this mode,
static RAM draws less than 1 or 2 microamps, or about 0.001 milliamps, which a
lithium battery can easily supply for many years.  In contrast, a single AAA
battery can usually supply around 20 to 50 milliamps for a couple hours.
Alkaline batteries are much better for high current applications, and lithium
is bettery for super-stable, continuous output of low currents.

Newer technologies like lithium AAA and AA batteries use different current
output methods, of which I'm not quite familiar with.  But needless to say they
combine the current output capabilities of alkaline with the stability of
lithium, creating a "super battery."  This is the basis for the high-capacity
(and VERY expensive) lithium-ion and NiMH batteries used in the latest laptops.
 The battery used in the latest gateway laptop costs over $200, just to give
you an idea of the technology used...

-Mel


Follow-Ups: References: