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From: fredm@media.mit.edu (Fred G Martin)
Subject: Re: Miniboard with 9-volt battery?
Message-ID: <1993Jul8.210945.17452@news.media.mit.edu>
Sender: news@news.media.mit.edu (USENET News System)
Organization: MIT Media Laboratory
References: <1993Jul8.102229.8542@msuvx1.memst.edu> <C9uzA9.FH0@ns1.nodak.edu>
Date: Thu, 8 Jul 1993 21:09:45 GMT
Lines: 51

In article <C9uzA9.FH0@ns1.nodak.edu> altenbur@plains (Karl Altenburg) writes:

> The typical alkaline 9-volt battery has a very low capacity.  It is
> contructed of several small cells (six 1.5V cells).  So, although a
> 9-volt battery would allow the MiniBoard to run, you wouldn't be able
> to run even a small motor for very long.  The C-cells have many more
> times the capacity (for a rough comparsion, think of the difference in
> weight between a 9-volt and six C-cells.)  If weight is problem, you
> could use AAs, but there again you have about one third to one fourth
> the capacity of the C-cells.

One further thing about batteries while we're on the topic...

Not only is the overall capacity of the battery important (e.g., 9v
batteries won't last long simply because they don't hold that much
charge), but a battery's ability to -deliver current-.  This
characteristic is typically measured as a battery's "internal
resistance."   When you try to draw lots of current out of a battery,
the cells' characteristic internal resistance limits how much current
can actually be delivered---as you try to draw more and more, voltage
is lost against this resistance until you can't draw any more current.

An interesting experiment, if you have a VOM that can handle it, is to
short-circuit a new AA alkaline battery and measure the current it
delivers with your VOM.  You'll need a 10 amp scale, because it turns
out that a AA alkaline delivers 8 amps of short circuit current.  This
is a the maximum current that you'll ever get out of a AA cell---if
the cell were actually powering something, you'd get less current,
because you'd need it to be delivering voltage as well as current.  In
the short-circuit case, the voltage output is zero, so it's not going
to be an effective use of the battery.

This whole discussion is relevant if you're trying to power a motor
off of a 9v battery, assuming you've already decided it doesn't matter
if the battery doesn't last very long.  Since the 9v's cells are
small, their current-delivering capacity is also small.  The bottom
line is you can only run low-current draw (i.e., weak) motors off of a
9v battery.  I haven't done the short-circuit-VOM test will a 9v
battery, but I would guess it maxes out at around 1 amp.

One other thing: *don't* try the short circuit test with NiCad or lead
acid batteries!  Their internal resistance is much lower than alkaline
cells and hence their short circuit current is much higher.  I've seen
too many melted down wires and overheated cells when lead acids are
inadvertently shorted.  Remember, these babies are used to start cars.

cheers,
Fred
-- 
Fred Martin | fredm@media.mit.edu | (617) 253-7143 | 20 Ames St. Rm. E15-301
Epistemology and Learning Group, MIT Media Lab     | Cambridge, MA 02139 USA
