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Power optimization and some (stupid?) LED questions

From: Demetrius Anger   dangerwaste.org
Date: Wed, 18 Aug 2004 09:42:12 -0700

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I don't know if these questions are way off topic or not - it's not really
about robotics.  It is just the beginning of learning for me though so I'm
trying to start simple.  Let me know if it is totally off-topic.

I have some questions about battery power.  I'm making lighting for a
scultpure that uses LEDs.  I'm going to (try to) make a dark-activated
switch so it only runs at night, but it needs to run for about a week.  I
can always go switch out batteries but I'd rather just make something that
doesn't need any attention so I'm trying to figure out if there are
different ways to do things to optimize the power use.

I got a couple 6 volt lantern batteries that I was going to use.


Okay stupid question(?) first - if the battery is 6V and the LEDs have a 2
volt drop and want to run on 20mA do I need to put a resistor in there if I
use three LEDs in series?  This is totally confusing me.  Do I calculate the
resistance as if the LEDs weren't there?  Like the ideal LED is without
resistance right, but then there's a voltage drop so then it's got to be
doing something right?  I am feeling stupid about this because I think
someone has explained this to me before.
Aaa!

Also, does it make a difference if I set the batteries up in series or
parallel in terms of power use?  Or does it make a difference in power how
much the LEDs are in parallel or series?

I want to run about 20 of the LEDs.  I'm just trying to get a general idea
of whether or not it matters how I set it up.  Like 2 batteries in series
with parallel sets of 6 LEDs in series.  Or 2 batteries in parallel with
parallel sets of 3 LEDs.  Or should I put every single LED in parallel for
some reason?


Anyone done battery powered stuff?  Should I get more batteries or are my 2
6V batteries going to last the week?  I can't find any information about how
to calculate battery life.

The only thing I can find is that battery life is longer at lower rates of
discharge.  I went to some of the battery manufacturer sites and can't find
anything about amp/hours (which is what someone told me how battery
discharge was measured) or battery life at all.

Thanks for any help!  If these are too many questions or whatever anyone
have a source I can read up on?

-aaron


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