Alt-BEAM Archive
Message #00271
To: BEAM mailing list beam@corp.sgi.com
From: Michael Kennan kennanm@efn.org
Date: Sun, 7 Feb 1999 21:00:25 -0800 (PST)
Subject: [alt-beam] Re: Current vs. Voltage
On Sun, 7 Feb 1999 TurtleTek@aol.com wrote:
> Now, I have a few questions about this that never got answered the last time:
> 1)First of, Is this true? Or is it just another "BEAM myth"?
> 2)If it's true, then why is it a mistake so often made? I see tons of bots
> with mis-matched cells connected in parallel on "My bot" sites.
> 3)Why does this happen? I guess the formula isn't "output current is sum of
> cell currents", eh?
> 4) Supply any other information you think we may need.
Ok, I'll bite.
1) Depends on the cells. One thing to keep in mind is that the rules that
govern current and voltage mostly rely on everything else remaining
constant... A solar cell is a semiconductor, and like all semiconductors
it is going to do different things depending on which way it is biased and
what it is hooked to. Think about 2 different batteries, with different
voltages and current capabilities. If you hook these up in parallel, what
will happen? Well, the smaller battery might just think it is in a
charger, if the difference is extreme enough. Then the larger battery is
not just driving the circuit, it is also driving the little battery. With
solar cells, you can have a similar situation, where the smaller solar
cell actually stops producing current since the larger cell is trying to
force a current through it the wrong way.
2) It may or may not be a problem for any specific pair of cells.
Experiment and see for yourself before you make the bot... just hook up
the cells with a dummy load and measure the voltage and current.
3) See (1), plus this: output current is the sum of the currents,
provided that you cannot drive any of the current sources backwards. With
a solar cell, you can to some extent, so there is a nonlinearity. Look at
the VI curves for the cells (perhaps they are in the PDF file mentioned a
couple of days ago?) and you will probably see that for certain currents,
the voltage drops considerably.
MK
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