Calculating Solar Panel Amperage - Page 2

 On Mon, 08 Oct 2007 16:21:34 +0100, Eeyore



Thanks poobear. That explains it. Well done!
 -

Bob

 

As with so many things in life... "it depends."

If you just hook the panel directly to the battery then you'll never get
more than short-circuit current from the panel to the battery.  And
considering that the panel isn't short-circuited when connected to the
battery, you won't even get that.

The I-V curve is the key as 'R.H.Allen' pointed out.  There is a 'maximum
power' operating point for the panel.  As you draw more current from the
panel in a given sun situation, the voltage won't drop hardly at all up to a
point.  Then the voltage will drop very quickly if you draw current beyond
that point.

So, getting back to your battery, "it depends" a lot on what kind of charge
controller you have.  There is a category called MPPT (Maximum Power Point
Tracking).  These will use an internal DC to DC converter to transfer the DC
power from the voltage of the panel to DC power at the voltage needed for
your battery.  And in the process actually boost the current level in many
situations.  By allowing the panel to operate at a different voltage than
the load, it can get the maximum *power* out of the panel (note that maximum
power and maximum current aren't the same thing).

So if the maximum power point for your panel happened to be 5.5 amps at 18V
(~100W) and you wanted 14V output to your battery and electrical system, you
could actually get 7.14 amps.

If on the other hand you have a pretty 'dumb' charger, or no charger at all,
then you would only get 5.5 amps whether the voltage was 12V, 13V 15V or
even 18V.

daestrom

 On Wed, 10 Oct 2007 19:24:25 -0400, "daestrom"



Daestrom,

Well,,, it was just a general question. I wasn't trying to split hairs
or get deeply into theory.

I was just wondering whether my battery would get closer to 5.x
amps@14v from the controller, or closer to 7.x amps@14v. I was
erroneously thinking that the step down in voltage could possibly be
traded for increased amperage, while not exceeding the original panel
wattage of 100w.

It's  perfectly logical that since the panel is under load, the 20v
output can no longer be used to calculate controller's output
amperage. It's a like a catch 22, or a circular argument. :D

Thanks!!!
 -

Bob

 

Like I said, "it depends."

If you have a MPPT controller, then yes you can get closer to 7 amps @ 14V.

If you have a simple controller, then no you would only get about 5 amps @
14V.

daestrom