[aprssig] (Slightly OT) What is solar panel power output?

[Steve Noskowicz]

Bob,
   Correct me if I am wrong, but aren't good solar cells closer to a (Pretty) constant voltage *UNTIL* you get close to the current limit, then they supply *near* constant current down to zero volts? 
  There must be some "softness" to this, In other words, *some* voltage decrease as the load increases, probably some IR losses.
  Then as load increases further, you have a knee where the Voltage starts dropping faster and faster as the current increases slightly to Isc.  [or is there some foldback?]
 However,  I believe this is what you are saying.

-- 
73, Steve, K9DCI


--- On Tue, 9/9/08, bob evinger <wd9eka at evinger.com> wrote:

> From: bob evinger <wd9eka at evinger.com>
> Subject: Re: [aprssig] (Slightly OT) What is solar panel power output?
> To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
> Date: Tuesday, September 9, 2008, 5:45 PM
> actually those numbers tell a lot. Solar panels are current
> sources. That 
> amp rating is short ciruit current, and the current it will
> provide into a 
> battery with proper sized wire should be slightly less.
> 
> The preferred testing method of a new panel when recieved,
> and also one of 
> the easiest ways to verify solar alignment is to disconnect
> it fron the 
> battery and put an ammeter directly across the panels
> leads(ammeter has to 
> be rated for greater than the peak output of the panel)
> then adjust angle 
> for highest current at noon, assuming a fixed panel mount.
> If the panels 
> are adjustable then it is best to do this for at least
> summer winter. If 
> the panels are not adjustable, or in a place one cant get
> to in the 
> winter, then the more vertical the panel for winter time
> the better. Snow 
> will fall off quicker(if in florida disregard :) ).
> 
> take the current and you might as well assume 12 volts,
> that is the power 
> available effectively if you are using a low end
> regulator,Pulse width 
> modulated.
> 
> If you have enough panels, it can be worth getting an
> MPPT(maximum power 
> point tracking) charge controller but they will cost more
> than a PWM 
> controller. They will also harvest another 20-25% more
> power out of the 
> panels. THis is because they are dc-dc converters and will
> keep the panels 
> voltage at maximum power. With a PWM controller, the
> FET's open up and the 
> batteries pull the panel voltage down. On a low battery
> that may be 11.5 
> volts. An mppt controller will keep the panels voltage at
> or near its 
> rated 16.5 volts and give more available power.
> 
> simplest example.  assume a panel capable of 10 amps
> with a pwm and the batteries sitting at 11.5 volts there
> will only be 115 
> watts available and 10 amps will flow into the battery.
> 
> MPPT controller at 16.5 volts and 10 amps going into the
> controller you 
> will have 165 watts available, on the output side of the
> MPPT controller 
> under these conditions there should be close to 14.3 amps
> flowing into 
> the battery. As the battery charges the current out will
> drop off.
> 
> An MPPT controller works quite well. I used PWM controllers
> for years but 
> this spring doubled my PV capacity to almost 1000 watts of
> panels and 
> replaced my pwm controller with an mppt.
> 
> I have about 54 amps of current availalbe if it was just a
> pwm controller. 
> I have seen up to about 72 amps charging with the mppt
> controller during 
> the 10am - 2pm prime power window. The technology helps
> more on partly 
> overcast days and non-peak hours.
> 
> Sorry probably way more than most on here wanted to know.
> 
> On Tue, 9 Sep 2008, Mark Cheavens wrote:
> 
> > Neither of those numbers tell us anything. You need to
> know the
> > output in WATTS (or amps @ a given voltage).
> >
> > Put a variable load (carbon pile) across the leads and
> measure the
> > amp output @ 13volts while in direct sunlight. While
> you are doing
> > that you can also measure the output at different
> angles (less than
> > ideal) from direct sunlight. Most start to drop
> significantly @ about
> > 15degrees in any direction.
> >
> > Mark
> > KC5EVE
> > At 03:15 PM 9/9/2008, you wrote:
> >> I am getting closer to setting up my APRS weather
> and 6 meter
> >> PropNET stations far enough from the house to make
> AC power
> >> impractical.  I have three old solar panels (cost
> me about $100 used
> >> 15 years ago) that I want to put into service.  In
> full sunlight,
> >> two of them in parallel put out  19.88v open
> circuit  and  4.78a
> >> short circuit.  How does that translate to power
> available at 13.2v
> >> nominal for running the rigs and charging the
> battery?
> >> -- 73 --
> >> J. Gary Bender, WS5N
> >> DM54rp
> >> Fence Lake, New Mexico USA
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