# Slighlty Different Voltages

Discussion in 'Home Power and Microgeneration' started by Danno, Aug 6, 2004.

1. ### DannoGuest

I know I saw a post similar to this recently, but I can't dredge it up...
I currently have an ICP 75W panel, it apparently pushes at 15V. I would like to
wire a Sharp panel in parallel with it, but the Sharp panels push at 17.2V.
Will it be OK to hook these up like this? I use a standard (non MPPT) charge
controller, but would eventually like to get into an MPPT. Would the differing
voltages affect the efficiency?

2. ### Vaughn SimonGuest

I have dissimilar panels wired together and find it to be a non-issue.
A simple test is to connect them both together with an ammeter in the
circuit. If you can remove the wire from either panel and see the charge
current decrease (and I think you will) , you are home free.
Doubtless, it would be better if they were the same, but you are
probably better off with the two dissimilar panels wired together than with
either one of the two by itself.

Vaughn

3. ### DannoGuest

Thanks for the responses. My concern was that, in my zest to increase my
charging capacity, I would ruin my current (limited) ability. I am running
my current system through a "standard" charge controller (ICP CC20), with a
float of 14.4V. It is capable of handling the increased amperage of the new
panel. I guess I'll have to put the MPPT idea on the back shelf for a while...
Am I correct in assuming that the maximum voltage of the Sharp
will not be reached; that it will be held at 15V by the first panel. Or does
the charge controller limit both to 14.4V? I would like to take a look at the
output curve of the panel to see just how much power I'd actually be making.

4. ### WinstonGuest

Are the panels rated for the same amount of current?
If so, what would happen if you put them in series and drove the controller at
32.2 V? Ferinstance, these MPPTs have a maximum array voltage of 50 V:

http://www.solarseller.com/solar_converters_mppt_charge_controllers_maximum_power_point_tracking.htm

--Winston, the Solar Noob

5. ### DannoGuest

I had been thinking along the same lines, but that is too far off in the
future for me to worry about right now. They are not rated for the same current
anyway, so I doubt it would work here. Had considered maybe using a DC-DC
transformer to lower the voltage of the 17.2V to 15V, but then the power output
is not that different between 15V & 17.2V on the panel anyway.

6. ### DannoGuest

Good stuff, I think this is enough info for me to safely justify purchasing
the new panel. Thanks a lot!
Danno.

7. ### StarbaseGuest

I asked the same question a while back and got a few conflicting answers so
I just wired them all up and it seems to work OK to me, the main thing seems
to be that the battery bank dominates what the voltage is and if the battery
is not at ~13.8 volts (14.2v) then the charge controller will just whack the
power through, the panel voltage can't go above battery voltage if the
battery will accept a charge and all my panels are quite capable of
supplying the required equalization voltage. I have diodes on the panels
(just one on each) as all the panels are not pointing south or at the same
angle. As I get into it more I will learn more and make improvements but I
have not bust anything yet and always treat my batteries with the utmost
respect.

I can see that if the battery was fully charged and the charge controller
shorting the PV array then the higher voltage panels might reverse bias the
lower voltage panels but no more than a battery would on a night without a
blocking diode, would this harm the panel?

One other thing which confuses me a little and I have more to learn about is
the maximum amperage which a panel can handle (how would that happen in
parallel connections, if a panel is shaded does it's resistance not go up?,
is a panel not a diode itself?), a blocking diode should prevent current
from one panel flowing through another? Using a charge controller means you
do not normally have to use a blocking diode (which wastes about 0.6 volts x
X amps = X watts) but as I mix my panels I used them just to be safe.

These are just my thoughts on the subject and the way I am doing things at
present, things might (will) change as I learn more. I am grid connected and
treat it purely as a hobby.

Regards

Chris

8. ### DannoGuest

I have also tried to be gentle with my setup; it would seem there are no
inexpensive pieces to "take chances" with <grin>. ICP's setup is pretty slick,
I'm quite certain that they have the protective diodes built in. They market
their panels (or "systems", actually) with the "Plug'n'Play" branding, which
allows a total noob like me to just plug an additional panel into a specifically
shaped outlet on the bottom of the first panel. I'v got three 15-watt PV-film
panels spliced into an extra PnP wire I had, and it's working well; will do
the same with the Sharp 123 when I get it.

<grin>. I started that way last August... I think I am addicted... The
question for me, now, is: Can I make this work year-round in my 3rd fl, west-
facing, Canadian apartment? Summertime sun is awesome, it practically sets in
north, lots of afternoon and evening sun, but wintertime is, IIRC, 6 hour days
hanging very low in the southern sky. Now, the winter wind, OTOH...