Series connection Imp difference 33%

SolarT

This is definitely a noob question. I have read a bunch of threads and the golden rule of Series connection is <10%.

What would happen if the 2 PV listed below has a series connection to MPPT charger?

STC (W) 185 130
PTC (W) 165.60 116.37
Vmpp 37.5 17.5
Impp 5 7.43
Voc 45.4 21.2
Isc 5.27 8.02

Will one blow the one apart or the Imp will not go over the lower value? Or something else.

BB.

Re: Series connection Imp difference 33%

Solar panels, are roughly, current sources...

They have a soft enough I*V curve at Pmax (Vmp*Imp) that if they match within 10%, the curves shift a little bit and all is still OK.

For placing a Imp = 5a and 7.43a in series... Basically, the lower current panel will be pressed much farther down the IV curve to where Imp of the 5 amp panel will be closer to 7 amps. Or the other panel will see its 7 amps drop to 5.43 amps at a slightly higher Vout...

And if you look at the data sheet (if you have one), you will probably see that the lower Imp panel will have Vout ~ 0 volts (or even less if they where to graph negative voltage)... I.e., the panel diodes will will simply and the ~2.43 amps and add the ~5amps at zero volts for a total ~7.43amps.

It may not be that bad (I never have played with solar panels to see what would happen), but there is a very real chance that the power of both panels together will be around the power of the 7.43 amp panel by itself (or even a bit less because of diode losses).

With a "perfect current source", you cannot never get any other value of current other than the 5 amps (at full sun). The IV curve is =5amps at any voltage (between 0 and Vmp for a solar panel).

Of course, it is not a perfect current source--but it is not that far away from one.

It is not like a battery (or solar panels) where they will share current when connected in parallel.

Many times with "mixed panel" arrays--the owner is left with the tough decision of:

1. not using one or more unmatched panels
2. getting a second (or third) charge controller that can have its own Array Imp/Vmp
3. getting more panels to mix and match Vmp/Imp into something that will work.
4. Getting new panels because new panels are so much less expensive these days and selling the old panels on EBay or Craig's List.

Add to the confusion about whether you are using a PWM or MPPT charge controller and what battery bank voltage... That also can change how to mix/match panels too.

It is not a "simple" question.
-Bill

SolarT

Re: Series connection Imp difference 33%

I should have 2 MPPT controller Monday. This is just a learning project. I can run them on different MPPT with 2 sets of batteries. Or does it makes sense to have one battery bank and charge it with two MPPT.

What is PV IV(W)curve? I don't have any data sheet to look at.

I read some where that it makes no diff mono or poly crystaline panel. Any effect on the IV curve?

BB.

Re: Series connection Imp difference 33%

IV curve. Current on one axis and voltage on the other axis.

You are correct, poly or mono have the same shape curves.

You can get a curve from another vendor and change the numbers to match those vmp/imp of your panel.

You can parallel several controllers of mixed types on one battery bank.

Choose controllers based on panels and battery bank ratings.

200 watt and smaller arrays may want pwm as it costs less.

-Bill