Please help to understand this non sense

[gargouri2001]

Hello

First i want to tell a big hello to the all community here , i am reading this forum for years already .

Now here is the question , i have 2 diy solar panel :

1) - made from 42 cell (4w each) . output voltage open circuit 23,1v and current 6.3 A [145.53w].
2) - made from 48 cell (4w each) . output voltage open circuit 26.2v and current 7.1 A [186.02w].

When i wired those 2 panel in series i got 49.3v and 7.1A . .

as far as i know in series connection voltage should add but current should be equal to the lowest panel.
even more strange that overall output in watt is even higher , using each panel alone give overall of 331.32 w , but using them on series gives 350.03w ,
what the hell is going on here .

[inetdog]

Pretty simple, I think.

The current you measure is Isc, right, rather than a current under calculated load such as Imp?

And you built your DIY panels including bypass diodes, right?

What you are seeing is the current from the 7.1A panel flowing through the bypass diodes on the other panel rather than being restricted by that panel's lower current.

If you did not test and match your cells according to current when assembling your panels, you may have another surprise in store for you.

Suppose you have divided each panel into two bypass diode protected sections. In one section of one panel you have a cell that can only deliver 4.2A under the same conditions as the other cells can deliver 7.1A.
When you measure Voc all cells contribute.
When you measure Isc the section with the low cell will be bypassed and you will think that you have a 7.1A (Isc) panel.
Now try connecting your panel to a CC, either PWM or MPPT, though the visible effect will be greater with MPPT.
Imp will turn out to be a little less than 4.2A.
Ouch!

Let me know if my guess matches your conditions.

[sensij]

You can't multiply Voc by Isc to get panel power, so that 331 vs 350 W comparison is meaningless. You need to put a load on it.

[gargouri2001]

Pretty simple, I think.

The current you measure is Isc, right, rather than a current under calculated load such as Imp?

it is open circuit no load at all

And you built your DIY panels including bypass diodes, right?

YES
i have 3 diode in each panel cell arranged as 6 x 8 and 6 x7 .


i will try this weekend to hook those panel to mppt charger and report back .

is there a fast way to put them under load without PWM or MPPT charger

[inetdog]

it is open circuit no load at all


YES
i have 3 diode in each panel cell arranged as 6 x 8 and 6 x7 .


i will try this weekend to hook those panel to mppt charger and report back .

is there a fast way to put them under load without PWM or MPPT charger

Sure. Use power resistors or light bulbs to make a "load bank" that will pull roughly the predicted Imp when you apply roughly the Vmp voltage to it.
As a rough rule of thumb for silicon cells Vmp is about 80% of Voc and Imp is about 90% of Isc.

For your panels, testing one at a time, you would want about a 4 ohm resistance.
For the two panels in series you would want about an 8 ohm resistance.

If you are able to vary the light level (aiming the panel at different angles away from the sun, for example), you can plot a voltage versus current curve using a fixed resistance.
If the bypass diodes are coming into play, you should notice a jump upward in the voltage at the point where the current drops below the Isc of the worst cell in the panel.

[Sunking]

it is open circuit no load at all

You canno tmeasure current in an OPEN CIRUIT because there is no current in an OPEN CIRCUIT. Current onluy flows in a complete or short circuit. That is why they call Isc Current Short Circuit. It is also why they call Voc Voltage Open Circuit because the circuit is OPEN.

[inetdog]

You canno tmeasure current in an OPEN CIRUIT because there is no current in an OPEN CIRCUIT. Current onluy flows in a complete or short circuit. That is why they call Isc Current Short Circuit. It is also why they call Voc Voltage Open Circuit because the circuit is OPEN.

So that is why I keep getting a Vsc of zero for all the panels I test?!?