Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
- Messages
- 14,271
I suppose I could buy a solar panel tester like Will reviewed, but that doesn't feel DIY enough to me.
Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way.
I'm presently working on an underperforming string of Sharp 165W Polysilicon. Underperforming like this 12s string is putting out about 68% the current of an identical paralleled 12s string.
I don't find Voc varying much.
Isc, lowest panel is only putting out 66% of highest.
When measuring Isc, you want to avoid arcing and burning contacts. You can use a suitable switch (visible blade knife switch would be good), or cover the panel or rotate it away from the sun.
In my case, Isc only flagged 2 panels as underperforming. It missed two other underperformers, and one marginal.
Measuring Imp requires a circuit like MPPT controller which finds optimum point on I/V curve. What I did was calculate what resistance would put the panel at this point. Could have used power rating of panel (PTC is about 85% of STC), but since I had observed Vmp on GT inverter while operating in a string and measured Imp of the good string, I used those values instead. I wanted 7 ohms, and a couple space heaters in parallel gave 6.5 ohms so I used that.
Average I(load) of 12 panels was 3.97A and lowest was 2.72A
Power (with 6.5 ohm Rload), one was 80% of average. Four were 70% or less. Seven were 94% and higher.
Recomputing with just the "good" panels, average is 121W (measured between Noon and 2:00 PM, some haze and smoke in California)
Good panel wattage ranged from 93% to 108% of average.
One poor panel was 81% of average, bad were 38%, 41%, 58%, 71% of average power
(This doesn't mean their maximum power was that low, just power when loaded with 6.5 ohms. )
Important note: Isc was not low for the three panels with measured load (into 6.5 ohms) that were 58%, 71%, 81% of average good panels. It was measurement of I(load) and V(load) which detected them.
I have spares I will measure (got late in the day and haze caused them to read low), and swap those in. These are used panels with connectors cut off, so I have to install new MC3.
Then, I will look into warranty. I bought these 18 panels new in about 2004. Sharp's performance warranty is 25 years, 10 years 90% output and 15 more years 80% output. It says ship to authorized dealer (locator doesn't find one within maximum radius). It doesn't say what they would do for me.
For anyone putting up a "new" array of used panels, once you have made a list of V(load) & I(load) (or Isc) for your 13 panels, you can select which four to put in each string. Current from each string will be approximately your lowest measured I(load) of panels in that string. Don't worry so much about Voc or Vmp (Vload) variations; the 4s strings should be close enough for that not to matter. If there is much current variation among panels, matching within strings should increase array power.
Details to follow ...
Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way.
I'm presently working on an underperforming string of Sharp 165W Polysilicon. Underperforming like this 12s string is putting out about 68% the current of an identical paralleled 12s string.
I don't find Voc varying much.
Isc, lowest panel is only putting out 66% of highest.
When measuring Isc, you want to avoid arcing and burning contacts. You can use a suitable switch (visible blade knife switch would be good), or cover the panel or rotate it away from the sun.
In my case, Isc only flagged 2 panels as underperforming. It missed two other underperformers, and one marginal.
Measuring Imp requires a circuit like MPPT controller which finds optimum point on I/V curve. What I did was calculate what resistance would put the panel at this point. Could have used power rating of panel (PTC is about 85% of STC), but since I had observed Vmp on GT inverter while operating in a string and measured Imp of the good string, I used those values instead. I wanted 7 ohms, and a couple space heaters in parallel gave 6.5 ohms so I used that.
Average I(load) of 12 panels was 3.97A and lowest was 2.72A
Power (with 6.5 ohm Rload), one was 80% of average. Four were 70% or less. Seven were 94% and higher.
Recomputing with just the "good" panels, average is 121W (measured between Noon and 2:00 PM, some haze and smoke in California)
Good panel wattage ranged from 93% to 108% of average.
One poor panel was 81% of average, bad were 38%, 41%, 58%, 71% of average power
(This doesn't mean their maximum power was that low, just power when loaded with 6.5 ohms. )
Important note: Isc was not low for the three panels with measured load (into 6.5 ohms) that were 58%, 71%, 81% of average good panels. It was measurement of I(load) and V(load) which detected them.
I have spares I will measure (got late in the day and haze caused them to read low), and swap those in. These are used panels with connectors cut off, so I have to install new MC3.
Then, I will look into warranty. I bought these 18 panels new in about 2004. Sharp's performance warranty is 25 years, 10 years 90% output and 15 more years 80% output. It says ship to authorized dealer (locator doesn't find one within maximum radius). It doesn't say what they would do for me.
For anyone putting up a "new" array of used panels, once you have made a list of V(load) & I(load) (or Isc) for your 13 panels, you can select which four to put in each string. Current from each string will be approximately your lowest measured I(load) of panels in that string. Don't worry so much about Voc or Vmp (Vload) variations; the 4s strings should be close enough for that not to matter. If there is much current variation among panels, matching within strings should increase array power.
Details to follow ...
