Quick background: Bought an off-grid cabin in Dec 2020 in the Pacific NW with an existing DIY solar power system. It works fine for our minimal needs as a weekend/vacation place (our well pump runs, our phones stay charged, our little refrigerator stays cold). Since the sun has come back, I've started exploring it with my multimeters mostly to educate myself on how it works, but also so that eventually I can plan out some upgrades so we can spend more consecutive days out here (especially once the sun leaves again for the winter). While working on the array, I ran into a situation I didn't understand and am curious what y'all think about it.
The array has a combined nameplate rating of about 2000W, but it's composed of a variety of modules of different ratings. They are rather mismatched, and I've never gotten more than about 800 to 900 watts out of it. Here are the nameplate specs:
These were all wired in parallel EXCEPT 6+5 are in series and 4+3 are in series, I assume to get their combined voltage up enough to be useful. These all fed into a Midnite Classic 150 which is charging a 24V 400AH lead acid battery bank.
One thing I observed is that when the batteries are full and the Midnite is in float, the array voltage rises to the 33-35V range. At this point, the Midnite is only sending about 1A through the batteries, but my DC clamp meter shows that the higher voltage panels are still producing several amps, most of which is flowing backwards through the tyns panels (because the array voltage is higher than their open circuit voltage?). I imagine this is not good for them, but I wasn't sure how serious a problem this is.
While exploring that, I completely disconnected all the panels to measure their Voc and Isc, on a moderately sunny day where temps were in the mid 50s. The tyns panels all measured about 6-7A Isc, and Voc was around 31V (as opposed to the 33V nameplate). Is this within the normal variation from the lab conditions on the nameplate, or indicative that the tyns panels are damaged?
Finally, as a second experiment on the tyns panels, I tried connecting four of them into a 2x2 array, and connected this to the Midnite (all other panels were still disconnected). At the time I did this, my batteries had not yet completed bulk charging, so the Midnite began charging in MPPT mode. It saw that the open circuit voltage of this 2x2 array was 62V as expected, but still found that the maximum power point was about 26V -- I think this is the lowest it could go while charging the batteries -- yielding about 350W. When I measured the voltage across each individual panel, one in each string had dropped to 2-3V, with the other panel in the string carrying most of that 26V. I didn't understand this at all, but it didn't seem good either. Are there other ways I can check these out to understand what they're doing?
Since then, I've split the 5 tyns panels onto a Trace C40 charge controller that the previous owners left, to keep them from dissipating power from the other panels (which are still on the Midnite). They seem to be able to help out with bulk charging this way, but gradually drop off as the battery voltage rises.
The array has a combined nameplate rating of about 2000W, but it's composed of a variety of modules of different ratings. They are rather mismatched, and I've never gotten more than about 800 to 900 watts out of it. Here are the nameplate specs:
Panel # | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
Brand | Sharp | ?? | ?? | ?? | ?? | ?? | BP Solar | Evergreen Solar | Evergreen Solar | BP Solar | MX Solar USA | MX Solar USA |
Model | NT180U1B | tyns-195S6 | tyns-195S6 | tyns-195S6 | tyns-195S6 | tyns-195S6 | BP590 | EC-102-GL | EC-102-GL | BP585 | MX 60 | MX 60 |
Pmax | 180 | 195 | 195 | 195 | 196 | 195 | 90 | 102 | 102 | 85 | 235 | 235 |
Vmp | 35.86 | 25.05 | 25.14 | 25.08 | 24.97 | 25.19 | 18.5 | 16.7 | 16.7 | 18 | 29.3 | 29.3 |
Imp | 5.02 | 7.81 | 7.76 | 7.78 | 7.86 | 7.77 | 4.86 | 6.11 | 6.11 | 4.72 | 7.84 | 7.84 |
Voc | 44.08 | 33.14 | 33.25 | 33.18 | 33.09 | 33.29 | 22.3 | 21.3 | 21.3 | 22.03 | 36.8 | 36.8 |
Isc | 5.6 | 8.63 | 8.6 | 8.57 | 8.62 | 8.62 | 5.2 | 7.32 | 7.32 | 5 | 8.36 | 8.36 |
One thing I observed is that when the batteries are full and the Midnite is in float, the array voltage rises to the 33-35V range. At this point, the Midnite is only sending about 1A through the batteries, but my DC clamp meter shows that the higher voltage panels are still producing several amps, most of which is flowing backwards through the tyns panels (because the array voltage is higher than their open circuit voltage?). I imagine this is not good for them, but I wasn't sure how serious a problem this is.
While exploring that, I completely disconnected all the panels to measure their Voc and Isc, on a moderately sunny day where temps were in the mid 50s. The tyns panels all measured about 6-7A Isc, and Voc was around 31V (as opposed to the 33V nameplate). Is this within the normal variation from the lab conditions on the nameplate, or indicative that the tyns panels are damaged?
Finally, as a second experiment on the tyns panels, I tried connecting four of them into a 2x2 array, and connected this to the Midnite (all other panels were still disconnected). At the time I did this, my batteries had not yet completed bulk charging, so the Midnite began charging in MPPT mode. It saw that the open circuit voltage of this 2x2 array was 62V as expected, but still found that the maximum power point was about 26V -- I think this is the lowest it could go while charging the batteries -- yielding about 350W. When I measured the voltage across each individual panel, one in each string had dropped to 2-3V, with the other panel in the string carrying most of that 26V. I didn't understand this at all, but it didn't seem good either. Are there other ways I can check these out to understand what they're doing?
Since then, I've split the 5 tyns panels onto a Trace C40 charge controller that the previous owners left, to keep them from dissipating power from the other panels (which are still on the Midnite). They seem to be able to help out with bulk charging this way, but gradually drop off as the battery voltage rises.
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