I have two Sunpower 50-watt panels connected in series, creating a 100-watt setup. I mounted them to a DIY frame that I can place on my small Teardrop trailer and remove and place in the sun if needed. The battery is a 240 ah lithium. When I first put this to use in 2022, I got a solid 75 watts plus on sunny days when positioning the panels at optimal angles toward the sun and, on numerous occasions, got it up to over 90 watts. Once the battery reached full, the system would kick into the Absorb state and radically reduce the wattage going into the battery, which I believe is normal behavior. This summer, I’ve not been able to get more than 40 watts from the panels, no matter how well I’ve placed them. Then, this last weekend, I had my battery fully charged before driving to a 5-hour destination. While driving on a full sunny day, mid-way there, I checked the app and noticed the battery voltage was 14.5 volts and still the Bulk state. The wattage coming in was around 30 watts. I’m not sure what this means, but I think it means it’s not operating correctly. Note that the system seems to top off the battery enough each day, so reserved power storage isn’t an issue. I run a Dometic CFX-55 12v fridge, charge our phones, and use normal LED illumination in the evening hours. I have Firmware vs 1.64. Thoughts? Thanks, Devin
If the battery voltage is 14.5, then it looks fully charged.
The system can revert to bulk state if the charge is interrupted or the battery voltage drops briefly.
Thing to do is use Victron Connect to check the chargers voltage parameters.
Hi Mike. Thank you for your follow-up so many months ago. I’m sorry I never responded. Especially since I’m the one in need of help. I hope you’re still willing to share your knowledge.
Now that summer is here and I’ve had the camper out, I’m interested in returning to this issue. I’ve attached some screenshots, hoping it may mean something more to you. Again, what I’m wanting to understand is why I never see the solar wattage exceed 35 watts, give or take? Is it because the battery is always charged just enough that the controller limits the wattage intake? If you recall, I have 240 amp hours in a small teardrop trailer. We have a 55-liter 12v fridge, which pulls power around the clock. Other than that, we don’t use any other power until after the sun sets, when we use the LED lights, which we really only use for crawling into the cabin to prepare for bed or illuminating the galley area at the back of the camper for short periods of time. On warmer evenings, we run our 12v fan throughout the night and my spouse and I plug in our phones for an overnight charge. We have an inverter that never gets used because we don’t camp with 110-volt devices. I’d love to hear more of your thoughts and if I need to be concerned. Thank You! 
Let us know your solar panel specs, Voc, Vmp, Isc and Imp. That is open circuit voltage, max power voltage, max power current and max power current.
Then confirm that they are in series.
Also, please can you post a pic of the MPPT settings page?
The above shows that the PV voltage is 24.88: this is dependent on the panel type, so we need the Voc, Vmp, and Isc and Imp data from the module label.
Battery voltage is 13.3V, and state is in bulk, so the only thing looking like stopping the charge is either under performing panels, or bad panel wiring.
If you have a good multimeter, then with the panels in full sun, disconnect the wires from the MPPT, and measure the Voc from the modules. Then if possible, measure the Isc of the modules to see if these values agree with the module data. Both should initially be measures as is - i.e. in series. From this i’m tending to suspect either damaged modules or a wiring fault.
I took a multimeter to the panels. Each panel measures out a bit over 17 volts when individually tested. When combined in series, it measured around 34-35 volts, give or take a point or two. I measured at the end of the 50-foot 10-gauge cable and it tested around 33-34 volts. I then tested it at the input of the controller, and it was 22-24 volts. I do have a 20 amp fuse in place, so I tested it before the fuse and it popped back up to 35-ish volts. So I removed the fuse, and I’m now testing in the 34-35 volt range. However, as soon as I connected the battery back to the system, the voltage bounces between 16-17 volts to over 30 volts and back down again. Finally, today is overcast and so I may need to wait until a clear day for further testing.
I think these are what you were asking for. Please let me know if there’s somewhere else in the app with more info I can provide. Thank You!
This is caused by the variable load of the MPPT testing.
However, you do need to disconnect the PV cables from the MPPT, and measure the short circuit current from the panels, with full sun. This will show up any high resistance connections in the pv cables. The 17 volts sounds low for a 12V panel - these are usually ~20V open circuit, and 17.5V at Max power point. Do check against the data on the back of the panel. If these are very old, then they may be worth replacing.
The battery voltage settings are fine - though the equalisation voltage may be a bit high - see your battery data for the right voltage.
I finally got a full sunny day since we last spoke. I measured 2.2 amps at the panel connectors and 2.0 at the end of the 60-foot connection.
The panels are 2 years old. They are flexible and they catch a lot of wind resistance when traveling at freeway speeds. Not sure if that means anything. But it does seem they’re underperforming since I first started using them. They weren’t that expensive, so not a big deal to replace if needed.
Thoughts?
Your panels certainly seem to be down on performance. The long cables are dropping the performance but not by much so you do not have high resistance or poor connections.
These Sunpower panels are high quality, I have 4 x 170W on my river/ canal boat, 5 years old and still see >650W peak, but these only travel at 4mph. If yours see a lot of vibration when travelling they may well develop some fatigue cracking.
I think you’re right about the fatigue cracking. It’s the only thing that seems to make sense. I just ordered a pair of 50-watt rigid panels to replace this setup. Hopefully, they’ll withstand the wind resistance when traveling at 70 mph.
I finally had time to test my new panels from Callsun. I purchased two 50-watt panels, wired them in series, like I did with my original setup. It seems I’ve been dealing with faulty panels and didn’t want to accept that was the case. These new panels are doing great, and for now, I’m happy as a clam! Thanks for all your help. You too, Mike.
