I've got a Bluetti AC300 that allows a maximum solar input of 150V, and I live in a place that experiences severe cold for a few weeks each winter. Here's what I've worked out: My panels are rated at 25°C (77°F) with an open-circuit voltage (Voc) of 45.5V and a maximum voltage (Vmax) of 37V. The short-circuit current (Isc) is 8.85A, and the maximum current (Imax) is 8.38A. The temperature coefficient is at -0.32.
I'm wiring these panels in a 3S2P configuration. I've done the math to consider the temperature coefficient, and I believe I'm fine as long as temperatures stay above 25°F (-4°C). But I'm concerned about what happens if the temps drop below that. At 0°F (-18°C), using Voc, I calculate a risky 155V. However, if I go by the Vmax, I'd be at 125V. This situation might occur for a week or two every other year.
So, I have two questions:
1. Am I correct in using Voc in my calculations here? Does Vmax factor into this? 2. Should I set an alarm to disconnect one panel from each series if the temperature goes below 25°F (-4°C), or is that overkill?
Oh, and I realized I've changed my setup from 3S2P to 2S3P for the same panels, same wattage, but lower voltage. Just wanted to get that out there!
3 Answers
Totally agree with the risky voltages! The 155V could definitely be too much for your inverter/controller. It's important to stay below that Voc number as a rule of thumb.
If you know extreme cold is coming up, consider just not using your system until it warms up. Switching to lower voltage panels could save you from a headache later on.
For your first question, yes, use the Voc for safety and planning. It represents the worst-case scenario when temperatures drop. If you exceed the 150V limit due to cold, you might indeed face damage to your Bluetti.
As for your setup, moving to lower voltage panels would definitely help. There are good options out there in the 400-450W range that fit well without risking overloads.
Regarding your alarm, it's a good idea, especially if you expect extreme cold. Monitoring could prevent damage if things start getting risky!
Great points from everyone! Just to add, if you play around with your panel configuration, watch out for the amp limits too. Your MPPT's 12A cap might restrict your overall output as well—as you pointed out with your calculations. Keep tweaking, and you'll nail down the best setup for your needs!