I have 3 panels, 200W each, in series. The voltage from them (open) is 72.7v, 8.6A
When I connect to the controller I’m getting 61.4v, 8.6A
I know there’s a drop when under load, but why is the voltage dropping so much? Is there a setting I need to change?
Related question- I have 436W coming into the controller, but the battery’s BMS says there’s only about 403W going in. Another rather large discrepancy- am I expecting too much?
Hi Rick. That is a normal drop for most MPPTs. I have one at 218v and drops to 180v. There is ALWAYS going to be a discrepancy between what the controller says and the BMS as the system will use some watts to run, produce heat due to resistance. Just remember one cardinal rule…NEVER go over 100 volts VOC and include at least 10-12% buffer in the winter. Look at your panel temperature coefficient and there are tools out there to calculate that as well.
So does this mean my 200W panels will only produce 200W in open circuit testing?
So disappointing..
Losses in heat conversions efficiency and wiring. (30w)
At this voltage no amps are made. Onky useful to check you string number etc.
Awesome as long as no connections are heating unti the point where they melt.
And yes it is normal for them to drop quite a bit in some cases underload. Power is wattage which is is volt*amps
It will never produce power in open circuit.
Only in the perfect circumstances will you see 200W(so yes maybe occasionally)
In 25°C in 1000w/ms is usually the conditions on the spec sheet, something tha is rare in real life.
You are getting 88% though of installed power that is pretty decent.
Here is a plot of voltage vs power for various light levels for a panel about half the voltage of yours. Open circuit is 42V but the peak power voltage is about 34V, which is in line with your drop. Then if the light level varies, the peak power voltage does not vary much, but the current will. Hopefully this is useful to visualise the operation. The voltage has to fall below the open circuit voltage to provide the voltage difference to develop the current flow through the cells.
