Hello,
I have designed a solar system for my RV that is focused to be as efficient as possible at low sun exposure as I live in Scandinavia and most often sun exposure is not great.
Current setup in short (for my particular usecase):
Solar array:
3x enjoysolar® PERC Monocrystalline Solarmodul 110W 12V connected in serial providing a operational voltage at full sun exposure with an ambient temperature of about 20 degrees centigrade just short of of 70-ish volts. Max voltage I seen is 75-ish volts.
Solar charge controller:
SmartSolar MPPT 100/20 (48V capable version).
Battery system:
95Ah AGM 12V Battery.
This system generates energy even at very low sun exposure and that I'm very pleased with. BUT as I designed the solar system I have done so with the plan to replace the 12V AGM battery, when it have served its purpose which is has at this point, with a either a 24V or a 48V lithium battery system and all the infrastructure around that. Naturally I like to go with a 48V battery for many reason but I believe I have come across a show stopper if l like to have as high efficiency as possible at low sun exposure in the future as well.
Monitoring the solar system today, through the Victorn Connect app, it seems like the solar charger will generate energy down to about 30-ish vols coming from the solar array. The solar array drops to low to no voltage with a quite steep curve when dropping below 30-ish volts. This gives that for a 12V and 24V battery system the solar charger will generate energy until the quick voltage drop below 30-ish volt coming of the solar array. As the solar charger requires a higher voltage coming from the solar array then the battery system it is to charge, a 48V battery system will, as it seems, not be able to gain any energy at low sun exposure and my solar array.
Question:
Am my assumption correct that a 48V battery system will not be able to gain any energy from the solar charger as its input voltage from the solar array drops below a 48V battery systems voltage, as it can sit at 58.4V fully charged, quite quick being not fully exposed by the sun? Or will the MPPT algorithm take the battery system voltage into consideration and keep the power point higher in voltage for a longer time before dropping it? I have not been able to test with any other battery system voltages to gain any further understanding on this topic. Hopefully someone here can help me understand this better. :-)
Please note that I do care for this energy generated at low sun exposure with my current system and as I see that a 24V battery system also would be gaining from my current setup. I know that for larger systems this is a no issue but for this small RV system it makes a difference. Another note is that it would be very hard to fit one more solar panel to up the array output voltage close to the solar chargers maximum voltage due to lack of roof space on my RV.
With kind regards
Toby
Sunny this morning and varying cloud in the afternoon. The mppt never made Absorb, so tracked all day, from ~105V to 90V (I think that's warming). Except for the periods at the start and finish when there's no direct sun and it drops to ~60V. Not much current happens then though, but like you, I think it all helps. And this 60V thing happens every day, all year. 
