My system has been up for 9 days. The voltage reached a high of 2.57 after three days of operation. It is in partial shade from large trees for part of the day. We have had clouds for the last 4 days and my voltage has dropped to between 2.41 - 2.38 for the last two days. The highest solar radiation I’ve received during that period is 172W/m2. I asked CS about this (they responded very quickly) and said I would need to receive 350W/m2 to charge. However, when I have received brief periods of sun over the last two days (172W/m2 being the max received) the system does seem to be either charging or treading water. It would seem the charging begin when any level of direct sunlight is received. Any thoughts on this?
Does it matter what the number is ? Really ?
There are several topics discussing battery voltages and charging and using artificial light to charge the Tempest. Search this forum using the magnifying glass for more answers.
This post by David explains what Weatherflow have worked out:
https://community.tempest.earth/t/power-booster-for-tempest-owners/15712/18
cheers Ian 
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When the solar panels start to charge, your battery will not immediately jump to higher voltage. Depending on how ‘empty’ they are it might take several hours to increase like 0.2V. The closer you get to 2.6V the quicker it will go higher as they are near plenty and every little extra does push voltage really quickly up.
2.38V is already low and it will need at least a full day of good sun to charge up. This period of the year, count several days of sun before it reaches 2.6V and higher.