Have you got a public link to your station?
So because I'm on my third Tempest in less than 12 months I'm a self-appointed expert on these things haha, but to be honest your battery graph looks very similar to mine as it was in the process of failing. At our place today we had way less sunshine than you did and for much less duration yet in the space of an hour or so my battery increased from 2.63v to 2.78v, whereas yours increased by 0.01v.
It's probably worth reaching out to their support team and they will be able to tell straight away if it's ok or not, but yeah that definitely looks familiar. It only took my initial email to them querying it for them to say it wasn't right and for a replacement to be sent.