The lithium battery in the ISS isn't chargeable and doesn't charge. You should check the voltage of the lithium battery and replace it if necessary to restore evening data.
If the lithium battery wasn't nearly expired when you put it in (very unlikely, but still a slight possibility), then your solar panel connector may not be making a good connection at the connector OR more likely, your super capacitor has a problem.
I'm not sure if there is any easier way to diagnose a super cap problem other than removing the white cover and physically examining it for corrosion or leakage. If it is leaking, you can replace it yourself if you have the skills, you can call Davis for a repair, or just use lithium batteries like crazy. Normally a bad super cap will drain the battery and it happens over a shorter period than an entire winter and spring . People have cut one of the leads to it in that case to gain longer battery life