Hello…
The Davis Vantage Pro2 requires a 3.0 volt Lithium Battery (Non Rechargeable).
The difference between Lithium and Lithium Ion is that the Lithium is NON Rechargeable,…and the Lithium Ion is rechargeable.
The solar panel on the Davis Unit ISS charges the Capacitor, not the battery…once the capacitor is out of juice…the battery is there for backup power. on most days the solar panel can charge the capacitor enough to make it through the entire night time hours, this is why the battery also last so long.
most likely, and I am guessing here…during the winter time when we get down to around 9 hours of daylight around X-mas time, is when the capacitor comes close to getting a good enough charge during the daytime to make it through the entire night.
I dont think it is a wise idea to place a 3.7 volt or any other volt… than the recommended 3.0 volt that the Davis ISS Requires.
…Chris