Flooded cell batteries can freeze -19 F (40% charge) and -98 F (fully charged)] You might want to consider an AGM battery.
How big is the risk of frozen batteries?
My car was sitting for 1.5 weeks (due to snowy road conditions) and during the entire time, the average temperature has been around -3C (i.e. below freezing). I don't predict it will go above freezing for a few weeks, at least.
This weekend it was finally snow-scarce enough and cold enough that the roads were dry, but when I tried to give my car a start it was just click-click-click (radio and immo worked). Based on my reserach of other forum threads, this is typical for an unused battery in cold conditions.
So my plan is to jump it, using my VW Golf 2.0 TDI (diesel). I read both the Elise and Golf owner's manuals, and the Golf owner's manual indicates there may be a difference in voltage between diesel and unleaded cars, and also that a battery that is frozen can be a high safety risk as it thaws. Meanwhile, the Elise manual's tone seems to be strongly against jump-starting.
I left the car unlocked (in its locked, unheated parking garage) to make my life easier in case the battery fully dies out. In that garage, it's impossible to heat unless I wait for Spring (i.e. Mother Nature's heating).
So, among others, questions raised:
1. Can I do the jump-start with the car I have?
2. Am I going to risk life and limb in the process?
3. Is there an easier way? My garage has a power source. Could I just trickle-charge the battery up via the cigarette lighter, for example? My desire to drive the car is not urgent so I'm happy to wait.
4. It's a 2005. I have, in the past, gone 3 weeks without driving it and the battery worked great. From some threads, I've found out that typical battery life is 2 years. Should I just do a replacement?
I live in Estonia, not far from Helsinki and a few hours' drive from the border with Russia, if that gives you an idea of the climate conditions.