We are happy pensionnars (that's why a Jazz?) and we stay in winter a couple of months in the sun in Spain. The battery of our previous car (Toyota corolla verso) had no problem with that. But I was afraid that with a Jazz hybrid and a small 12v battery it could be a different story. So I was very curious how it would go. Here is a report for anyone interested.
The Jazz is parked in a garage, where unfortunately there is no electricity. So no possibility for a trickel charger.
On departure, the battery condition was measured with a multi-meter: 12.8V. (and the traction battery had 5 blocks). Furthermore, no special measures have been taken, so the keyless system has remained active (I assume).
When we returned after two and a half months, I checked the battery without starting the system (I had left the engine compartment unlocked). The meter read 11.9V. I have not tried to start the system on the battery, but I first connected a starting aid (this starting aid:
https://www.lidl.nl/p/ultimate-speed-mobiele-autostarthulp-met-powerbank/p100337742 ) and then started the car. That went fine. After releasing the starting aid I made a trip of more than an hour.
The next day I measured the voltage of the battery again and it was 12.6V.
My conclusion is that the small battery can hold a lot more than I expected and that it can last for a while. But after a year or two I'll replace the battery with an EFB one (assuming it's available for the Jazz). They are more resistant to deep discharge I understand, so that, even after a very cold winter and deep discharge, I don't have to worry about the condition of the battery after a very long standstill.