On my old Fiat Panda I just used to disconnect the battery when leaving it for a long time and reconnect when I returned. The battery seemed to be able to retain a large enough charge to start the car even after over a year. Would that be possible on the 12 volt battery of the Jazz to prevent drain from the sleeping electronics?
Disconnecting is not a good idea. If the 12v battery is discharging in less than 3 months there may be some DIY solutions. Regular topping using a mains smart charger. Or if the car is outdoors connecting a solar panel charger may give a trickle charge that exceeds ,or delays its natural parasitic electronics discharge.
If you regularly leave the car unused for 1-2 months and have no mains power for a smart charger it might be possible to temporarily connect a remotely located 12v battery to the cars 12v battery wired in parallel. This would give 12v but a greater reserve capacity. I am not an expert so there may be objections to this.
I carry a portable battery pack jump starter in the car The battery is only about the size of a paperback book. Its said to be able to start a conventional car with starter motor so my hope is in an emergency it will easily power up the 12v Jazz electronics long enough for the HV battery to start the car. But I have never yet needed to use it so cant guarantee it will work.
Bear in mind that most new cars, including conventional ICE engine ones with large 12v batteries ,have electronics running 24/7 ,and can flatten their batteries in a matter of weeks or days if they dont get regular long journeys.
The biggest obstacle to not using the car for long periods is not the 12v battery but the High Voltage battery. Unless the car is driven for at least 30 minutes every 3 months, to put the HV battery through some discharge/recharge cycles, the HV battery can be permanently damaged. And very expensive to replace. A 12 Volt battery can also be permanently damaged if allowed to fully discharge but relatively cheap and easy to replace.
A HV battery charger, costing £1000,s ,only viable for professional garages ,might be able to deal with cars in Long term shortage, but this is beyond the ability of domestic smart chargers etc.
