Rather than modifying the car (which is a Band-Aid for the real problem) I would suggest finding and correcting what's really wrong
Here is a summary of the 12V battery charging and discharging two weeks ago. (over one-week period)
Charged around 8 hours during the whole week (120V level 1 charger).
I keep the main battery at around 70% charge.
Drove the car about 40 minutes the whole week combined.
Discharge: around 60 hours had the back gate or door open. The car drains 153mA with a door open. (no key in an ignition) That is 9.2Ah.
the car discharges 13mA all the other time. 108 hours resulting in additional 1.4 Ah.
I didn’t drive it at all last week, the doors were closed. That is another 2.2 Ah.
The reason I am airing the car so much, it has a smell.
The original Mitsubishi battery is probably around 35Ah, so it should be ok, but it is not a deep cycle battery,
and it degraded fast over the one year. I think, that before it died, it didn’t even have 10Ah capacity.
When I was topping it up with a charger, the charger shut off in 15 minutes.
I don’t think, that there is anything wrong with the car, other then the high parasitic drain,
and I think, that these levels are likely similar across all i-Mievs.
I don’t see the mod as a band-aid. I can leave the back gate open the whole day, and it doesn’t drain any.
If I want to work on a car, I can switch the 12V battery off easily. I will store the car in the winter, The 12V switch makes life easier.