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If it was me I would discharge the traction battery to about 50% and connect an intelligent 12v battery charger/conditioner that has an automatic maintenance/trickle mode to the 12v battery while it is still connected to the car, and leave it connected.Owners manuals always say don't connect a charger to the battery while it's in the car but that's just to cover themselves in the event that the charger was faulty and subjected the electronics in the car to excessively high voltage (above about 15 volts) however the chances of that happening with a modern intelligent computer controlled charger are basically zero, if anything they are smarter than the relatively "dumb" charger in the car itself.If you leave the 12v battery disconnected for a few months the BMU will (apparently) lose all record of the state of health of the traction battery - I would definitely try to avoid this as it could take a long time for it to re-learn the SoH of the battery without a diagnostic session to perform a battery capacity measurement. If the SoH figure for the battery is drastically in error that means you will either run out of charge very suddenly when you think you have a lot of range left, or not be able to drive as far as you should, depending on which way the error is.As long as it remains fully topped up, a 12v lead acid battery will not freeze until -70C.One possible issue of leaving the car outside in -25C conditions for a long time is that the electronics of the 12v charger itself may have difficulties operating in those extremely low temperatures unless the idle power it consumes is enough keep itself warm. You might actually want to wrap up the charger and put it inside the car or under the bonnet - don't leave it exposed to the elements even if it's water proof...
If it was me I would discharge the traction battery to about 50% and connect an intelligent 12v battery charger/conditioner that has an automatic maintenance/trickle mode to the 12v battery while it is still connected to the car, and leave it connected.
Owners manuals always say don't connect a charger to the battery while it's in the car but that's just to cover themselves in the event that the charger was faulty and subjected the electronics in the car to excessively high voltage (above about 15 volts) however the chances of that happening with a modern intelligent computer controlled charger are basically zero, if anything they are smarter than the relatively "dumb" charger in the car itself.
If you leave the 12v battery disconnected for a few months the BMU will (apparently) lose all record of the state of health of the traction battery - I would definitely try to avoid this as it could take a long time for it to re-learn the SoH of the battery without a diagnostic session to perform a battery capacity measurement. If the SoH figure for the battery is drastically in error that means you will either run out of charge very suddenly when you think you have a lot of range left, or not be able to drive as far as you should, depending on which way the error is.
As long as it remains fully topped up, a 12v lead acid battery will not freeze until -70C.
One possible issue of leaving the car outside in -25C conditions for a long time is that the electronics of the 12v charger itself may have difficulties operating in those extremely low temperatures unless the idle power it consumes is enough keep itself warm. You might actually want to wrap up the charger and put it inside the car or under the bonnet - don't leave it exposed to the elements even if it's water proof...
