Leaving for a month

Yes, pull the 12v and bring it inside and soon as you get home after a month, start charging it... Then by the time you're ready to use the car it'll have a nice strong charge on it.

 

just unhook the negative. you will need a 12v source to pop the hatch when you get back, unless you don't mind climbing in.
make sure your mechanical key works.

 

If you park outside, you could try a solar charger. This one will also condition the 12v battery.
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Battery Tender 12V, 270mA, 5W Solar Battery Charger

Did you turn off the Smart Key (button under the steering wheel)? That draws a lot of power.

 

Deactivate the SKS using the button below the steering wheel. That alone should allow a good battery to last over a month.

 

With a car over 10 year old, I would be more concerned about HV battery than the 12 V one. After a gap of one month, that needs to have enough power to crank the engine.

Please make sure it is fully charged when you leave it. You can force charge the battery by using left foot on Brake and then using right foot on gas and putting the car in Drive to force charge the battery.

 

Solar chargers hooked straight to the 12v battery or jump points. Very low rate so no fear of overcharging.

 

With a solar panel inside the car, don't use the 12v sockets. They are not on when the car is not on.

The OBDII port is always live, so that is an easy to access point for temporary use.

 

Good tip! I will be doing that before going on vacation. That way the battery is fully charged when leaving it unattended for three weeks.

 

What mostly drains the battery is being connected to the car: the slow but inexorable discharge imposed by the car's background needs. Any of the above mentioned battery disconnection methods would be effective; either @bisco's disconnect the neg cable, or @PriusCamper 's bring it up to your apartment.

I'd suggest to get yourself a decent smart charger too. Our car's often idle for multiple days, then we maybe do just a short shopping trip. I use a CTEK 4.3 with a quick-connect, pretty much 'round the clock: keeps the battery like new.

 

With normale ICE cars I never had a problem with the 12V battery with the car standing 3 or 4 weeks. The 12V batteries in our hybrids are of course quite a bit smaller. Mine has a 45Ah battery whilst our ICE car has a 75Ah battery.