I have to leave my 2016 GL63 for four months and have no ability to plug in a maintainer. I do not believe there is a standby mode in the 2016, so I'm considering simply disconnecting the battery and reconnecting it when I return.

I also considered keeping a second battery in the rear area jumped to the main battery for double the AH - but I doubt even that's enough to last the four months and would rather not ruin the battery.

Any advice? Ok to disconnect for 4 months? Should I disconnect the aux as well? Will I face major hassles when I put it back in service?

Many thanks!

Unplugging the battery might reset certain system functions and needs to be calibrated and/or relearn. For example there might be a chance the windows need to be relearned to close properly (open the windows and hold it for seconds, then close it and hold down for seconds), there might also be an SRS aigbag light. The time needs to be set again, the transmission shifting behaviour might be different for the first few miles, etc.

Just curious -- in what kind of storage location are you parking it where there is no electrical power?

2 batteries and get someone to start it and let both charge.. maybe 3 times??

I have a three-car garage which opens into a large common parking area. The three-car garage will be filled with my wife's ML, and my GTS and SL63 - all of which I will have on maintainers. However, the low man on the totem pole (the GL63) will be out in the common parking garage which does not allow us to plug anything in to the outlets. So my only way to plug it in would be to rent out a spot somewhere that has power (which would be more hassle, I think, than the other ideas discussed here.

Sadly I won't have anyone around to start the car. Plus, I imagine to put a significant charge on the two-battery setup would require the car to be running for some hours - which would be frowned upon in the parking garage.

Good thought though - thank you!

Have you considered a solar charger? You should be able to find one that outputs enough current during daylight hours to keep your battery maintained. There are many from which to choose.

I'd thought of that, but don't have a good outdoor spot to park the SUV. Since it is in an underground garage, I am afraid a solar option would not be viable.

Does artificial lighting generate enough electricity to charge a 12V battery?

Four months of sitting should be fine if the garage is not freezing and the battery should be fine. Even if the main battery gets a bit weak, jump the car to start it and all will be fine. Because disconnecting the main battery for four months will kill the backup battery and reset all sorts of car functions that will need to be reprogrammed.

Are you talking about that aux battery? if so, not all MBs have that.

I'm far from an expert but I thought all X166s have the aux/backup battery.

Maybe check with your dealer's tech specialist on this. If there is absolutely no way to attach a maintainer, then I'd opt for the total battery disconnection -- main and aux (if there is one). If left connected, the battery management system should prevent the batteries from discharge to the point of irreversible damage, but this still leaves you with badly depleted batteries that you'll have to jump start and recharge.
Do you have any friends with space in their driveway and an extension cord?

Sorry for the confusion, I meant not all MBs have that, I admit I am not too familiar with the GL X166, since you are and you know all X166 GL has it, then it probably has it, thanks for your reply.