You place the positive and negative clips to the posts under the hood. It is directly connected to the 12 volt AGM and then over to the 48 volt battery via the voltage converter.

Toban

 

I think somewhere in the manual I read not to use what used to be the cigarette lighter if that's what you mean. I can't find that now, but the manual does say this about both battery types "When charging the battery and during starting assistance, always use the jump-start connection point in the engine compartment."

 

Yes, that is correct, you use the positive/negative posts under the engine hood. This is where you can hook up your CTEK charger. The 12 volt AGM battery is a constant draw on the 48 volt battery. On my GLE 450, the AGM will go down to 12.5 volts over a period of time and an hour later it’s back up to 12.8/9. This is why if the car sits for awhile, you should put a battery charger on to maintain the batteries.

Toban

 

Correct, as long as you use a "Maintainer," not a "Charger" or "Trickle Charger." Maybe "Smart Charger" is an appropriate term.
CTEK is only one brand. The long-term leader in the field is Deltran's Battery Tender. Despite the merchandising claims of CTEK, the features are relatively the same.