hello everybody, and thanks in advance for your support.
I have one W204 C250 2013. I use the car a few times in the year, because I usually another car. so, I connect the battery to my battery tender, to keep the battery charged, even when the engine don's start in the whole month.
the question is: I notice that my car has one auxiliary battery located in the trunk, used for eco start / stop system.
I want to know: does the battery tender charge the aux battery simultaneously, when charging the main battery? should I charge the aux battery separately to keep it charged?
any help will be appreciated,
thanks again,

Best thing you can do to the auxiliary battery is remove it and sell it .

When you are charging the main battery put your meter across the aux and see what the voltage is.