I think you are showing your lack of knowledge. The "nut" that you are talking about is just a post connected to the chassis of the car. It is made redundant by the the fact that there was an engine mount within reach which was , like the nut you talked about, connected to the chassis. All you need is a clean connection tot he chassis for the -ve and a connection to the +ve ( which is provided under the red plastic flap). In my car, a w222 s350 , i dont have the engine mount within reach and have to use the "nut"

Agree that user's manual suggested connecting to the negative terminal by removing the panel as marked, but this suggested negative terminal is connect to the car's chassis or ground anyway. There is no difference or problem by connecting to the car's chassis directly such as the engine mount which is also a true ground, especially it is more accessible. Don't object to using the owner manual's suggestion of course. Thanks.

Agree. The manufacture has to identify a negative terminal and label it for car owners to access. I am not opposing to using the manufacture's suggestion terminal for ground. As discussed above, the entire car's chassis is grounded and connected to the negative terminal of the battery. Technically and electrically, there is no difference between my suggested engine mount and the manufacture suggested negative ground. The engine mount location that I suggested is exposed, without having to remove any panels, and might be a little easier to access.

You could connect the charging cable directly to the primary battery, but you will have to remove the panel in the trunk to get to the battery.

the aux battery on the s550 w222 is not under the hood its in the right side panel of the trunk.

I got mine from either Amazon or Walmart, not sure. Just make sure the dimensions and power spec are consistent. Don't really need to buy it from Mercedes dealer. I had this Aux battery for 2.5 years now, and no problem so far.