We need a way to recharge the battery from “shore power.” That means mains outlets attached to the house. The easiest way is to run an extension cord through an open door, but that’s not very elegant, secure, or warm.
Some people add a full-on 30 amp or 50 amp circuit with a special plug. That’s the same plug as RV parks use for hook-ups. If you install the special outlet on the side of your house, you can use it to charge the battery faster or run more equipment than if you use a regular 15 amp outlet.
We didn’t want to do that, though. RV parks are not going to be part of our itinerary. Friends’ driveways might be though. They will most likely never have the special 30 amp outlet, but they’ll undoubtedly have a 15 amp one. Charging from 15 amps is slower than charging from 30 amps, but we’ll most likely have the time. For us, the common availability of the 15 amp outlets was more important than the faster charging time.
So, we just needed a way to run a suitable cable from outside the van to inside the van without compromising on security.
The answer: drill a hole in the floor between the inside and outside walls of the van near the back door.
Then, run a 12 AWG extension cable from under the van, through the rocker panel, through the hole (first threading on the exterior parts of a waterproof grommet), and then into the switch panel where it gets its own special circuit breaker to be code-compliant.
Mercedes makes some nice access holes in the rocker panel, so it was pretty easy to find a suitable location. I put a hinged cover on the hole. We used this one from Amazon and cut the enclosed back away from it. The cover has a little sliding flap at the bottom so it can either be completely closed up, or it can be closed except for a small hole that lets the cable through. The cable can be pulled out far enough to just avoid scraping on the ground. Then, it’s easy to add a second extension cable between this and a regular wall outlet.
The inverter doubles as a charger. It can turn 12V DC from the battery into 120V AC, and it can also go the other way – turning 120V AC from the wall outlet into 12V DC for the battery to drink up. It’s even smarter than that, though. If we need to use power while the van’s plugged in, it’ll use the shore power first, but if that’s not enough it’ll add extra from the battery. It knows never to try and take more from the shore power than it should, so (in theory) it should never trip breakers inside the house.