It appears the ACC and battery wires on the Viofo HK3 hardwire kit need to come from the same voltage source in order to work properly. I installed the 100Ah battery in my car last night and ran two 12 AWG copper wires from my trunk where the battery is to the passenger cabin fuse panel which is located close to the steering column in my car. I figured I would wire the dash cam directly to the battery for now while I get everything else in place for the Victron DC/DC charger. I ran the 6AWG cable that will be going to the starter battery at the same time and left it on the passenger compartment side of the firewall for now.
I thought I was going to be able to leave the ACC cable hooked up to the fuse box and still use it as a trigger to put/take the dash cam into/out of parking mode. When I hooked the dash cam up to the positive and negative terminals of the 100Ah battery, it wouldn't turn on even when I turned my car on to give power to the ACC cable. The dash cam would only turn on when the ACC cable was getting power from the 100Ah battery. It seems I'll have to get a relay tied into an ACC circuit on my car to switch power from the 100Ah battery to the ACC cable on the hardwire kit when the car turns on and to cut the power when the car turns off. I wired the ACC and BATT cable of the HWK together and connected them to the battery for now. This means dash cam will just stay on constantly and not go into parking mode.
Here's some photos. I bought a steel battery tray to serve as the base. I wanted the battery to go into a container of sorts, so I bought a group 31 battery box. If I could have found a decent metal box, then I would have just bought a metal box for it. The battery box is a bit too long for the tray, so the rubber helps raise the box above the lip of the tray. I'm using straps right now to hold the tray in place.
I'll probably replace the straps by screwing the tray onto a piece of aluminum sheet. I'll then find some type of hardware to attach the sheet metal to the same metal rings the straps are attached to. Also, I have a piece of rubber protecting the straps from the tray, but I don't anticipate keeping the strap setup for too long. I'll probably end up doing away with the rubber strips as soon as I can find some suitable aluminum blocks to attach to the tray.
I have a couple of pieces of rubber sitting between the straps and the battery box.
I drilled two holes in the bottom of the battery box for the threaded rods of the tray. The angled part of the rods poke through slots on the side of the battery tray. The rubber strips help align the battery box with these slots as well. The tray came with some brackets that fit over the slotted rods, but they were too tall and I couldn't put the cover of the battery box on with them installed. I cut and drilled a piece of aluminum to size as a replacement to the brackets which hold the battery in place. The rods, piece of aluminum, and battery help to keep the battery box attached to the metal tray.. There's a washer and wingnut on each threaded rod. The threaded rods are just a tad bit too high for the battery box lid to close completely, so I'll need to put some additional material under the battery box to lift it a little bit higher.
Here's the temporary install of the 12AWG cables that I ran to my dash cam. I fused each side of the wire since I wasn't using the vehicle chassis for ground. I'll move the 12 AWG cables to a distribution block once I install the DC/DC converter. I'm thinking about replacing the current piece of aluminum with a wider piece that will give me a convenient place to mount the distribution blocks and the 40 amp MEGA fuse for the LiFePo4 battery. I plan on attaching two pieces of aluminum to the bottom of the batter tray that will extend out towards the open end of the trunk. From there, I'll attach some angle brackets that I will use to mount the DC/DC charger.
Here's the box with the rest of the stuff in my trunk. Eventually I might take the cart and emergency roadside kit out of my car. This would allow me to center the battery box in the back of my trunk right behind the seats. The stuff in the emergency road side kit I could put either in the compartment below the trunk where the spare tire is or in the storage bin.
I'll post some more pictures when I have the DC/DC charger mounted along with the distribution blocks.