Have started to put together that bms system. These are the components ..
The bits at the top are just to simulate the battery input for testing.
The processor with Can bus interface and the screw shield on top with the resistor network for voltage division. I measure each cell to the same ground, makes it easier.
Left are the 4 relays to engage the DC chargers for each cell with a current sensor giving feedback about the additional charge going in. The other board is an opto coupler board for the arduinos outputs to 12V because I am using a 12V relay board. I could use a 5V board directly as in other boxes I make, but I have a 12V relays spare. Missing from the image is the keypad and the display. I have ordered another 2 row display. I used the one I had for the levelling unit and I also have ordered a 3.2″ touch screen from 4D systems. The LCD display and the keypad is the cheap version but I wanted to try the 4D displays and their software because I am planning to have a few more satellite displays in the van with functions useful at different locations and especially the vehicle.  I plan to remove some of the discrete switches I have in the vehicle and use small touch screens instead.

I also have now decided how to carry all the signals from the van to the vehicle. I will have two more connections beside the trailer plug and the anderson power plug. I will have a Can bus wire for all switching and monitoring signals and a single fiber optic cable for all the cameras I want to bring forward. I will use some split tube around the suzi coils I have and attach the two cables inside the split tube to the coils. This makes it clean to connect and I do not need another coil.

The video converters for fiber optics are cheap and the cable is cheap too .. US$ 80 for a pair of 8 channel converters for up to 8 cameras is cheap enough. I enquired about a 15core suzi coil for all my video and switching signals and they wanted A$300 for that .. crazy .. and a lot of wiring and weight .. so I decided on the more convenient way of the Can bus and the optical cable. In the vehicle I have a video display processor anyway for the selection of the camera input(s) onto the display.

I can display 2×4 cameras on the 10″ display through AV1 and AV2, the VGA is connected to the onboard PC since years and the PAL input is also used. It can be switched between the DVD player and the dash camera output.

I am happy with that decision although it means a couple more boxes to make with processors, but I am getting real quick at that now ..