I had a transit FT 350 (2.4 L, 116 HP, 2011) converted to a camper and installed solar panel + solar battery. Because the camper has relatively long service life, would be practically the 2 on-board batteries (2 batteries under the driver seat) also to supply with solar charge. An acquaintance of mine meant that I had to attach a 3 way switch to the plus cable to the solar battery ( between charging module and solar battery), from which the plus cable to the on-board battery is the negative cable coming from the negative pole of the solar battery to the body. This could be changed, the solar battery and the onboard batteries charge. Is among us a professional here, who can confirm or correct this ?? And …… who knows which of the 2 batteries is responsible for starting ??