Thanks for the input.
I am trying another script, in Lua this time, based on SunsetF16.
Otherwise I am in Europe for supplies.
What I found so far (did not order yet) is 3.3v 40Ah Lithium battery for 60 Euros. It is quite smaller than an RV battery. My point was to be on battery only when the Mains power is down (night and week-ends). Then when mains power comes back then battery recharges and power supply to camera is switched to mains, all this without disturbing the camera. So it can last for 45 days or crash before. 45 days would be 64800 images if shot every mn.
I can imagine the camera will crash before that, easily.