I am commissioning a new boat which will have 3 x 160Ah AGM batteries for the 12V house bank. I have a 310W solar panel and propose using a SmartSolar MPPT 100/30 for this. I also have a "spare" 1200Wh 48V lithium battery and I'm wondering if I could use this as an alternative source for topping up the 12V system through the same solar regulator (having a switch to ensure only one source at a time). I would only plan to use this when sailing at night when my solar contributes nothing and I'm using a fair bit of power with fridge, instruments, lights and autopilot.
For the sake of this scenario I am not worried about getting the 48V battery recharged - that is another issue.
Any drawbacks in this arrangement?