I have a medium sized residential refrigerator in my motor home that I keep cool on trips by running the generator a bit every few hours. We are planning a long trip and I have installed a 1500 watt p...
Battery banks should be made of the same type of batteries and preferably installed at the same time. If you have one that does hold up the same as the rest, it will constantly draw power from the others.
I suspect it's the inverter and/or wiring that is your issue. If the fridge takes 1500w to start, that's only 125amps. A pair of 6v batteries wired for 12v should handle that easy. The bigger issue is how long they will handle it.