Does this all depend upon the specific use profile?
For a few amps, no or rather minimal problems. For the situation where the trailer bank is low, those batteries will pull power - possibly more power than the fusing on the umbilical will support.
Then, there is the charging profile, the battery state monitoring, and the lack of regulation associated with the proposed method.
In an emergency and with managed control, maybe. However, as a standard approach to normal use, this seems like a simple solution at first glance that gets more complicated as you consider the possibilities.
Backup battery bank - suggest you add a disconnect for the primary and also add a high amp plug connector. Then unplug the main when at minimum capacity and plug in the secondary. The disconnect eliminates the load for safe connection and the separate banks eliminate battery state conflict. Using 6 and 12 volt
batteries in a combined bank may work - 12 volt
is 12 volt
, but the interaction between cells is normally considered at a minimum poor practice.
Not an expert - suggest you spend some time consulting with one. I'll be interested to hear that information. Your question is an interesting one as the proposal is simple to implement. Just not convinced it is the right way to proceed. Maybe the solution is as simple as disconnecting the battery cables on the main bank while you use the backup. Pat