Originally Posted by Maine Sail
If your other bank is no longer receiving a charge the Echo will also shut down..
Well, yes and no. So long as the EchoCharge is sensing more than 13.0V on the main bank, including a "float level" of 13.2V or more, it will continue to pass some current to the auxiliary bank.
So, the answer to the OP's question is, YES, it will maintain the standby battery just fine.