I think you have found your problem.Thank you so much. My ESS hasn't worked for at least 6 months and I do have a smaller battery. I will try that.
From what I have read you can eliminate the aux battery by disconnecting it's negative terminal.
If you want as a test, I would disconnect the negative wire from the aux battery and see if your main battery can be charged and hold the charge for a few days, or you may just want to take the main battery and get a load test on it to determine its health.
With the negative wire removed from the aux battery your ESS will no longer function and you will probably get the message that it is not available.
Let us know how you make out.