Another Data Point
Here's another data point:
After sitting overnite with the EchoCharge connected to both battery banks but no charging current on, voltages this morning are:
Gel-cell house battery bank: 13.02
AGM start battery: 13.02
Charge current from the EchoCharge to the AGM battery: 0.25A
Because the resting voltage of the gels is above 12.9 or so, the EchoCharge light has been on all night and the device is still active, putting a tiny charge into the AGM "start battery".
Later today I plan to test the setup by putting a really high voltage on the gels for a short time, and see what the EchoCharge passes thru.
Stay tuned :-)