OK, here we go. Here is the latest series of tests.
The only difference in setup from yesterday is that the 3nd column shows "DC Amps to the House Bank" ...as supplied by the power supply...not the amps actually going into the EchoCharge.
The power supply used was a Mastech HY-5020E capable of putting out 0-50VDC at up to 20 amps. Maximum I could get it to when attached to the gelled bank was 16.0 volts, which is shown in the table.
As you can see, once the voltage reached and exceeded 15.0 VDC, the EchoCharge would not output more than 14.5 volts...even with 16.0 volts in. This parallels the experience with the EchoCharge on my boat whenever I am equalizing my batteries at 15.5 to 16.5VDC.
So....apparently, as you have noted, the Echo charge pretty much follows the input voltage, less approximately 0.4V, until the input voltage reaches around 15, whence it "cuts off" any further voltage increase to the start battery.
This is just fine for flooded and AGM batteries, which are typically undercharged anyway. Higher charging voltages are better, up to a point, as others have noted, including Trojan battery engineers.
Would be most interested in the results of your bench tests, Maine.