Voltage tested at battery is insufficient while at full load

I recently put in a new ECU due to my battery discharging over a week or so. I also installed a re-manufactured ECU. I cleaned the battery terminals and battery, cleaned the wire connections at the power distribution box, had the alternator pulled apart diodes and rotor short test performed which test fine. I had new brushes installed for the heck of it, I had the van scoped with the first re-manufactured ECU and everything tested fine including voltage drop tests etc. Except for the ecu driver was reported bad. The ecu had a lifetime limited warranty so the seller all computers inc. Sent a second ECU. After installation same problems. I tested the field wire on the ECU and it tested with engine running from cold no load 5.8v with engine revved it went down to 2.5v or close to it. The voltage measured 13.85v. Once at full load the field wire tested at 11.5 to 12.20v at idle and stayed there with the engine revved. I then tested the positive wire that runs to the alternator (the small one not the big one) at the ECU and it measured exactly what I got at the battery with the engine running under all loads and conditions. Also measured the negative ground into the ECU and read a 0.04v drop between what was at the battery and what was at the ECU. I also hooked a jump starting cable directly to the alternator case and the battery negative post with no changes in voltage both at the ECU wires and the voltage at the battery. I also removed the alternator wire from the power distribution box and left the battery cable hooked up thus running the engine on only battery power and eliminating the risk of blowing electronics. I started the engine and got 29v at idle and 50v with engine revved. I cannot figure out why the voltage output is not at least 13.8v under full load. Does anyone have any ideas or is it just a bad driver again? What is a ECU driver anyway? Is that the software program it uses or something else?